Skip to content. Skip to navigation
Sections
Personal tools
You are here: Home
Featured Journal
Navigation
Site Search
 
Search only the current folder (and sub-folders)
Log in


Forgot your password?
New user?
Check out our New Publishers' Select for Free Articles
Journal Search

Earth and Planetary Science Letters (v.241, #1-2)

Editorial Board (pp. iii).
Editorial Board (pp. iii).

The early thermal and magnetic state of the cratered highlands of Mars by Javier Ruiz; Patrick J. McGovern; Rosa Tejero (pp. 2-10).
Surface heat flows are calculated from elastic lithosphere thicknesses for the heavy cratered highlands of Mars, in terms of the fraction of the surface heat flow derived from crustal heat sources. Previous heat flow estimations for Mars used linear thermal gradients, which is equivalent to ignoring the existence of heat sources within the crust. We compute surface heat flows following a methodology that relates effective thickness and curvature of an elastic plate with the strength envelope of the lithosphere, and assuming crustal heat sources homogeneously distributed in a radioactive element-rich layer 20 or 60 km thick. The obtained results show that the surface heat flow increases with the proportion of heat sources within the crust, and with the decrease of both radioactive element-rich layer thickness and surface temperature. Also, the results permit us to calculate representative temperatures for the crust base, rock strength for the upper mantle, and lower and upper limits to the crustal magnetization depth and intensity, respectively. For Terra Cimmeria, an effective elastic thickness of 12 km implies between 30% and 80% of heat sources located within the crust. In this case the uppermost mantle would be weak at the time of loading, and temperatures in the lower crust cold enough to favor unrelaxed crustal thickness variations and to permit deep Curie depths in the highlands, as suggested by the observational evidence.

Keywords: Mars; elastic thickness; heat flow; thermal structure; Curie depth


The early thermal and magnetic state of the cratered highlands of Mars by Javier Ruiz; Patrick J. McGovern; Rosa Tejero (pp. 2-10).
Surface heat flows are calculated from elastic lithosphere thicknesses for the heavy cratered highlands of Mars, in terms of the fraction of the surface heat flow derived from crustal heat sources. Previous heat flow estimations for Mars used linear thermal gradients, which is equivalent to ignoring the existence of heat sources within the crust. We compute surface heat flows following a methodology that relates effective thickness and curvature of an elastic plate with the strength envelope of the lithosphere, and assuming crustal heat sources homogeneously distributed in a radioactive element-rich layer 20 or 60 km thick. The obtained results show that the surface heat flow increases with the proportion of heat sources within the crust, and with the decrease of both radioactive element-rich layer thickness and surface temperature. Also, the results permit us to calculate representative temperatures for the crust base, rock strength for the upper mantle, and lower and upper limits to the crustal magnetization depth and intensity, respectively. For Terra Cimmeria, an effective elastic thickness of 12 km implies between 30% and 80% of heat sources located within the crust. In this case the uppermost mantle would be weak at the time of loading, and temperatures in the lower crust cold enough to favor unrelaxed crustal thickness variations and to permit deep Curie depths in the highlands, as suggested by the observational evidence.

Keywords: Mars; elastic thickness; heat flow; thermal structure; Curie depth


The 3.26–3.24 Ga Barberton asteroid impact cluster: Tests of tectonic and magmatic consequences, Pilbara Craton, Western Australia by Andrew Glikson; John Vickers (pp. 11-20).
The location in the Barberton Greenstone Belt (Kaapvaal Craton) of ∼3.26–3.24 Ga asteroid impact ejecta units at, and immediately above, a sharp break between a >12 km-thick mafic–ultramafic volcanic crust (Onverwacht Group ∼3.55–3.26 Ga, including the ∼3.298>3.258 Ga Mendon Formation) and a turbidite–felsic volcanic rift-facies association (Fig Tree Group ∼3.258–3.225 Ga), potentially represents the first documented example of cause–effect relations between extraterrestrial bombardment and major tectonic and igneous events [D.R. Lowe, G.R. Byerly, F. Asaro, F.T. Kyte, Geological and geochemical record of 3400 Ma old terrestrial meteorite impacts, Science 245 (1989) 959–962; D.R. Lowe, G.R. Byerly, F.T. Kyte, A. Shukolyukov, F. Asaro, A. Krull, Spherule beds 3.47–3.34 Ga-old in the Barberton greenstone belt, South Africa: a record of large meteorite impacts and their influence on early crustal and biological evolution, Astrobiology 3 (2003) 7–48; A.Y. Glikson, The astronomical connection of terrestrial evolution: crustal effects of post-3.8 Ga mega-impact clusters and evidence for major 3.2±0.1 Ga bombardment of the Earth–Moon system, J. Geodyn. 32 (2001) 205–229]. Here we correlate this boundary with a contemporaneous break and peak magmatic and faulting events in the Pilbara Craton, represented by the truncation of a 3.255–3.235 Ga-old volcanic sequence (Sulphur Springs Group—SSG) by a turbidite-banded iron formation–felsic volcanic association (Pincunah Hill Formation, basal Gorge Creek Group). These events are accompanied by ∼3.252–3.235 Ga granitoids (Cleland plutonic suite). The top of the komatiite–tholeiite–rhyolite sequence of the SSG is associated with a marker chert defined at 3.238±3–3.235±3 Ga, abruptly overlain by an olistostrome consisting of mega-clasts of felsic volcanics, chert and siltstone up to 250×150 m-large, intercalated with siliciclastic sedimentary rocks and felsic volcanics (Pincunah Hill Formation-basal Gorge Creek Group-GCG [R. M. Hill, Stratigraphy, structure and alteration of hanging wall sedimentary rocks at the Sulphur Springs volcanogenic massive sulphide (VMS) prospect, east Pilbara Craton, Western Australia. B.Sc Hon. Thesis, University of Western Australia (1997) 67 pp.; M.J. Van Kranendonk, A.H. Hickman, R.H. Smithies, D.R. Nelson, Geology and tectonic evolution of the Archaean north Pilbara terrain, Pilbara Craton, Western Australia, Econ. Geol. 97 (2002) 695–732; M.J. Van Kranendonk, Geology of the North Shaw 1:100000 Sheet. Geological Survey Western Australia 1:100000 Geological Series (2000) 86 pp., R. Buick, C.A.W. Brauhart, P. Morant, J.R. Thornett, J.G. Maniew, J.G. Archibald, M.G. Doepel, I.R. Fletcher, A.L. Pickard, J.B. Smith, M.B. Barley, N.J. McNaughton, D.I. Groves, Geochronology and stratigraphic relations of the Sulphur Springs Group and Strelley Granite: a temporally distinct igneous province in the Archaean Pilbara Craton, Australia, Precambrian Res. 114 (2002) 87–120]). The structure and scale of the olistostrome, not seen elsewhere in the Pilbara Craton, is interpreted in terms of intense faulting and rifting, supported by topographic relief represented by deep incision of overlying arenites (Corboy Formation) into underlying units [M.J. Van Kranendonk, Geology of the North Shaw 1:100000 Sheet. Geological Survey Western Australia 1:100000 Geological Series (2000) 86 pp.]. The age overlaps between (1) 3.255±4–3.235±3 Ga peak igneous activity represented by the SSG and the Cleland plutonic suite (Pilbara Craton) and the 3.258±3 Ga S2 Barberton impact unit, and (2) 3.235±3 Ga top SSG break and associated faulting and the 3.243±4 S3–S4 Barberton impact units may not be accidental. Should correlations between the Barberton S2–S4 impact units and magmatic and tectonic events in the Pilbara Craton be confirmed, they would imply impact-triggered reactivation of mantle convection, crustal anatexis, faulting and strong vertical movements in Archaean granite–greenstone terrains associated with large asteroid impacts, culminating in transformation from sima-dominated crust to continental rift environments.

Keywords: Archaean; Pilbara; asteroid; impact; olistostrome


The 3.26–3.24 Ga Barberton asteroid impact cluster: Tests of tectonic and magmatic consequences, Pilbara Craton, Western Australia by Andrew Glikson; John Vickers (pp. 11-20).
The location in the Barberton Greenstone Belt (Kaapvaal Craton) of ∼3.26–3.24 Ga asteroid impact ejecta units at, and immediately above, a sharp break between a >12 km-thick mafic–ultramafic volcanic crust (Onverwacht Group ∼3.55–3.26 Ga, including the ∼3.298>3.258 Ga Mendon Formation) and a turbidite–felsic volcanic rift-facies association (Fig Tree Group ∼3.258–3.225 Ga), potentially represents the first documented example of cause–effect relations between extraterrestrial bombardment and major tectonic and igneous events [D.R. Lowe, G.R. Byerly, F. Asaro, F.T. Kyte, Geological and geochemical record of 3400 Ma old terrestrial meteorite impacts, Science 245 (1989) 959–962; D.R. Lowe, G.R. Byerly, F.T. Kyte, A. Shukolyukov, F. Asaro, A. Krull, Spherule beds 3.47–3.34 Ga-old in the Barberton greenstone belt, South Africa: a record of large meteorite impacts and their influence on early crustal and biological evolution, Astrobiology 3 (2003) 7–48; A.Y. Glikson, The astronomical connection of terrestrial evolution: crustal effects of post-3.8 Ga mega-impact clusters and evidence for major 3.2±0.1 Ga bombardment of the Earth–Moon system, J. Geodyn. 32 (2001) 205–229]. Here we correlate this boundary with a contemporaneous break and peak magmatic and faulting events in the Pilbara Craton, represented by the truncation of a 3.255–3.235 Ga-old volcanic sequence (Sulphur Springs Group—SSG) by a turbidite-banded iron formation–felsic volcanic association (Pincunah Hill Formation, basal Gorge Creek Group). These events are accompanied by ∼3.252–3.235 Ga granitoids (Cleland plutonic suite). The top of the komatiite–tholeiite–rhyolite sequence of the SSG is associated with a marker chert defined at 3.238±3–3.235±3 Ga, abruptly overlain by an olistostrome consisting of mega-clasts of felsic volcanics, chert and siltstone up to 250×150 m-large, intercalated with siliciclastic sedimentary rocks and felsic volcanics (Pincunah Hill Formation-basal Gorge Creek Group-GCG [R. M. Hill, Stratigraphy, structure and alteration of hanging wall sedimentary rocks at the Sulphur Springs volcanogenic massive sulphide (VMS) prospect, east Pilbara Craton, Western Australia. B.Sc Hon. Thesis, University of Western Australia (1997) 67 pp.; M.J. Van Kranendonk, A.H. Hickman, R.H. Smithies, D.R. Nelson, Geology and tectonic evolution of the Archaean north Pilbara terrain, Pilbara Craton, Western Australia, Econ. Geol. 97 (2002) 695–732; M.J. Van Kranendonk, Geology of the North Shaw 1:100000 Sheet. Geological Survey Western Australia 1:100000 Geological Series (2000) 86 pp., R. Buick, C.A.W. Brauhart, P. Morant, J.R. Thornett, J.G. Maniew, J.G. Archibald, M.G. Doepel, I.R. Fletcher, A.L. Pickard, J.B. Smith, M.B. Barley, N.J. McNaughton, D.I. Groves, Geochronology and stratigraphic relations of the Sulphur Springs Group and Strelley Granite: a temporally distinct igneous province in the Archaean Pilbara Craton, Australia, Precambrian Res. 114 (2002) 87–120]). The structure and scale of the olistostrome, not seen elsewhere in the Pilbara Craton, is interpreted in terms of intense faulting and rifting, supported by topographic relief represented by deep incision of overlying arenites (Corboy Formation) into underlying units [M.J. Van Kranendonk, Geology of the North Shaw 1:100000 Sheet. Geological Survey Western Australia 1:100000 Geological Series (2000) 86 pp.]. The age overlaps between (1) 3.255±4–3.235±3 Ga peak igneous activity represented by the SSG and the Cleland plutonic suite (Pilbara Craton) and the 3.258±3 Ga S2 Barberton impact unit, and (2) 3.235±3 Ga top SSG break and associated faulting and the 3.243±4 S3–S4 Barberton impact units may not be accidental. Should correlations between the Barberton S2–S4 impact units and magmatic and tectonic events in the Pilbara Craton be confirmed, they would imply impact-triggered reactivation of mantle convection, crustal anatexis, faulting and strong vertical movements in Archaean granite–greenstone terrains associated with large asteroid impacts, culminating in transformation from sima-dominated crust to continental rift environments.

Keywords: Archaean; Pilbara; asteroid; impact; olistostrome


Chemical divides and evaporite assemblages on Mars by Nicholas J. Tosca; Scott M. McLennan (pp. 21-31).
Assemblages of evaporite minerals record detailed physical and chemical characteristics of ancient surficial environments. Accordingly, newly discovered regions of saline minerals on Mars are high priority targets for exploration. The chemical divide concept of evaporite mineral formation is used successfully to predict evaporite mineralogy and brine evolution on Earth. However, basaltic weathering largely controls fluid compositions on Mars and the robust predictive capabilities of terrestrial chemical divides cannot be used to interpret Martian evaporites. Here we present a new chemical divide system that predicts evaporite assemblages identified in SNC-type meteorites, ancient evaporites discovered on Meridiani Planum by the Opportunity rover, and Mars Express OMEGA data. We suggest that a common fluid type that has been buffered to different pH levels by basaltic weathering controls the variability among Martian evaporite assemblages and that evaporite mineralogy and brine evolution is essentially established by the initial composition of the dilute evaporating fluid.

Keywords: Mars; Mars exploration rovers; evaporites; geochemistry; sulfates; carbonates


Chemical divides and evaporite assemblages on Mars by Nicholas J. Tosca; Scott M. McLennan (pp. 21-31).
Assemblages of evaporite minerals record detailed physical and chemical characteristics of ancient surficial environments. Accordingly, newly discovered regions of saline minerals on Mars are high priority targets for exploration. The chemical divide concept of evaporite mineral formation is used successfully to predict evaporite mineralogy and brine evolution on Earth. However, basaltic weathering largely controls fluid compositions on Mars and the robust predictive capabilities of terrestrial chemical divides cannot be used to interpret Martian evaporites. Here we present a new chemical divide system that predicts evaporite assemblages identified in SNC-type meteorites, ancient evaporites discovered on Meridiani Planum by the Opportunity rover, and Mars Express OMEGA data. We suggest that a common fluid type that has been buffered to different pH levels by basaltic weathering controls the variability among Martian evaporite assemblages and that evaporite mineralogy and brine evolution is essentially established by the initial composition of the dilute evaporating fluid.

Keywords: Mars; Mars exploration rovers; evaporites; geochemistry; sulfates; carbonates


Shock reequilibration of fluid inclusions in Coconino sandstone from Meteor Crater, Arizona by Megan E. Elwood Madden; David A. Kring; Robert J. Bodnar (pp. 32-46).
This study examines the effects of natural shock metamorphism on fluid inclusions trapped in porous sedimentary target rocks and compares these results to previous experimental work on single crystal quartz. Samples of shock metamorphosed Coconino sandstone were collected from Barringer Meteorite Crater (Meteor Crater, Arizona) and classified based on their shock features into the six shock stages described by Kieffer [S.W. Kieffer, 1971. Shock metamorphism of the Coconino sandstone at Meteor Crater, Arizona, Journal of Geophysical Research 76, 5449-5473.]. The frequency of two-phase fluid inclusions decreases dramatically from unshocked samples of Coconino sandstone through shock stages 1a, 1b, and 2. No two-phase fluid inclusions were observed in shock stage 3 or 4 samples. However, the total number of grains containing fluid inclusions remains approximately the same for shock stages 1a–2, suggesting that two-phase fluid inclusions reequilibrated during impact to form single-phase inclusions. In shock stages 3 and 4, the total number of inclusions also decreases, indicating that at these higher shock pressures fluid inclusions are destroyed by plastic deformation and phase changes within the host mineral. Entrained quartz grains within a shock stage 5 sample contain two-phase inclusions, emphasizing the short duration of melting associated with the impact and the heterogeneous nature of impact processes. These results are similar to those observed in single-crystal experiments, although inclusions survive to slightly higher shock pressures in samples of naturally shocked Coconino sandstone. Results of this study suggest that the rarity of fluid inclusions in meteorites does not preclude the presence of fluids on meteorite parent bodies. Instead, fluid inclusions trapped during alteration events may have been destroyed due to shock processing. In addition, loss of fluids from inclusion vesicles along fractures and microcracks may lead to shock devolatilization, even in unsaturated target rocks.

Keywords: fluid inclusions; Meteor Crater; meteorites; shock metamorphism; volatile loss


Shock reequilibration of fluid inclusions in Coconino sandstone from Meteor Crater, Arizona by Megan E. Elwood Madden; David A. Kring; Robert J. Bodnar (pp. 32-46).
This study examines the effects of natural shock metamorphism on fluid inclusions trapped in porous sedimentary target rocks and compares these results to previous experimental work on single crystal quartz. Samples of shock metamorphosed Coconino sandstone were collected from Barringer Meteorite Crater (Meteor Crater, Arizona) and classified based on their shock features into the six shock stages described by Kieffer [S.W. Kieffer, 1971. Shock metamorphism of the Coconino sandstone at Meteor Crater, Arizona, Journal of Geophysical Research 76, 5449-5473.]. The frequency of two-phase fluid inclusions decreases dramatically from unshocked samples of Coconino sandstone through shock stages 1a, 1b, and 2. No two-phase fluid inclusions were observed in shock stage 3 or 4 samples. However, the total number of grains containing fluid inclusions remains approximately the same for shock stages 1a–2, suggesting that two-phase fluid inclusions reequilibrated during impact to form single-phase inclusions. In shock stages 3 and 4, the total number of inclusions also decreases, indicating that at these higher shock pressures fluid inclusions are destroyed by plastic deformation and phase changes within the host mineral. Entrained quartz grains within a shock stage 5 sample contain two-phase inclusions, emphasizing the short duration of melting associated with the impact and the heterogeneous nature of impact processes. These results are similar to those observed in single-crystal experiments, although inclusions survive to slightly higher shock pressures in samples of naturally shocked Coconino sandstone. Results of this study suggest that the rarity of fluid inclusions in meteorites does not preclude the presence of fluids on meteorite parent bodies. Instead, fluid inclusions trapped during alteration events may have been destroyed due to shock processing. In addition, loss of fluids from inclusion vesicles along fractures and microcracks may lead to shock devolatilization, even in unsaturated target rocks.

Keywords: fluid inclusions; Meteor Crater; meteorites; shock metamorphism; volatile loss


Os–Hf–Sr–Nd isotope and PGE systematics of spinel peridotite xenoliths from Tok, SE Siberian craton: Effects of pervasive metasomatism in shallow refractory mantle by Dmitri A. Ionov; Steven B. Shirey; Dominique Weis; Gerhard Brgmann (pp. 47-64).
Os–Hf–Sr–Nd isotopes and PGE were determined in peridotite xenoliths carried to the surface by Quaternary alkali basaltic magmas in the Tokinsky Stanovik Range on the Aldan shield. These data constrain the timing and nature of partial melting and metasomatism in the lithospheric mantle beneath SE Siberian craton. The xenoliths range from the rare fertile spinel lherzolites to the more abundant, strongly metasomatised olivine-rich (70–84%) rocks. Hf–Sr–Nd isotope compositions of the xenoliths are mainly within the fields of oceanic basalts. Most metasomatised xenoliths have lower143Nd/144Nd and176Hf/177Hf and higher87Sr/86Sr than the host basalts indicating that the metasomatism is older and has distinct sources. A few xenoliths have elevated176Hf/177Hf (up to 0.2838) and plot above the Hf–Nd mantle array defined by oceanic basalts.187Os/188Os in the poorly metasomatised, fertile to moderately refractory (Al2O3≥1.6%) Tok peridotites range from 0.1156 to 0.1282, with oldest rhenium depletion ages being about 2 Ga. The187Os/188Os in these rocks show good correlations with partial melting indices (e.g. Al2O3, modal cpx); the intercept of the Al–187Os/188Os correlation with lowest Al2O3 estimates for melting residues (∼0.3–0.5%) has a187Os/188Os of ∼0.109 suggesting that these peridotites may have experienced melt extraction as early as 2.8 Gy ago.187Os/188Os in the strongly metasomatised, olivine-rich xenoliths (0.6–1.3% Al2O3) ranges from 0.1164 to 0.1275 and shows no apparent links to modal or chemical compositions. Convex-upward REE patterns and high abundances of heavy to middle REE in these refractory rocks indicate equilibration with evolved silicate melts at high melt/rock ratios, which may have also variably elevated their187Os/188Os. This inference is supported by enrichments in Pd and Pt on chondrite-normalised PGE abundance patterns in some of the rocks. The melt extraction ages for the Tok suite of 2.0 to 2.8 Ga are younger than oldest Os ages reported for central Siberian craton, but they must be considered minimum estimates because of the extensive metasomatism of the most refractory Tok peridotites. This metasomatism could have occurred in the late Mesozoic to early Cenozoic when the Tok region was close to the subduction-related Pacific margin of Siberia and experienced large-scale tectonic and magmatic activity. This study indicates that metasomatic effects on the Re–Os system in the shallow lithospheric mantle can be dramatic.

Keywords: Siberia; mantle peridotite; metasomatism; Os isotopes; Hf–Nd–Sr isotopes; PGE


Os–Hf–Sr–Nd isotope and PGE systematics of spinel peridotite xenoliths from Tok, SE Siberian craton: Effects of pervasive metasomatism in shallow refractory mantle by Dmitri A. Ionov; Steven B. Shirey; Dominique Weis; Gerhard Brügmann (pp. 47-64).
Os–Hf–Sr–Nd isotopes and PGE were determined in peridotite xenoliths carried to the surface by Quaternary alkali basaltic magmas in the Tokinsky Stanovik Range on the Aldan shield. These data constrain the timing and nature of partial melting and metasomatism in the lithospheric mantle beneath SE Siberian craton. The xenoliths range from the rare fertile spinel lherzolites to the more abundant, strongly metasomatised olivine-rich (70–84%) rocks. Hf–Sr–Nd isotope compositions of the xenoliths are mainly within the fields of oceanic basalts. Most metasomatised xenoliths have lower143Nd/144Nd and176Hf/177Hf and higher87Sr/86Sr than the host basalts indicating that the metasomatism is older and has distinct sources. A few xenoliths have elevated176Hf/177Hf (up to 0.2838) and plot above the Hf–Nd mantle array defined by oceanic basalts.187Os/188Os in the poorly metasomatised, fertile to moderately refractory (Al2O3≥1.6%) Tok peridotites range from 0.1156 to 0.1282, with oldest rhenium depletion ages being about 2 Ga. The187Os/188Os in these rocks show good correlations with partial melting indices (e.g. Al2O3, modal cpx); the intercept of the Al–187Os/188Os correlation with lowest Al2O3 estimates for melting residues (∼0.3–0.5%) has a187Os/188Os of ∼0.109 suggesting that these peridotites may have experienced melt extraction as early as 2.8 Gy ago.187Os/188Os in the strongly metasomatised, olivine-rich xenoliths (0.6–1.3% Al2O3) ranges from 0.1164 to 0.1275 and shows no apparent links to modal or chemical compositions. Convex-upward REE patterns and high abundances of heavy to middle REE in these refractory rocks indicate equilibration with evolved silicate melts at high melt/rock ratios, which may have also variably elevated their187Os/188Os. This inference is supported by enrichments in Pd and Pt on chondrite-normalised PGE abundance patterns in some of the rocks. The melt extraction ages for the Tok suite of 2.0 to 2.8 Ga are younger than oldest Os ages reported for central Siberian craton, but they must be considered minimum estimates because of the extensive metasomatism of the most refractory Tok peridotites. This metasomatism could have occurred in the late Mesozoic to early Cenozoic when the Tok region was close to the subduction-related Pacific margin of Siberia and experienced large-scale tectonic and magmatic activity. This study indicates that metasomatic effects on the Re–Os system in the shallow lithospheric mantle can be dramatic.

Keywords: Siberia; mantle peridotite; metasomatism; Os isotopes; Hf–Nd–Sr isotopes; PGE


Primitive neon and helium isotopic compositions of high-MgO basalts from the Kerguelen Archipelago, Indian Ocean by Sonia Doucet; Manuel Moreira; Dominique Weis; James S. Scoates; Andr Giret; Claude Allgre (pp. 65-79).
The geochemical characteristics of mildly alkalic basalts (24–25 Ma) erupted in the southeastern Kerguelen Archipelago are considered to represent the best estimate for the composition of the enriched Kerguelen plume end-member. A recent study of picrites and high-MgO basalts from this part of the archipelago highlighted the Pb and Hf isotopic variations and suggested the presence of mantle heterogeneities within the Kerguelen plume itself. We present new helium and neon isotopic compositions for olivines from these picrites and high-MgO basalts (6–17 wt.% MgO) both to constrain the enriched composition of the Kerguelen plume and to determine the origin of isotopic heterogeneities involved in the genesis of Kerguelen plume-related basalts. The olivine phenocrysts have extremely variable4He/3He compositions between MORB and primitive values observed in OIB (∼90,000 to 40,000; i.e., R/ Ra ∼8 to 18) and they show primitive neon isotopic ratios (average21Ne/21Neext ∼0.044). The neon isotopic systematics and the4He/3He ratios that are lower than MORB values for the Kerguelen basalts clearly suggest that the Kerguelen hotspot belongs to the family of primitive hotspots, such as Iceland and Hawaii. The rare gas signature for the Kerguelen samples, intermediate between MORB and solar, is apparently inconsistent with mixing of a primitive component with a MORB-like source, but may result from sampling a heterogeneous part of the mantle with solar3He/22Ne and with a higher (U,Th)/3He ratio compared to typically high R/ Ra hotspot basalts such as those from Iceland and Hawaii.

Keywords: helium and neon isotopes; Kerguelen plume; high-MgO basalts; olivine; mantle


Primitive neon and helium isotopic compositions of high-MgO basalts from the Kerguelen Archipelago, Indian Ocean by Sonia Doucet; Manuel Moreira; Dominique Weis; James S. Scoates; André Giret; Claude Allègre (pp. 65-79).
The geochemical characteristics of mildly alkalic basalts (24–25 Ma) erupted in the southeastern Kerguelen Archipelago are considered to represent the best estimate for the composition of the enriched Kerguelen plume end-member. A recent study of picrites and high-MgO basalts from this part of the archipelago highlighted the Pb and Hf isotopic variations and suggested the presence of mantle heterogeneities within the Kerguelen plume itself. We present new helium and neon isotopic compositions for olivines from these picrites and high-MgO basalts (6–17 wt.% MgO) both to constrain the enriched composition of the Kerguelen plume and to determine the origin of isotopic heterogeneities involved in the genesis of Kerguelen plume-related basalts. The olivine phenocrysts have extremely variable4He/3He compositions between MORB and primitive values observed in OIB (∼90,000 to 40,000; i.e., R/ Ra ∼8 to 18) and they show primitive neon isotopic ratios (average21Ne/21Neext ∼0.044). The neon isotopic systematics and the4He/3He ratios that are lower than MORB values for the Kerguelen basalts clearly suggest that the Kerguelen hotspot belongs to the family of primitive hotspots, such as Iceland and Hawaii. The rare gas signature for the Kerguelen samples, intermediate between MORB and solar, is apparently inconsistent with mixing of a primitive component with a MORB-like source, but may result from sampling a heterogeneous part of the mantle with solar3He/22Ne and with a higher (U,Th)/3He ratio compared to typically high R/ Ra hotspot basalts such as those from Iceland and Hawaii.

Keywords: helium and neon isotopes; Kerguelen plume; high-MgO basalts; olivine; mantle


40Ar/39Ar geochronology constraints on late miocene weathering rates in Minas Gerais, Brazil by Isabela de Oliveira Carmo; Paulo Marcos Vasconcelos (pp. 80-94).
40Ar/39Ar incremental heating ages for twenty one grains of cryptomelane, collected at 0, 42, 45, and 60 m depths in the Cachoeira Mine weathering profile, Minas Gerais, permit calculating long-term (10 Ma time scale) weathering rate (saprolitization rate) in SE Brazil. Pure well-crystallized cryptomelane grains with high K contents (3–5 wt.%) yield reliable geochronological results. The40Ar/39Ar plateau ages obtained decrease from the top to the bottom of the profile (12.7±0.1 to 7.6±0.1 Ma at surface; 7.6±0.2 to 6.1±0.2 Ma at 42 m; and 7.1±0.2 to 5.9±0.1 Ma at 45 m; 6.6±0.1 to 5.2±0.1 Ma at 60 m), yielding a weathering front propagation rate of 8.9±1.1 m/m.y. From the geochronological results and the mineral transformations implicit by the current mineralogy in the weathering profiles, it is possible to calculate the saprolitization rate for the Cachoeira Mine lithologies and for adjacent weathering profiles developed on granodiorites and schists. The measured weathering front propagation rate yields a saprolitization rate of 24.9±3.1 t/km2/yr. This average long-term (>10 Ma) saprolitization rate is consistent with mass balance calculations results for present saprolitization rates in weathering watersheds. These results are also consistent with long-term saprolitization rates estimated by combining cosmogenic isotope denudation rates with mass balance calculations.

Keywords: Weathering rate; chemical denudation; 40; Ar; /; 39; Ar geochronology; Miocene; SE Brazil


40Ar/39Ar geochronology constraints on late miocene weathering rates in Minas Gerais, Brazil by Isabela de Oliveira Carmo; Paulo Marcos Vasconcelos (pp. 80-94).
40Ar/39Ar incremental heating ages for twenty one grains of cryptomelane, collected at 0, 42, 45, and 60 m depths in the Cachoeira Mine weathering profile, Minas Gerais, permit calculating long-term (10 Ma time scale) weathering rate (saprolitization rate) in SE Brazil. Pure well-crystallized cryptomelane grains with high K contents (3–5 wt.%) yield reliable geochronological results. The40Ar/39Ar plateau ages obtained decrease from the top to the bottom of the profile (12.7±0.1 to 7.6±0.1 Ma at surface; 7.6±0.2 to 6.1±0.2 Ma at 42 m; and 7.1±0.2 to 5.9±0.1 Ma at 45 m; 6.6±0.1 to 5.2±0.1 Ma at 60 m), yielding a weathering front propagation rate of 8.9±1.1 m/m.y. From the geochronological results and the mineral transformations implicit by the current mineralogy in the weathering profiles, it is possible to calculate the saprolitization rate for the Cachoeira Mine lithologies and for adjacent weathering profiles developed on granodiorites and schists. The measured weathering front propagation rate yields a saprolitization rate of 24.9±3.1 t/km2/yr. This average long-term (>10 Ma) saprolitization rate is consistent with mass balance calculations results for present saprolitization rates in weathering watersheds. These results are also consistent with long-term saprolitization rates estimated by combining cosmogenic isotope denudation rates with mass balance calculations.

Keywords: Weathering rate; chemical denudation; 40; Ar; /; 39; Ar geochronology; Miocene; SE Brazil


High-pressure phases of CaCO3: Crystal structure prediction and experiment by Artem R. Oganov; Colin W. Glass; Shigeaki Ono (pp. 95-103).
Post-aragonite phase of CaCO3, experimentally known to be stable above 40 GPa [S. Ono, T. Kikegawa, Y. Ohishi, J. Tsuchiya, Post-aragonite phase transformation in CaCO3 at 40 GPa, Am. Mineral. 90 (2005) 667–671], is believed to be a major carbon-containing mineral in the Earth's mantle. Crystal structure of this mineral phase could not be solved using experimental data or traditional theoretical simulation methods and remained a controversial issue. Using a combination of advanced ab initio simulation techniques and high-pressure experiment, we have been able to determine the crystal structure of CaCO3 post-aragonite. Here, we performed simulations with the USPEX code [C.W. Glass, A.R. Oganov, and N. Hansen, (in preparation). USPEX: a universal structure prediction program], which is based on an evolutionary algorithm using ab initio free energy as the fitness function. This novel methodology for crystal structure prediction, which uses only the chemical composition as input, is described in detail. For CaCO3, we identify a number of energetically competitive structures, the most stable of which closely matches the experimental powder diffraction pattern and, in agreement with experiment, becomes more stable than aragonite above 42 GPa. This structure belongs to a new structure type, which is also adopted by the high-pressure post-aragonite phases of SrCO3 and BaCO3. It has 2 formula units in the orthorhombic unit cell (space group Pmmn) and contains triangular CO32− ions and Ca2+ ions in the 12-fold coordination. Above 137 GPa, a pyroxene-type structure (space group C2221) with chains of CO44− tetrahedra becomes more stable than post-aragonite. For MgCO3, this structure becomes more stable than magnesite above 106 GPa and is a good candidate structure for MgCO3 post-magnesite.

Keywords: CaCO; 3; post-aragonite; MgCO; 3; post-magnesite; D″ layer; high pressure; crystal structure prediction; evolutionary algorithm; ab initio; density functional theory


High-pressure phases of CaCO3: Crystal structure prediction and experiment by Artem R. Oganov; Colin W. Glass; Shigeaki Ono (pp. 95-103).
Post-aragonite phase of CaCO3, experimentally known to be stable above 40 GPa [S. Ono, T. Kikegawa, Y. Ohishi, J. Tsuchiya, Post-aragonite phase transformation in CaCO3 at 40 GPa, Am. Mineral. 90 (2005) 667–671], is believed to be a major carbon-containing mineral in the Earth's mantle. Crystal structure of this mineral phase could not be solved using experimental data or traditional theoretical simulation methods and remained a controversial issue. Using a combination of advanced ab initio simulation techniques and high-pressure experiment, we have been able to determine the crystal structure of CaCO3 post-aragonite. Here, we performed simulations with the USPEX code [C.W. Glass, A.R. Oganov, and N. Hansen, (in preparation). USPEX: a universal structure prediction program], which is based on an evolutionary algorithm using ab initio free energy as the fitness function. This novel methodology for crystal structure prediction, which uses only the chemical composition as input, is described in detail. For CaCO3, we identify a number of energetically competitive structures, the most stable of which closely matches the experimental powder diffraction pattern and, in agreement with experiment, becomes more stable than aragonite above 42 GPa. This structure belongs to a new structure type, which is also adopted by the high-pressure post-aragonite phases of SrCO3 and BaCO3. It has 2 formula units in the orthorhombic unit cell (space group Pmmn) and contains triangular CO32− ions and Ca2+ ions in the 12-fold coordination. Above 137 GPa, a pyroxene-type structure (space group C2221) with chains of CO44− tetrahedra becomes more stable than post-aragonite. For MgCO3, this structure becomes more stable than magnesite above 106 GPa and is a good candidate structure for MgCO3 post-magnesite.

Keywords: CaCO; 3; post-aragonite; MgCO; 3; post-magnesite; D″ layer; high pressure; crystal structure prediction; evolutionary algorithm; ab initio; density functional theory


Silicate and carbonate melt inclusions associated with diamonds in deeply subducted carbonate rocks by Andrey V. Korsakov; Jrg Hermann (pp. 104-118).
Deeply subducted carbonate rocks from the Kokchetav massif (Northern Kazakhstan) recrystallised within the diamond stability field ( P=4.5–6.0 GPa; T≈1000 °C) and preserve evidence for ultra high-pressure carbonate and silicate melts. The carbonate rocks consist of garnet and K-bearing clinopyroxene embedded in a dolomite or magnesian calcite matrix. Polycrystalline magnesian calcite and polyphase carbonate–silicate inclusions occurring in garnet and clinopyroxene show textural features of former melt inclusions. The trace element composition of such carbonate inclusions is enriched in Ba and light rare earth elements and depleted in heavy rare earth elements with respect to the matrix carbonates providing further evidence that the inclusions represent trapped carbonate melt. Polyphase inclusions in garnet and clinopyroxene within a magnesian calcite marble, consisting mainly of a tight intergrowth of biotite+K-feldspar and biotite+zoisite+titanite, are interpreted to represent two different types of K-rich silicate melts. Both melt types show high contents of large ion lithophile elements but contrasting contents of rare earth elements. The Ca-rich inclusions display high REE contents similar to the carbonate inclusions and show a general trace element characteristic compatible with a hydrous granitic origin. Low SiO2 content in the silicate melts indicates that they represent residual melts after extensive interaction with carbonates. These observations suggest that hydrous granitic melts derived from the adjacent metapelites reacted with dolomite at ultra high-pressure conditions to form garnet, clinopyroxene – a hydrous carbonate melt – and residual silicate melts. Silicate and carbonate melt inclusions contain diamond, providing evidence that such an interaction promotes diamond growth. The finding of carbonate melts in deeply subducted crust might have important consequences for recycling of trace elements and especially C from the slab to the mantle wedge.

Keywords: diamond; carbonate and silicate melts; UHP metamorphism


Silicate and carbonate melt inclusions associated with diamonds in deeply subducted carbonate rocks by Andrey V. Korsakov; Jörg Hermann (pp. 104-118).
Deeply subducted carbonate rocks from the Kokchetav massif (Northern Kazakhstan) recrystallised within the diamond stability field ( P=4.5–6.0 GPa; T≈1000 °C) and preserve evidence for ultra high-pressure carbonate and silicate melts. The carbonate rocks consist of garnet and K-bearing clinopyroxene embedded in a dolomite or magnesian calcite matrix. Polycrystalline magnesian calcite and polyphase carbonate–silicate inclusions occurring in garnet and clinopyroxene show textural features of former melt inclusions. The trace element composition of such carbonate inclusions is enriched in Ba and light rare earth elements and depleted in heavy rare earth elements with respect to the matrix carbonates providing further evidence that the inclusions represent trapped carbonate melt. Polyphase inclusions in garnet and clinopyroxene within a magnesian calcite marble, consisting mainly of a tight intergrowth of biotite+K-feldspar and biotite+zoisite+titanite, are interpreted to represent two different types of K-rich silicate melts. Both melt types show high contents of large ion lithophile elements but contrasting contents of rare earth elements. The Ca-rich inclusions display high REE contents similar to the carbonate inclusions and show a general trace element characteristic compatible with a hydrous granitic origin. Low SiO2 content in the silicate melts indicates that they represent residual melts after extensive interaction with carbonates. These observations suggest that hydrous granitic melts derived from the adjacent metapelites reacted with dolomite at ultra high-pressure conditions to form garnet, clinopyroxene – a hydrous carbonate melt – and residual silicate melts. Silicate and carbonate melt inclusions contain diamond, providing evidence that such an interaction promotes diamond growth. The finding of carbonate melts in deeply subducted crust might have important consequences for recycling of trace elements and especially C from the slab to the mantle wedge.

Keywords: diamond; carbonate and silicate melts; UHP metamorphism


Magnetite dissolution, diachronous greigite formation, and secondary magnetizations from pyrite oxidation: Unravelling complex magnetizations in Neogene marine sediments from New Zealand by Christopher J. Rowan; Andrew P. Roberts (pp. 119-137).
Detailed rock magnetic and electron microscope analyses indicate that the magnetic signature of Neogene marine sediments from the east coast of New Zealand is dominated by the authigenic iron sulphide greigite. The greigite is present as a mixed population of stable single domain and superparamagnetic grains, which is consistent with authigenic growth from solution. This growth can result from pyritization reactions soon after deposition, which also leads to dissolution of most detrital magnetite; however, where constrained by field tests, our data suggest that remanence acquisition can occur >1 Myr after deposition, and can vary in timing at the outcrop scale. Strong viscous overprints result from oxidation of the iron sulphides, probably during percolation of oxic ground water. This process can sometimes destroy any ancient remanent magnetization. This complex magnetic behaviour, particularly the presence of late-forming magnetizations carried by greigite, means that the remanence in New Zealand Cenozoic sediments, and in similar sediments elsewhere, cannot be assumed to be primary without confirmation by field tests. The reversals test should be employed with caution in such sediments, as patchy remagnetizations can lead to false polarity stratigraphies.

Keywords: magnetite; dissolution; greigite; pyrite; hematite; remagnetization; New Zealand


Magnetite dissolution, diachronous greigite formation, and secondary magnetizations from pyrite oxidation: Unravelling complex magnetizations in Neogene marine sediments from New Zealand by Christopher J. Rowan; Andrew P. Roberts (pp. 119-137).
Detailed rock magnetic and electron microscope analyses indicate that the magnetic signature of Neogene marine sediments from the east coast of New Zealand is dominated by the authigenic iron sulphide greigite. The greigite is present as a mixed population of stable single domain and superparamagnetic grains, which is consistent with authigenic growth from solution. This growth can result from pyritization reactions soon after deposition, which also leads to dissolution of most detrital magnetite; however, where constrained by field tests, our data suggest that remanence acquisition can occur >1 Myr after deposition, and can vary in timing at the outcrop scale. Strong viscous overprints result from oxidation of the iron sulphides, probably during percolation of oxic ground water. This process can sometimes destroy any ancient remanent magnetization. This complex magnetic behaviour, particularly the presence of late-forming magnetizations carried by greigite, means that the remanence in New Zealand Cenozoic sediments, and in similar sediments elsewhere, cannot be assumed to be primary without confirmation by field tests. The reversals test should be employed with caution in such sediments, as patchy remagnetizations can lead to false polarity stratigraphies.

Keywords: magnetite; dissolution; greigite; pyrite; hematite; remagnetization; New Zealand


Magma dynamics at mid-ocean ridges by noble gas kinetic fractionation: Assessment of magmatic ascent rates by A. Paonita; M. Martelli (pp. 138-158).
Despite its impact in understanding oceanic crust formation and eruptive styles of related volcanism, magma dynamics at mid-ocean ridges are poorly known. Here, we propose a new method to assess ascent rates of mid-ocean ridge basalt (MORB) magmas, as well as their pre- and sin-eruptive dynamics. It is based on the idea that a rising magma can reach a variable degree of both CO2 supersaturation in melt and kinetic fractionation among noble gases in vesicles in relation to its ascent rate through the crust. To quantify the relationship, we have used a model of multicomponent bubble growth in MORB melts, developed by extending the single-component model of Proussevitch and Sahagian [A.A. Proussevitch, D.L. Sahagian, Dynamics and energetics of bubble growth in magmas: analytical formulation and numerical modeling, J. Geophys. Res. 103 (1998), 18223–18251.] to CO2–He–Ar gas mixtures. After proper parameterization, we have applied it to published suites of data having the required features (glasses from Pito Seamount and mid-Atlantic ridges). Our results highlight that the investigated MORB magmas display very different ranges of ascent rates: slow rises of popping rock forming-magmas that cross the crust (0.01–0.5 m/s), slightly faster rates of energetic effusions (0.1–1 m/s), up to rates of 1–10 m/s which fall on the edge between lava effusion and Hawaiian activity. Inside a single plumbing system, very dissimilar magma dynamics highlight the large differences in compressive stress of the oceanic crust on a small scale. Constraints on how the systems of ridges work, as well as the characteristics of the magmatic source, can also be obtained. Our model shows how measurements of both the dissolved gas concentration in melt and the volatile composition of vesicles in the same sample are crucial in recognizing the kinetic effects and definitively assessing magma dynamics. An effort should be made to correctly set the studied samples in the sequence of volcanic submarine deposits where they are collected. Enhanced knowledge of a number of physical properties of gas-bearing MOR magmas is also required, mainly noble gas diffusivities, to describe multicomponent bubble growth at a higher confidence level.

Keywords: Bubble growth; MORB; Noble gas; Kinetic fractionation; Modeling


Magma dynamics at mid-ocean ridges by noble gas kinetic fractionation: Assessment of magmatic ascent rates by A. Paonita; M. Martelli (pp. 138-158).
Despite its impact in understanding oceanic crust formation and eruptive styles of related volcanism, magma dynamics at mid-ocean ridges are poorly known. Here, we propose a new method to assess ascent rates of mid-ocean ridge basalt (MORB) magmas, as well as their pre- and sin-eruptive dynamics. It is based on the idea that a rising magma can reach a variable degree of both CO2 supersaturation in melt and kinetic fractionation among noble gases in vesicles in relation to its ascent rate through the crust. To quantify the relationship, we have used a model of multicomponent bubble growth in MORB melts, developed by extending the single-component model of Proussevitch and Sahagian [A.A. Proussevitch, D.L. Sahagian, Dynamics and energetics of bubble growth in magmas: analytical formulation and numerical modeling, J. Geophys. Res. 103 (1998), 18223–18251.] to CO2–He–Ar gas mixtures. After proper parameterization, we have applied it to published suites of data having the required features (glasses from Pito Seamount and mid-Atlantic ridges). Our results highlight that the investigated MORB magmas display very different ranges of ascent rates: slow rises of popping rock forming-magmas that cross the crust (0.01–0.5 m/s), slightly faster rates of energetic effusions (0.1–1 m/s), up to rates of 1–10 m/s which fall on the edge between lava effusion and Hawaiian activity. Inside a single plumbing system, very dissimilar magma dynamics highlight the large differences in compressive stress of the oceanic crust on a small scale. Constraints on how the systems of ridges work, as well as the characteristics of the magmatic source, can also be obtained. Our model shows how measurements of both the dissolved gas concentration in melt and the volatile composition of vesicles in the same sample are crucial in recognizing the kinetic effects and definitively assessing magma dynamics. An effort should be made to correctly set the studied samples in the sequence of volcanic submarine deposits where they are collected. Enhanced knowledge of a number of physical properties of gas-bearing MOR magmas is also required, mainly noble gas diffusivities, to describe multicomponent bubble growth at a higher confidence level.

Keywords: Bubble growth; MORB; Noble gas; Kinetic fractionation; Modeling


High-temperature archeointensity measurements from Mesopotamia by Yves Gallet; Maxime Le Goff (pp. 159-173).
We present new archeointensity results obtained from 127 potsherds and baked brick fragments dated from the last four millennia BC which were collected from different Syrian archeological excavations. High temperature magnetization measurements were carried out using a laboratory-built triaxial vibrating sample magnetometer (Triaxe), and ancient field intensity determinations were derived from the experimental procedure described by Le Goff and Gallet [Le Goff and Gallet. Earth Planet. Sci. Lett. 229 (2004) 31–43]. As some of the studied samples were previously analyzed using the classical Thellier and Thellier [Thellier and Thellier . Ann. Geophys. 15 (1959) 285–376] method revised by Coe [Coe. J. Geophys. Res. 72 (1967) 3247–3262], a comparison of the results is made from the two methods. The differences both at the fragment and site levels are mostly within ±5%, which strengthens the validity of the experimental procedure developed for the Triaxe. The new data help to better constrain the geomagnetic field intensity variations in Mesopotamia during archeological times, with the probable occurrence of an archeomagnetic jerk around 2800–2600 BC.

Keywords: archeointensity; high temperature magnetization measurements; Triaxe; Thellier and Thellier method; Mesopotamia


High-temperature archeointensity measurements from Mesopotamia by Yves Gallet; Maxime Le Goff (pp. 159-173).
We present new archeointensity results obtained from 127 potsherds and baked brick fragments dated from the last four millennia BC which were collected from different Syrian archeological excavations. High temperature magnetization measurements were carried out using a laboratory-built triaxial vibrating sample magnetometer (Triaxe), and ancient field intensity determinations were derived from the experimental procedure described by Le Goff and Gallet [Le Goff and Gallet. Earth Planet. Sci. Lett. 229 (2004) 31–43]. As some of the studied samples were previously analyzed using the classical Thellier and Thellier [Thellier and Thellier . Ann. Geophys. 15 (1959) 285–376] method revised by Coe [Coe. J. Geophys. Res. 72 (1967) 3247–3262], a comparison of the results is made from the two methods. The differences both at the fragment and site levels are mostly within ±5%, which strengthens the validity of the experimental procedure developed for the Triaxe. The new data help to better constrain the geomagnetic field intensity variations in Mesopotamia during archeological times, with the probable occurrence of an archeomagnetic jerk around 2800–2600 BC.

Keywords: archeointensity; high temperature magnetization measurements; Triaxe; Thellier and Thellier method; Mesopotamia


Seasonality intensification and long-term winter cooling as a part of the Late Pliocene climate development by Stefan Klotz; Sverine Fauquette; Nathalie Combourieu-Nebout; Dieter Uhl; Jean-Pierre Suc; Volker Mosbrugger (pp. 174-187).
A mutual climatic range method is applied to the Mediterranean marine pollen record of Semaforo (Vrica section, Calabria, Italy) covering the period from ∼2.46 Ma to ∼2.11 Ma. The method yields detailed information on summer, annual and winter temperatures and on precipitation during the nine obliquity and precession-controlled ‘glacial’ periods (marine isotope stages 96 to 80) and eight ‘interglacial’ periods (marine isotope stages 95 to 81) characterising this time interval. The reconstruction reveals higher temperatures of at least 2.8 °C in mean annual and 2.2 °C in winter temperatures, and 500 mm in precipitation during the ‘interglacials’ as compared to the present-day climate in the study area. During the ‘glacials’, temperatures are generally lower as compared to the present-day climate in the region, but precipitation is equivalent. Along the consecutive ‘interglacials’, a trend toward a reduction in annual and winter temperatures by more than 2.3 °C, and toward a higher seasonality is observed. Along the consecutive ‘glacials’, a trend toward a strong reduction in all temperature parameters of at least 1.6 °C is reconstructed. Climatic amplitudes of ‘interglacial–glacial’ transitions increase from the older to the younger cycles for summer and annual temperatures. The cross-spectral analyses suggest obliquity related warm/humid–cold/dry ‘interglacial–glacial’ cycles which are superimposed by precession related warm/dry– cold/humid cycles. A time displacement in the development of temperatures and precipitation is indicated for the obliquity band by temperatures generally leading precipitation change at ∼4 kyr, and on the precession band of ∼9.6 kyr in maximum.

Keywords: Late Pliocene; obliquity; precession; palaeoclimate; pollen


Seasonality intensification and long-term winter cooling as a part of the Late Pliocene climate development by Stefan Klotz; Séverine Fauquette; Nathalie Combourieu-Nebout; Dieter Uhl; Jean-Pierre Suc; Volker Mosbrugger (pp. 174-187).
A mutual climatic range method is applied to the Mediterranean marine pollen record of Semaforo (Vrica section, Calabria, Italy) covering the period from ∼2.46 Ma to ∼2.11 Ma. The method yields detailed information on summer, annual and winter temperatures and on precipitation during the nine obliquity and precession-controlled ‘glacial’ periods (marine isotope stages 96 to 80) and eight ‘interglacial’ periods (marine isotope stages 95 to 81) characterising this time interval. The reconstruction reveals higher temperatures of at least 2.8 °C in mean annual and 2.2 °C in winter temperatures, and 500 mm in precipitation during the ‘interglacials’ as compared to the present-day climate in the study area. During the ‘glacials’, temperatures are generally lower as compared to the present-day climate in the region, but precipitation is equivalent. Along the consecutive ‘interglacials’, a trend toward a reduction in annual and winter temperatures by more than 2.3 °C, and toward a higher seasonality is observed. Along the consecutive ‘glacials’, a trend toward a strong reduction in all temperature parameters of at least 1.6 °C is reconstructed. Climatic amplitudes of ‘interglacial–glacial’ transitions increase from the older to the younger cycles for summer and annual temperatures. The cross-spectral analyses suggest obliquity related warm/humid–cold/dry ‘interglacial–glacial’ cycles which are superimposed by precession related warm/dry– cold/humid cycles. A time displacement in the development of temperatures and precipitation is indicated for the obliquity band by temperatures generally leading precipitation change at ∼4 kyr, and on the precession band of ∼9.6 kyr in maximum.

Keywords: Late Pliocene; obliquity; precession; palaeoclimate; pollen


Effects of the Santorini (Thera) eruption on manganese behavior in Holocene sediments of the eastern Mediterranean by Anja Reitz; John Thomson; Gert J. de Lange; Darryl R.H. Green; Caroline P. Slomp; A. Catalina Gebhardt (pp. 188-201).
The explosive eruption on the island of Santorini in ∼1630 B.C. in Minoan times had a large environmental impact over the eastern Mediterranean region. It has even been suggested that the Mn-enriched layer (the “Marker Bed?) above the most recent sapropel (S1) in sediments of a crestal area of the Mediterranean Ridge gained Mn from a hydrothermal source related to the Santorini eruption. Radiocarbon dating of two cores from this area sampled at high resolution demonstrate that this large Mn peak in fact pre-dates the Santorini event by ∼2.8 ky and forms part of a pattern seen in Mn profiles from all over the eastern Mediterranean. This same Mn profile shape is altered in areas that experienced substantial deposits of either the tephra layer emitted by the Santorini eruption or the turbidites that were triggered by it. Evidence of both of these perturbations is readily identified from compositional element/Al and Sr/Ca profiles that are distinct from those of the enclosing sediments. In one core with a 37 cm thick Santorini ash layer an oxidation front succeeded in penetrating the whole ash layer after emplacement to form a Mn peak but is now retreating. In cores where thin (<15 cm) Santorini turbidites or ash layers lie above S1, oxidation fronts initially form additional Mn peaks on top of the turbidites and subsequently alter the characteristic double peaked Mn profile shape usually observed above sapropel S1.

Keywords: manganese; Santorini; diagenesis; sapropel S1; eastern Mediterranean; molybdenum


Effects of the Santorini (Thera) eruption on manganese behavior in Holocene sediments of the eastern Mediterranean by Anja Reitz; John Thomson; Gert J. de Lange; Darryl R.H. Green; Caroline P. Slomp; A. Catalina Gebhardt (pp. 188-201).
The explosive eruption on the island of Santorini in ∼1630 B.C. in Minoan times had a large environmental impact over the eastern Mediterranean region. It has even been suggested that the Mn-enriched layer (the “Marker Bed”) above the most recent sapropel (S1) in sediments of a crestal area of the Mediterranean Ridge gained Mn from a hydrothermal source related to the Santorini eruption. Radiocarbon dating of two cores from this area sampled at high resolution demonstrate that this large Mn peak in fact pre-dates the Santorini event by ∼2.8 ky and forms part of a pattern seen in Mn profiles from all over the eastern Mediterranean. This same Mn profile shape is altered in areas that experienced substantial deposits of either the tephra layer emitted by the Santorini eruption or the turbidites that were triggered by it. Evidence of both of these perturbations is readily identified from compositional element/Al and Sr/Ca profiles that are distinct from those of the enclosing sediments. In one core with a 37 cm thick Santorini ash layer an oxidation front succeeded in penetrating the whole ash layer after emplacement to form a Mn peak but is now retreating. In cores where thin (<15 cm) Santorini turbidites or ash layers lie above S1, oxidation fronts initially form additional Mn peaks on top of the turbidites and subsequently alter the characteristic double peaked Mn profile shape usually observed above sapropel S1.

Keywords: manganese; Santorini; diagenesis; sapropel S1; eastern Mediterranean; molybdenum


Plume heat flow is much lower than CMB heat flow by Eric Mittelstaedt; Paul J. Tackley (pp. 202-210).
Plumes rising from the core–mantle boundary (CMB) are often assumed to transport most, or all, of the heat conducted across the CMB. Here this assumption is explored using numerical convection models in idealized geometries that lead to a single plume under steady-state or near steady state conditions. Plume heat transport is calculated for different internal heating rates using two methods and compared to the CMB heat flux. For these conditions, it is found that the heat flux transported by plumes in the upper mantle is only a fraction of the core heat flux and, thus, core heat flow estimates derived from observed hotspots could be multiplied by a factor of several.

Keywords: mantle convection; plumes; heat flow; core mantle boundary; hotspot


Plume heat flow is much lower than CMB heat flow by Eric Mittelstaedt; Paul J. Tackley (pp. 202-210).
Plumes rising from the core–mantle boundary (CMB) are often assumed to transport most, or all, of the heat conducted across the CMB. Here this assumption is explored using numerical convection models in idealized geometries that lead to a single plume under steady-state or near steady state conditions. Plume heat transport is calculated for different internal heating rates using two methods and compared to the CMB heat flux. For these conditions, it is found that the heat flux transported by plumes in the upper mantle is only a fraction of the core heat flux and, thus, core heat flow estimates derived from observed hotspots could be multiplied by a factor of several.

Keywords: mantle convection; plumes; heat flow; core mantle boundary; hotspot


Passing gas through the hydrate stability zone at southern Hydrate Ridge, offshore Oregon by Xiaoli Liu; Peter B. Flemings (pp. 211-226).
We present an equilibrium model of methane venting through the hydrate stability zone at southern Hydrate Ridge, offshore Oregon. Free gas supplied from below forms hydrate, depletes water, and elevates salinity until pore water is too saline for further hydrate formation. This system self-generates local three-phase equilibrium and allows free gas migration to the seafloor. Log and core data from Ocean Drilling Program (ODP) Site 1249 show that from the seafloor to 50 m below seafloor (mbsf), pore water salinity is elevated to the point where liquid water, hydrate and free gas coexist. The elevated pore water salinity provides a mechanism for vertical migration of free gas through the regional hydrate stability zone (RHSZ). This process may drive gas venting through hydrate stability zones around the world. Significant amount of gaseous methane can bypass the RHSZ by shifting local thermodynamic conditions.

Keywords: Gas hydrate; Hydrate ridge; Free gas migration; Salinity


Passing gas through the hydrate stability zone at southern Hydrate Ridge, offshore Oregon by Xiaoli Liu; Peter B. Flemings (pp. 211-226).
We present an equilibrium model of methane venting through the hydrate stability zone at southern Hydrate Ridge, offshore Oregon. Free gas supplied from below forms hydrate, depletes water, and elevates salinity until pore water is too saline for further hydrate formation. This system self-generates local three-phase equilibrium and allows free gas migration to the seafloor. Log and core data from Ocean Drilling Program (ODP) Site 1249 show that from the seafloor to 50 m below seafloor (mbsf), pore water salinity is elevated to the point where liquid water, hydrate and free gas coexist. The elevated pore water salinity provides a mechanism for vertical migration of free gas through the regional hydrate stability zone (RHSZ). This process may drive gas venting through hydrate stability zones around the world. Significant amount of gaseous methane can bypass the RHSZ by shifting local thermodynamic conditions.

Keywords: Gas hydrate; Hydrate ridge; Free gas migration; Salinity


High resolution transmission electron microscopic study of synthetic nanocrystalline mackinawite by Hiroaki Ohfuji; David Rickard (pp. 227-233).
Direct observation using high resolution transmission electron microscopy reveals that precipitated iron(II) monosulfide, FeS, consists of nanocrystalline mackinawite particles. The individual nanocrystals are laminar rectilinear prisms displaying a continuum of particle sizes from 2 to 5.7 nm in thickness (the direction parallel to the c axis) and from 3 to 10.8 nm in length. The corresponding mean specific surface area is estimated to be to 38010 m2/g. The d001 of mackinawite nanocrystals obtained from precipitated FeS and freeze-dried FeS by electron diffraction are 5.19 and 5.08 , respectively. The effect of water on the nanoparticle structures is indicated by the formation of curved structures and infrequent dislocations in an anhydrous environment. The apparent disorder suggested by absent or weaker lattice spacings is symptomatic of the breakdown of Braggian systematics at these particle sizes. The results confirm that nanoparticulate materials do not behave simply as small fragments of their bulk crystalline counterparts. The results contribute to understanding the behavior of nanoparticulate materials on planetary surfaces and in the biosphere in general and the nature and properties of FeS in anoxic aqueous environments in particular.

Keywords: FeS; Mackinawite; Nanoparticle; Particle size; Specific surface area


High resolution transmission electron microscopic study of synthetic nanocrystalline mackinawite by Hiroaki Ohfuji; David Rickard (pp. 227-233).
Direct observation using high resolution transmission electron microscopy reveals that precipitated iron(II) monosulfide, FeS, consists of nanocrystalline mackinawite particles. The individual nanocrystals are laminar rectilinear prisms displaying a continuum of particle sizes from 2 to 5.7 nm in thickness (the direction parallel to the c axis) and from 3 to 10.8 nm in length. The corresponding mean specific surface area is estimated to be to 380±10 m2/g. The d001 of mackinawite nanocrystals obtained from precipitated FeS and freeze-dried FeS by electron diffraction are 5.19 and 5.08 Å, respectively. The effect of water on the nanoparticle structures is indicated by the formation of curved structures and infrequent dislocations in an anhydrous environment. The apparent disorder suggested by absent or weaker lattice spacings is symptomatic of the breakdown of Braggian systematics at these particle sizes. The results confirm that nanoparticulate materials do not behave simply as small fragments of their bulk crystalline counterparts. The results contribute to understanding the behavior of nanoparticulate materials on planetary surfaces and in the biosphere in general and the nature and properties of FeS in anoxic aqueous environments in particular.

Keywords: FeS; Mackinawite; Nanoparticle; Particle size; Specific surface area


Paleomagnetic evidence for a mid-Miocene clockwise rotation of about 25 of the Guide Basin area in NE Tibet by Maodu Yan; Rob VanderVoo; Xiao-min Fang; Josep M. Pars; David K. Rea (pp. 234-247).
Ten sections of Neogene molasse-type sediments were sampled in the Guide Basin of northeastern Tibet for magnetostratigraphy [X.M. Fang, M.D. Yan, R. Van der Voo, D.R., Rea, C. Song, J.M. Parés, J. Gao, J. Nie, S. Dai, Late Cenozoic deformation and uplift of the NE Tibetan plateau: evidence from high resolution magnetostratigraphy of the Guide Basin, Qinghai Province, China, Geol. Soc. America Bull. 107 (2005) 1208–1225 [1]], but they also yield seven well-dated formation-mean directions that reveal changing declinations as rotations occurred in response to crustal deformation north of the India–Asia collision zone. Three formations are of early Miocene and Oligocene age, as indicated by fossils and magnetic reversal records, whereas four younger formations yield late Miocene and Pliocene ages. The dual-polarity magnetizations are typically antipodal, but reveal inclinations that are too shallow, most likely because of post-depositional inclination flattening. The late Miocene and younger directions show formation-mean declinations between 354° and 7°, whereas three early Miocene and late Oligocene mean declinations range from 31° to 44°. This indicates that a clockwise rotation of 25.1±4.6° took place during the middle part of the Miocene (best estimate 11–17 Ma). No rotations appear to have occurred, during that time, in the Xining, Lanzhou, Linxia and Jingning basins (Longzhong Basin) to the northeast and east of the Guide Basin; however, a rotation of similar magnitude has been documented by Dupont-Nivet and colleagues for pre-Miocene (>29 Ma) time in these areas. Collectively, these results show that the basins in NE Tibet have had independently evolving structural histories.

Keywords: Tibet; Neogene; Rotations; Paleomagnetism; Guide Basin


Paleomagnetic evidence for a mid-Miocene clockwise rotation of about 25° of the Guide Basin area in NE Tibet by Maodu Yan; Rob VanderVoo; Xiao-min Fang; Josep M. Parés; David K. Rea (pp. 234-247).
Ten sections of Neogene molasse-type sediments were sampled in the Guide Basin of northeastern Tibet for magnetostratigraphy [X.M. Fang, M.D. Yan, R. Van der Voo, D.R., Rea, C. Song, J.M. Parés, J. Gao, J. Nie, S. Dai, Late Cenozoic deformation and uplift of the NE Tibetan plateau: evidence from high resolution magnetostratigraphy of the Guide Basin, Qinghai Province, China, Geol. Soc. America Bull. 107 (2005) 1208–1225 [1]], but they also yield seven well-dated formation-mean directions that reveal changing declinations as rotations occurred in response to crustal deformation north of the India–Asia collision zone. Three formations are of early Miocene and Oligocene age, as indicated by fossils and magnetic reversal records, whereas four younger formations yield late Miocene and Pliocene ages. The dual-polarity magnetizations are typically antipodal, but reveal inclinations that are too shallow, most likely because of post-depositional inclination flattening. The late Miocene and younger directions show formation-mean declinations between 354° and 7°, whereas three early Miocene and late Oligocene mean declinations range from 31° to 44°. This indicates that a clockwise rotation of 25.1±4.6° took place during the middle part of the Miocene (best estimate 11–17 Ma). No rotations appear to have occurred, during that time, in the Xining, Lanzhou, Linxia and Jingning basins (Longzhong Basin) to the northeast and east of the Guide Basin; however, a rotation of similar magnitude has been documented by Dupont-Nivet and colleagues for pre-Miocene (>29 Ma) time in these areas. Collectively, these results show that the basins in NE Tibet have had independently evolving structural histories.

Keywords: Tibet; Neogene; Rotations; Paleomagnetism; Guide Basin


Mineral magnetic variation of the Jingbian loess/paleosol sequence in the northern Loess Plateau of China: Implications for Quaternary development of Asian aridification and cooling by Chenglong Deng; John Shaw; Qingsong Liu; Yongxin Pan; Rixiang Zhu (pp. 248-259).
A high-resolution mineral magnetic investigation has been carried out on the Jingbian loess/paleosol sequence at the northern extremity of the Chinese Loess Plateau. Results show that the magnetic assemblage is dominated by large pseudo-single domain and multidomain-like magnetite with associated maghemite and hematite. Variations in the ratios of SIRM100mT/SIRM, SIRM100mT/SIRM30mT and SIRM100mT/SIRM60mT (SIRM is the saturation isothermal remanent magnetization; SIRM nmT represents the residual SIRM after an n mT alternating field demagnetization) have been used to document regional paleoclimate change in the Asian interior by correlating the mineral magnetic record with the composite δ18O record in deep-sea sediments. The long-term up-section decreasing trend in those ratios in both loess and paleosol units has been attributed to a long-term decrease in the relative contributions of eolian hematite during glacial extrema and of pedogenic hematite during interglacial extrema, respectively, which reveals a long-term decreasing trend in chemical weathering intensity in both glacial-stage source region (the Gobi and deserts in northwestern China) and interglacial-stage depositional area (the Loess Plateau region). We further relate this long-timescale variation to long-term increasing aridification and cooling, during both glacial extrema in the dust source region and interglacial extrema in the depositional area, over the Quaternary period. Changes in those ratios are most likely due to Quaternary aridification and cooling driven by ongoing global cooling, expansion of the Arctic ice-sheet, and progressive uplift of the Himalayan–Tibetan complex during this period.

Keywords: Chinese Loess Plateau; loess; paleosol; mineral magnetism; magnetoclimatology; Asian aridification and cooling


Mineral magnetic variation of the Jingbian loess/paleosol sequence in the northern Loess Plateau of China: Implications for Quaternary development of Asian aridification and cooling by Chenglong Deng; John Shaw; Qingsong Liu; Yongxin Pan; Rixiang Zhu (pp. 248-259).
A high-resolution mineral magnetic investigation has been carried out on the Jingbian loess/paleosol sequence at the northern extremity of the Chinese Loess Plateau. Results show that the magnetic assemblage is dominated by large pseudo-single domain and multidomain-like magnetite with associated maghemite and hematite. Variations in the ratios of SIRM100mT/SIRM, SIRM100mT/SIRM30mT and SIRM100mT/SIRM60mT (SIRM is the saturation isothermal remanent magnetization; SIRM nmT represents the residual SIRM after an n mT alternating field demagnetization) have been used to document regional paleoclimate change in the Asian interior by correlating the mineral magnetic record with the composite δ18O record in deep-sea sediments. The long-term up-section decreasing trend in those ratios in both loess and paleosol units has been attributed to a long-term decrease in the relative contributions of eolian hematite during glacial extrema and of pedogenic hematite during interglacial extrema, respectively, which reveals a long-term decreasing trend in chemical weathering intensity in both glacial-stage source region (the Gobi and deserts in northwestern China) and interglacial-stage depositional area (the Loess Plateau region). We further relate this long-timescale variation to long-term increasing aridification and cooling, during both glacial extrema in the dust source region and interglacial extrema in the depositional area, over the Quaternary period. Changes in those ratios are most likely due to Quaternary aridification and cooling driven by ongoing global cooling, expansion of the Arctic ice-sheet, and progressive uplift of the Himalayan–Tibetan complex during this period.

Keywords: Chinese Loess Plateau; loess; paleosol; mineral magnetism; magnetoclimatology; Asian aridification and cooling


Entrainment of meltwaters in hyperpycnal flows during deglaciation superfloods in the Gulf of Mexico by Paul Aharon (pp. 260-270).
Northern Gulf of Mexico is a key archival site of the Laurentide Ice Sheet melting history and the only margins containing evidence of meltwater outflows contemporaneous with global meltwater pulse mwp-1A. Inconsistencies between these meltwater floods and the absence of strong climate responses to large freshwater fluxes predicted by climate models raised questions concerning the validity of planktonic foraminiferal δ18O record in the Gulf of Mexico as a reliable index of freshwater fluxes. Isotope records of depth-stratified foraminifera document unusual trends compatible with entrainment of18O-depleted freshwaters in hyperpycnal flows reaching the seafloor and rising to the surface as buoyant plumes. These exceptionally large outbursts of floodwaters, coeval with a prominent erosion intensity peak in continental paleohydrology, were catastrophically released in a rapid succession over a period of ca. 500 yr (14.7–14.2 kyr). Correspondence in time and magnitude between the torrential floods and the mwp-1A event supports the view that LIS was likely a substantial freshwater source. Drainage of meltwaters through torrential hyperpycnal flows that underwent substantial mixing with seawater may resolve the problem with ocean modeling results that predict a much stronger climate response to large freshwater hypopycnal fluxes into the ocean.

Keywords: deglaciation; mwp-1A paradox; Gulf of Mexico; hyperpycnal flows; stable isotopes; Laurentide Ice Sheet; superfloods


Entrainment of meltwaters in hyperpycnal flows during deglaciation superfloods in the Gulf of Mexico by Paul Aharon (pp. 260-270).
Northern Gulf of Mexico is a key archival site of the Laurentide Ice Sheet melting history and the only margins containing evidence of meltwater outflows contemporaneous with global meltwater pulse mwp-1A. Inconsistencies between these meltwater floods and the absence of strong climate responses to large freshwater fluxes predicted by climate models raised questions concerning the validity of planktonic foraminiferal δ18O record in the Gulf of Mexico as a reliable index of freshwater fluxes. Isotope records of depth-stratified foraminifera document unusual trends compatible with entrainment of18O-depleted freshwaters in hyperpycnal flows reaching the seafloor and rising to the surface as buoyant plumes. These exceptionally large outbursts of floodwaters, coeval with a prominent erosion intensity peak in continental paleohydrology, were catastrophically released in a rapid succession over a period of ca. 500 yr (14.7–14.2 kyr). Correspondence in time and magnitude between the torrential floods and the mwp-1A event supports the view that LIS was likely a substantial freshwater source. Drainage of meltwaters through torrential hyperpycnal flows that underwent substantial mixing with seawater may resolve the problem with ocean modeling results that predict a much stronger climate response to large freshwater hypopycnal fluxes into the ocean.

Keywords: deglaciation; mwp-1A paradox; Gulf of Mexico; hyperpycnal flows; stable isotopes; Laurentide Ice Sheet; superfloods


Evidence from P-to-S mantle converted waves for a flat “660-km? discontinuity beneath Iceland by Z. Du; L.P. Vinnik; G.R. Foulger (pp. 271-280).
Iceland is the type example of a ridge-centered hotspot. It is controversial whether the seismic anomaly beneath it originates in the lower mantle or the upper mantle. Some recent studies reported that the 660-km discontinuity beneath central Iceland is shallow relative to peripheral regions and this was interpreted as an effect of elevated temperature at that depth. We investigate topography of the major upper mantle discontinuities by separating the effects of the topography and volumetric velocity heterogeneity in P receiver functions from 55 seismograph stations. Our analysis demonstrates that a significant (at least 10-km) shallowing of the 660-km discontinuity is only possible in the case of improbably low seismic velocities in the mantle transition zone beneath central Iceland. If, as in previous studies, lateral velocity variations in the mantle transition zone are neglected, the data require a depressed rather than an uplifted 660-km discontinuity. For a reasonable S-wave velocity anomaly in the mantle transition zone (around −3%) no topography on the 660-km discontinuity is required. This can be explained by the lack of temperature anomaly or an effect of two phase transitions with opposite Clapeyron slopes.

Keywords: transition zone; receiver functions; Iceland; plume; mantle


Evidence from P-to-S mantle converted waves for a flat “660-km” discontinuity beneath Iceland by Z. Du; L.P. Vinnik; G.R. Foulger (pp. 271-280).
Iceland is the type example of a ridge-centered hotspot. It is controversial whether the seismic anomaly beneath it originates in the lower mantle or the upper mantle. Some recent studies reported that the 660-km discontinuity beneath central Iceland is shallow relative to peripheral regions and this was interpreted as an effect of elevated temperature at that depth. We investigate topography of the major upper mantle discontinuities by separating the effects of the topography and volumetric velocity heterogeneity in P receiver functions from 55 seismograph stations. Our analysis demonstrates that a significant (at least 10-km) shallowing of the 660-km discontinuity is only possible in the case of improbably low seismic velocities in the mantle transition zone beneath central Iceland. If, as in previous studies, lateral velocity variations in the mantle transition zone are neglected, the data require a depressed rather than an uplifted 660-km discontinuity. For a reasonable S-wave velocity anomaly in the mantle transition zone (around −3%) no topography on the 660-km discontinuity is required. This can be explained by the lack of temperature anomaly or an effect of two phase transitions with opposite Clapeyron slopes.

Keywords: transition zone; receiver functions; Iceland; plume; mantle


Offshore geodetic data conducive to the estimation of the afterslip distribution following the 2003 Tokachi-oki earthquake by Toshitaka Baba; Kenji Hirata; Takane Hori; Hide Sakaguchi (pp. 281-292).
A cable-based seafloor observatory located offshore of Hokkaido, Japan, provides the world's first direct observation of offshore post-seismic displacement. After removing thermal noise, this data is used to constrain the distribution of offshore afterslip from a great interplate earthquake ( Mw 8.0) that occurred during 2003 along the southern Kuril Trench. A checkerboard resolution test showed a great contribution of the offshore data to improving the spatial resolution on the plate interface slip near the trench. The 1-yr afterslip is distributed in a U-shaped pattern encircling its co-seismic slip area. This result indicates that the stick-slip frictional properties of the plate interface are distributed in a patch-like pattern as opposed to a banding pattern as previously assumed in young subduction zones.

Keywords: The 2003 Tokachi-oki earthquake; afterslip; frictional properties; geodetic inversion; offshore data


Offshore geodetic data conducive to the estimation of the afterslip distribution following the 2003 Tokachi-oki earthquake by Toshitaka Baba; Kenji Hirata; Takane Hori; Hide Sakaguchi (pp. 281-292).
A cable-based seafloor observatory located offshore of Hokkaido, Japan, provides the world's first direct observation of offshore post-seismic displacement. After removing thermal noise, this data is used to constrain the distribution of offshore afterslip from a great interplate earthquake ( Mw 8.0) that occurred during 2003 along the southern Kuril Trench. A checkerboard resolution test showed a great contribution of the offshore data to improving the spatial resolution on the plate interface slip near the trench. The 1-yr afterslip is distributed in a U-shaped pattern encircling its co-seismic slip area. This result indicates that the stick-slip frictional properties of the plate interface are distributed in a patch-like pattern as opposed to a banding pattern as previously assumed in young subduction zones.

Keywords: The 2003 Tokachi-oki earthquake; afterslip; frictional properties; geodetic inversion; offshore data


Constraining the long-term evolution of the slip rate for a major extensional fault system in the central Aegean, Greece, using thermochronology by Stephanie Brichau; Uwe Ring; Richard A. Ketcham; Andrew Carter; Daniel Stockli; Maurice Brunel (pp. 293-306).
The brittle/ductile transition is a major rheologic boundary in the crust yet little is known about how or if rates of tectonic processes are influenced by this boundary. In this study we examine the slip history of the large-scale Naxos/Paros extensional fault system (NPEFS), Cyclades, Greece, by comparing published slip rates for the ductile crust with new thermochronological constraints on slip rates in the brittle regime. Based on apatite and zircon fission-track (AFT and ZFT) and (U–Th)/He dating we observe variable slip rates across the brittle/ductile transition on Naxos. ZFT and AFT ages range from 11.8±0.8 to 9.7±0.8 Ma and 11.2±1.6 to 8.2±1.2 Ma and (U–Th)/He zircon and apatite ages are between 10.4±0.4 to 9.2±0.3 Ma and 10.7±1.0 to 8.9±0.6 Ma, respectively. On Paros, ZFT and AFT ages range from 13.1±1.4 Ma to 11.1±1.0 Ma and 12.7±2.8 Ma to 10.5±2.0 Ma while the (U–Th)/He zircon ages are slightly younger between 8.3±0.4 Ma and 9.8±0.3 Ma. All ages consistently decrease northwards in the direction of hanging wall transport. Most of our new thermochronological results and associated thermal modeling more strongly support the scenario of an identical fault dip and a constant or slightly accelerating slip rate of ∼6–8 km Myr−1 on the NPEFS across the brittle/ductile transition. Even the intrusion of a large granodiorite body into the narrowing fault zone at ∼12 Ma on Naxos does not seem to have affected the thermal structure of the area in a way that would significantly disturb the slip rate. The data also show that the NPEFS accomplished a minimum total offset of ∼50 km between ∼16 and 8 Ma.

Keywords: Aegean Sea; extensional faulting; slip rate; brittle/ductile transition


Constraining the long-term evolution of the slip rate for a major extensional fault system in the central Aegean, Greece, using thermochronology by Stephanie Brichau; Uwe Ring; Richard A. Ketcham; Andrew Carter; Daniel Stockli; Maurice Brunel (pp. 293-306).
The brittle/ductile transition is a major rheologic boundary in the crust yet little is known about how or if rates of tectonic processes are influenced by this boundary. In this study we examine the slip history of the large-scale Naxos/Paros extensional fault system (NPEFS), Cyclades, Greece, by comparing published slip rates for the ductile crust with new thermochronological constraints on slip rates in the brittle regime. Based on apatite and zircon fission-track (AFT and ZFT) and (U–Th)/He dating we observe variable slip rates across the brittle/ductile transition on Naxos. ZFT and AFT ages range from 11.8±0.8 to 9.7±0.8 Ma and 11.2±1.6 to 8.2±1.2 Ma and (U–Th)/He zircon and apatite ages are between 10.4±0.4 to 9.2±0.3 Ma and 10.7±1.0 to 8.9±0.6 Ma, respectively. On Paros, ZFT and AFT ages range from 13.1±1.4 Ma to 11.1±1.0 Ma and 12.7±2.8 Ma to 10.5±2.0 Ma while the (U–Th)/He zircon ages are slightly younger between 8.3±0.4 Ma and 9.8±0.3 Ma. All ages consistently decrease northwards in the direction of hanging wall transport. Most of our new thermochronological results and associated thermal modeling more strongly support the scenario of an identical fault dip and a constant or slightly accelerating slip rate of ∼6–8 km Myr−1 on the NPEFS across the brittle/ductile transition. Even the intrusion of a large granodiorite body into the narrowing fault zone at ∼12 Ma on Naxos does not seem to have affected the thermal structure of the area in a way that would significantly disturb the slip rate. The data also show that the NPEFS accomplished a minimum total offset of ∼50 km between ∼16 and 8 Ma.

Keywords: Aegean Sea; extensional faulting; slip rate; brittle/ductile transition


Basement control on dyke distribution in Large Igneous Provinces: Case study of the Karoo triple junction by F. Jourdan; G. Fraud; H. Bertrand; M.K. Watkeys; A.B. Kampunzu; B. Le Gall (pp. 307-322).
Continental flood basalts consist of vast quantities of lava, sills and giant dyke swarms that are associated with continental break-up. The commonly radiating geometry of dyke swarms in these provinces is generally interpreted as the result of the stress regime that affected the lithosphere during the initial stage of continental break-up or as the result of plume impact. On the other hand, structures in the basement may also control dyke orientations, though such control has not previously been documented. In order to test the role of pre-dyke structures, we investigated four major putative Karoo-aged dyke swarms that taken together represent a giant radiating dyke swarm (the so-called “triple-junction?) ascribed to the Jurassic Karoo continental flood basalt (>3×106 km2; southern Africa). One of the best tests to discriminate between neoformed and inherited dyke orientation is to detect Precambrian dykes in the Jurassic swarms. Accordingly, we efficiently distinguished between Jurassic and Precambrian dykes using abbreviated low resolution,40Ar/39Ar incremental heating schedules.Save-Limpopo dyke swarm samples ( n=19) yield either apparent Proterozoic (728–1683 Ma) or Mesozoic (131–179 Ma) integrated ages; the Olifants River swarm ( n=20) includes only Proterozoic (851–1731 Ma) and Archaean (2470–2872 Ma) dykes. The single age obtained on one N–S striking dyke (1464 Ma) suggests that the Lebombo dyke swarm includes Proterozoic dykes in the basement as well. These dates demonstrate the existence of pre-Karoo dykes in these swarms as previously hypothesized without supporting age data. In addition, aeromagnetic and air-photo interpretations indicate that: (1) dyke emplacement was largely controlled by major discontinuities such as the Zimbabwe and Kaapvaal craton boundaries, the orientation of the Limpopo mobile belt, and other pre-dyke structures including shear zones and (2) considering its polygenetic, pre-Mesozoic origin, the Olif ants River dyke swarm cannot be considered part of the Karoo magmatic event.This study, along with previous results obtained on the Okavango dyke swarm, shows that the apparent “triple junction? formed by radiating dyke swarms is not a Jurassic structure; rather, it reflects weakened lithospheric pathways that have controlled dyke orientations over hundreds of millions of years. One consequence is that the “triple-junction? geometry can no longer be unambiguously used as a mantle plume marker as previously proposed, although it does not preclude the possible existence of a mantle plume. More generally, we suggest that most Phanerozoic dyke swarms (including triple junctions) related to continental flood basalts were probably controlled in part by pre-existing lithospheric discontinuities.

Keywords: 40; Ar/; 39; Ar dating; dyke swarm; triple junction; Karoo; mantle plume; basement control; structural inheritance


Basement control on dyke distribution in Large Igneous Provinces: Case study of the Karoo triple junction by F. Jourdan; G. Féraud; H. Bertrand; M.K. Watkeys; A.B. Kampunzu; B. Le Gall (pp. 307-322).
Continental flood basalts consist of vast quantities of lava, sills and giant dyke swarms that are associated with continental break-up. The commonly radiating geometry of dyke swarms in these provinces is generally interpreted as the result of the stress regime that affected the lithosphere during the initial stage of continental break-up or as the result of plume impact. On the other hand, structures in the basement may also control dyke orientations, though such control has not previously been documented. In order to test the role of pre-dyke structures, we investigated four major putative Karoo-aged dyke swarms that taken together represent a giant radiating dyke swarm (the so-called “triple-junction”) ascribed to the Jurassic Karoo continental flood basalt (>3×106 km2; southern Africa). One of the best tests to discriminate between neoformed and inherited dyke orientation is to detect Precambrian dykes in the Jurassic swarms. Accordingly, we efficiently distinguished between Jurassic and Precambrian dykes using abbreviated low resolution,40Ar/39Ar incremental heating schedules.Save-Limpopo dyke swarm samples ( n=19) yield either apparent Proterozoic (728–1683 Ma) or Mesozoic (131–179 Ma) integrated ages; the Olifants River swarm ( n=20) includes only Proterozoic (851–1731 Ma) and Archaean (2470–2872 Ma) dykes. The single age obtained on one N–S striking dyke (1464 Ma) suggests that the Lebombo dyke swarm includes Proterozoic dykes in the basement as well. These dates demonstrate the existence of pre-Karoo dykes in these swarms as previously hypothesized without supporting age data. In addition, aeromagnetic and air-photo interpretations indicate that: (1) dyke emplacement was largely controlled by major discontinuities such as the Zimbabwe and Kaapvaal craton boundaries, the orientation of the Limpopo mobile belt, and other pre-dyke structures including shear zones and (2) considering its polygenetic, pre-Mesozoic origin, the Olif ants River dyke swarm cannot be considered part of the Karoo magmatic event.This study, along with previous results obtained on the Okavango dyke swarm, shows that the apparent “triple junction” formed by radiating dyke swarms is not a Jurassic structure; rather, it reflects weakened lithospheric pathways that have controlled dyke orientations over hundreds of millions of years. One consequence is that the “triple-junction” geometry can no longer be unambiguously used as a mantle plume marker as previously proposed, although it does not preclude the possible existence of a mantle plume. More generally, we suggest that most Phanerozoic dyke swarms (including triple junctions) related to continental flood basalts were probably controlled in part by pre-existing lithospheric discontinuities.

Keywords: 40; Ar/; 39; Ar dating; dyke swarm; triple junction; Karoo; mantle plume; basement control; structural inheritance


Tertiary strike-slip faulting in southeastern Mongolia and implications for Asian tectonics by L.E. Webb; C.L. Johnson (pp. 323-335).
Geologic maps have long portrayed the Late Cretaceous–Recent geologic history of southeastern Mongolia as tectonically quiescent. We present new data based on outcrop observations that indicate the northeast-trending East Gobi fault zone (EGFZ) was reactivated in the Cenozoic as a sinistral strike-slip fault system. Inversions of Cenozoic fault-slip data imply that faulting was associated with north–northwest subhorizontal shortening and east–northeast subhorizontal extension. We propose that faulting is Tertiary in age based on published interpretations of seismic reflection data which reveal that the mid-Cretaceous (∼100–95 Ma) unconformity is deformed by strike-slip faults, and based on field observation of strike-slip faults and fracture sets that cut Upper Cretaceous and Cenozoic strata but lack evidence for neotectonic activity. Published seismicity maps also appear to argue against significant Quaternary faulting within the EGFZ. These new data may lend credence to published models proposing a Middle Miocene or older kinematic linkage between the EGFZ and the Altyn Tagh fault in China. The recognition that the EGFZ has a history of left-lateral displacement in both the Early Mesozoic and Cenozoic means that currently available estimates of offset based on displaced Paleozoic rocks constrain total offset only. This reactivation history supports the notion that inherited lithospheric structures are important in controlling the location and, thus, modes of intracontinental deformation in Asia as a function of collisional far field effects and evolving boundary conditions of the Pacific margin.

Keywords: intraplate deformation; strike-slip faults; fault reactivation; Asian tectonics; Cenozoic


Tertiary strike-slip faulting in southeastern Mongolia and implications for Asian tectonics by L.E. Webb; C.L. Johnson (pp. 323-335).
Geologic maps have long portrayed the Late Cretaceous–Recent geologic history of southeastern Mongolia as tectonically quiescent. We present new data based on outcrop observations that indicate the northeast-trending East Gobi fault zone (EGFZ) was reactivated in the Cenozoic as a sinistral strike-slip fault system. Inversions of Cenozoic fault-slip data imply that faulting was associated with north–northwest subhorizontal shortening and east–northeast subhorizontal extension. We propose that faulting is Tertiary in age based on published interpretations of seismic reflection data which reveal that the mid-Cretaceous (∼100–95 Ma) unconformity is deformed by strike-slip faults, and based on field observation of strike-slip faults and fracture sets that cut Upper Cretaceous and Cenozoic strata but lack evidence for neotectonic activity. Published seismicity maps also appear to argue against significant Quaternary faulting within the EGFZ. These new data may lend credence to published models proposing a Middle Miocene or older kinematic linkage between the EGFZ and the Altyn Tagh fault in China. The recognition that the EGFZ has a history of left-lateral displacement in both the Early Mesozoic and Cenozoic means that currently available estimates of offset based on displaced Paleozoic rocks constrain total offset only. This reactivation history supports the notion that inherited lithospheric structures are important in controlling the location and, thus, modes of intracontinental deformation in Asia as a function of collisional far field effects and evolving boundary conditions of the Pacific margin.

Keywords: intraplate deformation; strike-slip faults; fault reactivation; Asian tectonics; Cenozoic


Effects of fault geometry and slip style on near-fault static displacements caused by the 1999 Chi-Chi, Taiwan earthquake by Shiann-Jong Lee; Kuo-Fong Ma; How-Wei Chen (pp. 336-350).
We investigated the fault geometry effects and the corresponding coseismic slip distribution using various proposed earthquake fault models for the Chi-Chi earthquake of 21 September 1999. The types of fault geometries are threefold: a simple planar fault plane, a two segmented planar fault plane and a three dimensional (3D) curved fault surface rupture propagation model. We derived the estimated spatial slip distribution from an inversion analysis of GPS coseismic displacement data and show that the 3D fault model is the preferred solution. The simple and segmented fault models lead to significant artificial slip distributions associated with the pre-defined fault geometry and the spatial distribution of GPS stations. The spatial distribution of coseismic slip deduced from the 3D fault model has three observable features: (1) the overall slip is concentrated at depth of less than 12 km, which may well correspond to a shallow-dipping detachment; (2) the maximum slip of about 10 m is located 45 km to the north of the epicenter; and (3) the slip vector is dominated by the dip-slip component. In addition, the results from the inversion of GPS data are consistent with those from the inversion analysis of teleseismic broadband data. A resolution analysis, further, demonstrates that the results are highly correlated with field GPS data studies when we used synthetic test data. The inversion of spatially distributed GPS data is highly sensitive to fault geometry. We conclude that the use of the 3D fault model is not only necessary but also certainly competent enough to well explain the inferred slip style and the observed static coseismic displacements.

Keywords: Chi-Chi earthquake; fault geometry; inversion; coseismic GPS displacement; slip distribution


Effects of fault geometry and slip style on near-fault static displacements caused by the 1999 Chi-Chi, Taiwan earthquake by Shiann-Jong Lee; Kuo-Fong Ma; How-Wei Chen (pp. 336-350).
We investigated the fault geometry effects and the corresponding coseismic slip distribution using various proposed earthquake fault models for the Chi-Chi earthquake of 21 September 1999. The types of fault geometries are threefold: a simple planar fault plane, a two segmented planar fault plane and a three dimensional (3D) curved fault surface rupture propagation model. We derived the estimated spatial slip distribution from an inversion analysis of GPS coseismic displacement data and show that the 3D fault model is the preferred solution. The simple and segmented fault models lead to significant artificial slip distributions associated with the pre-defined fault geometry and the spatial distribution of GPS stations. The spatial distribution of coseismic slip deduced from the 3D fault model has three observable features: (1) the overall slip is concentrated at depth of less than 12 km, which may well correspond to a shallow-dipping detachment; (2) the maximum slip of about 10 m is located 45 km to the north of the epicenter; and (3) the slip vector is dominated by the dip-slip component. In addition, the results from the inversion of GPS data are consistent with those from the inversion analysis of teleseismic broadband data. A resolution analysis, further, demonstrates that the results are highly correlated with field GPS data studies when we used synthetic test data. The inversion of spatially distributed GPS data is highly sensitive to fault geometry. We conclude that the use of the 3D fault model is not only necessary but also certainly competent enough to well explain the inferred slip style and the observed static coseismic displacements.

Keywords: Chi-Chi earthquake; fault geometry; inversion; coseismic GPS displacement; slip distribution

Erratum to “231Pa/230Th fractionation by ocean transport, biogenic particle flux and particle type? [Earth Planet. Sci. Lett. 237 (2005) 135–155] by Mark Siddall; Gideon M. Henderson; Neil R. Edwards; Martin Frank; Simon A. Mller; Thomas F. Stocker; Fortunat Joos (pp. 351-359).
Erratum to “231Pa/230Th fractionation by ocean transport, biogenic particle flux and particle type” [Earth Planet. Sci. Lett. 237 (2005) 135–155] by Mark Siddall; Gideon M. Henderson; Neil R. Edwards; Martin Frank; Simon A. Müller; Thomas F. Stocker; Fortunat Joos (pp. 351-359).
Featured Book
Web Search

Powered by Plone CMS, the Open Source Content Management System

This site conforms to the following standards: