Earth and Planetary Science Letters (v.243, #3-4)

Editorial Board (pp. co2).

Accelerator mass spectrometry: Is the future bigger or smaller? by A.J.T. Jull; G.S. Burr (pp. 305-325).

Since its inception in the late 1970s, accelerator mass spectrometry has become a powerful tool for measurement of trace amounts of natural radionuclides. In this paper, we review recent advancements in AMS and discuss future directions of this powerful technique. We highlight some recent developments, including the introduction of smaller accelerators, novel detection systems and the development of new analytical capabilities. We believe that the future prospects for AMS measurements are practically unlimited and that AMS has a vital role to play in the exploration of space, where radionuclide measurements are a key factor in the understanding of processes on other planets.

Keywords: accelerator mass spectrometry; cosmogenic radionuclides; radiocarbon dating

Isotope fractionation of cadmium in lunar material by S. Schediwy; K.J.R. Rosman; J.R. de Laeter (pp. 326-335).

The double spike technique has been used to measure the isotope fractionation and elemental abundance of Cd in nine lunar samples, the Brownfield meteorite and the Columbia River Basalt BCR-1, by thermal ionisation mass spectrometry. Lunar soil samples give a tightly grouped set of positive isotope fractionation values of between +0.42% and +0.50% per mass unit. Positive isotope fractionation implies that the heavy isotopes are enhanced with respect to those of the Laboratory Standard. A vesicular mare basalt gave zero isotope fractionation, indicating that the Cd isotopic composition of the Moon is identical to that of the Earth. A sample of orange glass from the Taurus-Littrow region gave a negative isotope fractionation of −0.23±0.06% per mass unit, presumably as a result of redeposition of Cd from the Cd-rich vapour cloud associated with volcanism. Cadmium is by far the heaviest element to show isotope fractionation effects in lunar samples. The volatile nature of Cd is of importance in explaining these isotope fractionation results. Although a number of mechanisms have been postulated to be the cause of isotope fractionation of certain elements in lunar soils, we believe that the most likely mechanisms are ion and particle bombardment of the lunar surface.

Keywords: cadmium; double spike; isotope fractionation; lunar samples; mass spectrometry

New constraints on the lithium isotope compositions of the Moon and terrestrial planets by Tomáš Magna; Uwe Wiechert; Alex N. Halliday (pp. 336-353).

High-precision lithium (Li) isotope data are reported for samples from the Earth, Moon, Mars and Vesta and provide evidence of broadly similar compositions that are slightly heavy relative to those of chondrites. Mare basalts exhibit a large range of Li isotope compositions ( δ⁷Li=+3.4‰ to +6.4‰) that correlate with indices of magmatic differentiation. Three samples (quartz-normative basalts and picritic orange glass) that are thought to have formed by melting of relatively primitive source regions yield a mean δ⁷Li=+3.8±0.4‰ taken as the best estimate for the average composition of the Moon. Other samples are isotopically heavier correlating with increases in Rb and Hf and probably reflecting transport of isotopically heavy Li that formed in specific high-Ti cumulate–melt layers during crystallisation of the magma ocean. The most extreme lunar δ⁷Li is found in a ferroan anorthosite (+8.9‰). Terrestrial mantle olivines fall into a tight range between +3.6‰ and +3.8‰. If these olivines reflect the composition of the bulk Earth, the Li isotope compositions of Earth and Moon are identical. The Li isotope compositions of samples from the Moon, Earth, Mars and Vesta provide no evidence for differences between large inner solar system mantle reservoirs. This in turn provides evidence that core formation, volatile loss and the presence of a crust and hydrosphere have not significantly influenced the bulk Li isotope composition of the mantles of these objects. The fact that chondrites are isotopically light compared with differentiated planetary bodies of the inner solar system is consistent with a small but significant Li isotope fractionation within the accretionary disc or chondrite parent bodies, the origin of which is presently unclear.

Keywords: lithium isotopes; trace elements; terrestrial planets; mantle; Earth; Moon

Methane discharge from a deep-sea submarine mud volcano into the upper water column by gas hydrate-coated methane bubbles by Eberhard J. Sauter; Sergey I. Muyakshin; Jean-Luc Charlou; Michael Schl�ter; Antje Boetius; Kerstin Jerosch; Ellen Damm; Jean-Paul Foucher; Michael Klages (pp. 354-365).

The assessment of climate change factors includes a constraint of methane sources and sinks. Although marine geological sources are recognized as significant, unfortunately, most submarine sources remain poorly quantified. Beside cold vents and coastal anoxic sediments, the large number of submarine mud volcanoes (SMV) may contribute significantly to the oceanic methane pool. Recent research suggests that methane primarily released diffusively from deep-sea SMVs is immediately oxidized and, thus, has little climatic impact.New hydro-acoustic, visual, and geochemical observations performed at the deep-sea mud volcano Håkon Mosby reveal the discharge of gas hydrate-coated methane bubbles and gas hydrate flakes forming huge methane plumes extending from the seabed in 1250 m depth up to 750 m high into the water column. This depth coincides with the upper limit of the temperature–pressure field of gas hydrate stability. Hydrographic evidence suggests bubble-induced upwelling within the plume and extending above the hydrate stability zone. Thus, we propose that a significant portion of the methane from discharged methane bubbles can reach the upper water column, which may be explained due to the formation of hydrate skins. As the water mass of the plume rises to shallow water depths, methane dissolved from hydrated bubbles may be transported towards the surface and released to the atmosphere. Repeated acoustic surveys performed in 2002 and 2003 suggest continuous methane emission to the ocean. From seafloor visual observations we estimated a gas flux of 0.2 (0.08–0.36) mol s⁻¹ which translates to several hundred tons yr⁻¹ under the assumption of a steady discharge. Besides, methane was observed to be released by diffusion from sediments as well as by focused outflow of methane-rich water. In contrast to the bubble discharge, emission rates of these two pathways are estimated to be in the range of several tons yr⁻¹ and, thus, to be of minor importance.Very low water column methane oxidation rates derived from incubation experiments with tritiated methane suggest that methane is distributed by currents rather than oxidized rapidly.

Keywords: methane sources; plume; methane budget; submarine mud volcanoes; gas bubbles; gas hydrates; hydro-acoustics

Basin-wide estimates of the input of methane from seeps and clathrates to the Black Sea by J.D. Kessler; W.S. Reeburgh; J. Southon; R. Seifert; W. Michaelis; S.C. Tyler (pp. 366-375).

Numerous methane-emitting bottom features, such as seeps, methane clathrate hydrates (clathrates), and mud volcanoes, have been identified recently in the Black Sea. The fluxes of methane from these sources averaged over large spatial scales are unknown. Here we take advantage of the fact that the Black Sea is a semi-enclosed basin with restricted deep water circulation to establish first-order estimates of basin-wide fluxes of methane from these sources to the water column and atmosphere. First, we measured the natural radiocarbon content of methane (¹⁴C–CH₄) dissolved in the water column and emitted from seeps. The¹⁴C–CH₄ results showed that the dominant source of methane to the water column is emitted from seeps and a smaller source is diagenetically produced in relatively modern sediments. The¹⁴C–CH₄ results were then used to partition a basin-wide total methane budget; this analysis estimated the basin-wide flux of methane from seeps and clathrates to the water column to be 3.60 to 4.28 Tg yr−¹. Second, a geochemical box model was used to calculate possible distributions of methane inputs from seeps and clathrates as well as provide additional estimates of the basin-wide flux of methane from seeps and clathrates to the water column (4.95 to 5.65 Tg yr−¹).

Keywords: methane; seeps; methane clathrate hydrates; Black Sea; methane radiocarbon; seep methane flux

Gravitational depletion of the early Earth's upper mantle and the viability of early plate tectonics by Geoffrey F. Davies (pp. 376-382).

Numerical modelling of stirring of subducted mafic (oceanic) crust in the hot early mantle yields an upper mantle strongly depleted of mafic components due to gravitational settling, while the lower mantle becomes marginally enriched, with a thin dense layer at the base. These results accord with the very early and strong depletion of incompatible trace elements in the mantle source of the oldest rocks. They support a recent geochemical argument that the depletion is due more to the extraction of mafic material rather than of continental crust. The strong depletion would yield a thin oceanic crust, despite the high temperature of the mantle, which would make early plate tectonics more viable.

Keywords: early mantle; trace-element depletion; plate tectonics; mantle convection

Detection of a gravitational oscillation in length-of-day by Jon E. Mound; Bruce A. Buffett (pp. 383-389).

A high resolution length-of-day data set spanning approximately the last 42 years is found to contain an oscillation with a period of 5.8±0.8 years. This oscillation is interpreted to represent a normal mode of the Earth that arises due to gravitational coupling between the mantle and inner core. In order to match the observed period of the oscillation, the strength of the gravitational coupling between the mantle and inner core must be ∼3.0×10²⁰N m. This coupling constant depends on the heterogeneous density distribution of the mantle, including flow-induced deformation of the core–mantle boundary. The existence of this normal mode requires that the relaxation time of the inner core be sufficiently long (on the order of a few years or more) that a perturbation from equilibrium results predominantly in solid-body rotation, not viscous deformation, of the inner core. The inferred lower bound for the viscosity of the inner core is of the order of 10¹⁷Pa s.

Keywords: length-of-day; core–mantle coupling; inner core; mantle structure

Reanalysis of ancient eclipse, astronomic and geodetic data: A possible route to resolving the enigma of global sea-level rise by Jerry X. Mitrovica; John Wahr; Isamu Matsuyama; Archie Paulson; Mark E. Tamisiea (pp. 390-399).

Predictions of the Earth's response to the ice age appear to simultaneously reconcile a set of astronomical, geodetic and ancient eclipse observations related to changes in rotation, thus ruling out ice melting as a major contributor to 20th century sea-level rise. We demonstrate that the reconciliation disappears when an improved theory of rotational stability is applied. Furthermore, our reanalysis of longer satellite records renders previous estimates of the secular change in rotation rate suspect. The updated ice-age predictions and observations permit an anomalous 20th century ice flux of ∼1 mm/yr equivalent sea-level rise. Thus, the full suite of Earth rotation observations are consistent with a connection between climatic warming and recent melting of ice reservoirs.

Keywords: ancient eclipses; polar wander; Earth rotation; global sea level; glacial rebound

Combining cosmogenic, stratigraphic, and paleomagnetic information using a Bayesian approach: General results and an application to Sterkfontein by Paul Muzikar; Darryl Granger (pp. 400-408).

In geochronology it is increasingly common to apply several methods to a set of samples. The task then arises of combining different types of data, with perhaps qualitatively different types of uncertainty, into a coherent age estimate which makes optimum use of the available information. In this paper we discuss a method which uses Bayesian reasoning to combine radiometric, stratigraphic, and paleomagnetic information when dating sediment layers. After presenting a general formulation, we derive an analytic formula for the probability distribution of the age of a sediment layer. We then apply this methodology to refine previous cosmogenic nuclide burial dating results for the age of a sediment layer containing a hominin fossil at Sterkfontein, South Africa.

Keywords: Bayesian statistics; cosmogenic nuclides; burial dating; paleomagnetism; geochronology

Magnetobiochronology of Lower Miocene (Ramblian) continental sediments from the Tudela Formation (western Ebro basin, Spain) by Juan C. Larrasoa�a; Xabier Murelaga; Miguel Garc�s (pp. 409-423).

The calibration of European Lower Miocene continental stages is poorly constrained due to the lack of robust magnetostratigraphic data from continental sediments of this age. In this paper, we present the first magnetostratigraphic results from the mammal fossil-bearing continental sediments of the Lower–Middle Miocene Tudela Formation (western Ebro basin, Spain). The study is based on 331 paleomagnetic sites distributed along a 790-m-thick composite section. The resulting magnetostratigraphy provides an independent correlation to the geomagnetic polarity time scale and allows accurate dating of previously known and new Ramblian (upper Lower Miocene) fossil localities of the Tudela Formation.The new age constraints derived for the Tudela Formation fossil localities provide, together with a revaluation of other Iberian faunas, the first magnetochronology for the Ramblian continental stage. The lower boundary of the Ramblian is placed at the lower part of chron C6r (ca. 20.4Ma), and the boundary between the lower and upper Ramblian is located at the middle of chron C6n (ca. 19.6Ma). The upper boundary of the Ramblian, and hence the base of the Aragonian, is placed at the upper middle part of chron C5Cr (16.8–17Ma). Our results also provide the first magnetostratigraphically dated MN3 faunas in Spain, which, together with a revaluation of other Iberian and European faunas, provide a revised calibration of European mammal zone MN3. The lower boundary of zone MN3 is placed at the boundary between chrons C6r and C6n (ca. 20.1Ma), and its upper boundary is located at the upper middle part of chron C5Cr (16.8–17Ma).

Keywords: Miocene; Ramblian; magnetostratigraphy; biostratigraphy; fossil mammals; Ebro basin

δ¹³C and δ¹⁵N in the ornithogenic sediments from the Antarctic maritime as palaeoecological proxies during the past 2000 yr by Xiao-Dong Liu; Hong-Chun Li; Li-Guang Sun; Xue-Bin Yin; San-Ping Zhao; Yu-Hong Wang (pp. 424-438).

In this paper, we have examined carbon and nitrogen isotopic compositions in two ornithogenic sediment profiles from the Ardley Island and Barton Peninsula of Antarctica for palaeoecological changes during the past 2000 yr. The δ¹³C values of the two sediment profiles range from −22.26‰ to −19.15‰ (PDB) in Core G and from −24.01‰ to −19.87‰ in profile A, showing that the predominant carbon source in the sediments plausibly comes from terrestrial and aquatic plants in Antarctic such as mosses, lichens, and algae in lakes. As these δ¹³C values are also close to those in the fresh lake sediments that are not influenced by penguin guano, one may not use the δ¹³C values as evidence for the influence of guano on the sediments. The δ¹⁵N values of the two profiles range from 4.75‰ to 18.34‰ (air) and from 5.17‰ to 10.38‰ for Core G and Core A, respectively. The δ¹⁵N variations have positive correlations with the trends of the bio-element contents in the sediments. As the levels of these bio-elements in ornithogenic sediments had been used to reconstruct the changes of historical penguin population and tundra vegetation abundance and diversity, we then suggest that the δ¹⁵N records can be utilized to study palaeoecological processes of penguin. Our results show that penguin population and activity has generally decreased over the past 2000 yr. From 1300 to 900 yr BP and from 1790 to 1860 AD, penguin population and activity experienced two strong decreases. It will be interesting to understand the cause of these decreases.

Keywords: ornithogenic sediments; carbon and nitrogen isotope; palaeoecology; penguin; Antarctica

Reconstruction of past precipitation δ¹⁸O using tree-ring cellulose δ¹⁸O and δ¹³C: A calibration study near Lac d'Annecy, France by P.A. Danis; V. Masson-Delmotte; M. Stievenard; M.T. Guillemin; V. Daux; Ph. Naveau; U. von Grafenstein (pp. 439-448).

Two sets of living oaks ( Quercus R.) growing under different hydrological situations were sampled near Lac d'Annecy, France. The stable isotope composition of the latewood cellulose was analysed in terms of oxygen and carbon isotopic composition for the period 1971–2001 and compared to the nearby monthly meteorological (temperature, relative humidity) and isotopic composition of precipitation records. A linear regression shows that 60% of the cellulose oxygen isotopic composition inter-annual variations are due to changes in precipitation isotopic composition and relative humidity. The small slope between cellulose versus precipitation oxygen isotopic composition suggests a significant contribution of local continental recycling and questions the common assumption that vapour oxygen isotopic composition is at equilibrium with precipitation isotopic composition in Europe during the growing season. Finally, the combined use of tree-ring oxygen and carbon isotopic composition records enables to account for leaf water enrichment due to stomata stress and to provide a reconstruction ( R²=0.64) of the summer precipitation isotopic composition.

Keywords: Quercus; tree rings; proxy; oxygen; carbon; stable isotopes; humidity

CO₂ release variations during the last 2000 years at the Colli Albani volcano (Roma, Italy) from speleothems studies by P. Tuccimei; G. Giordano; M. Tedeschi (pp. 449-462).

The Colli Albani is the quiescent volcano that dominates the southwestern skyline of Roma (Italy). The last eruption occurred during the Holocene, from the eccentric Albano maar, along its western slope. The volcano is presently affected by cyclic seismic swarms, ground uplift and diffuse CO₂ degassing. The degassing has caused several deadly incidents during the last years and constitutes a major civil protection concern, as the volcano slopes are densely inhabited. Nevertheless, the volcano does not have a permanent monitoring network, and the background level and anomalous CO₂ levels, the relationship between the gas release and the seismic and ground deformation activity at the Colli Albani are still to be defined. The aim of this work is to define the historical record of CO₂ release. Evidences of deep CO₂ periodic release during the last 2000 years in the area of Colli Albani volcano (Roma, Italy) are offered from speleothems studies. A Roman-age stone mine, now used for mushroom cultivation, is decorated with actively growing speleothems, characterised by depositional hiatuses. Different levels of four stalactites, separated by depositional unconformities, and several samples from a single depositional cycle belonging to a stalagmite have been dated by U/Th method and analysed for their O and C isotopic composition. Eight cycles of deposition have been identified from 90–110 A.D. to 1350–1370 A.D., some of which are recognised across different speleothems. The age gap dividing different growth layers is in the order of one to few hundred years giving a temporal span for periodic interruption of speleothems deposition. O and C isotopic analyses performed on the samples collected from a single cycle (the oldest) have shown that the composition of the mother solutions was initially mainly meteoric and that a progressive increase in the input of a deep component rich in CO₂ (up to a proportion of 20–30%) occurred just before the interruption of the speleothem deposition. This could be due to a progressive increase of the acidity of the water solutions that caused the undersaturation of fluids. If we extrapolate this mechanism to the other cycles of deposition, being characterised by analogue isotopic compositions, we can hypothesise periods of deposition interrupted by episodes of CO₂ release which in the Colli Albani volcano are often recorded in coincidence with earthquakes. Therefore we have correlated the hiatuses with some of the largest historical earthquakes interesting to the city of Rome.

Keywords: CO; ₂; hazard; Colli Albani volcano; Roma; Italy; speleothems; U/Th dating; stable isotopes; water chemistry

New Early to Middle Triassic U–Pb ages from South China: Calibration with ammonoid biochronozones and implications for the timing of the Triassic biotic recovery by Maria Ovtcharova; Hugo Bucher; Urs Schaltegger; Thomas Galfetti; Arnaud Brayard; Jean Guex (pp. 463-475).

New zircon U–Pb ages are proposed for late Early and Middle Triassic volcanic ash layers from the Luolou and Baifeng formations (northwestern Guangxi, South China). These ages are based on analyses of single, thermally annealed and chemically abraded zircons. Calibration with ammonoid ages indicate a 250.6±0.5 Ma age for the early Spathian Tirolites/ Columbites beds, a 248.1±0.4 Ma age for the late Spathian Neopopanoceras haugi Zone, a 246.9±0.4 Ma age for the early middle Anisian Acrochordiceras hyatti Zone, and a 244.6±0.5 Ma age for the late middle Anisian Balatonites shoshonensis Zone. The new dates and previously published U–Pb ages indicate a duration of ca. 3 my for the Spathian, and minimal durations of 4.5±0.6 my for the Early Triassic and of 6.6+0.7/−0.9 my for the Anisian. The new Spathian dates are in a better agreement with a 252.6±0.2 Ma age than with a 251.4±0.3 Ma age for the Permian–Triassic boundary. These dates also highlight the extremely uneven duration of the four Early Triassic substages (Griesbachian, Dienerian, Smithian, and Spathian), of which the Spathian exceeds half of the duration of the entire Early Triassic. The simplistic assumption of equal duration of the four Early Triassic subdivisions is no longer tenable for the reconstruction of recovery patterns following the end Permian mass extinction.

Keywords: U–Pb ages; zircon; Early Triassic; ammonoids; biotic recovery

High resolution evidence for linkages between NW European ice sheet instability and Atlantic Meridional Overturning Circulation by V.L. Peck; I.R. Hall; R. Zahn; H. Elderfield; F. Grousset; S.R. Hemming; J.D. Scourse (pp. 476-488).

Published studies show that ice rafted debris (IRD) deposition preceding Heinrich (H) events H1 and H2 in the NE Atlantic was derived from the NW European ice sheets (NWEIS), possibly offering clues about ice sheet sensitivity and stability, and the mechanisms that caused periodic collapse of the Laurentide Ice Sheet (LIS). We present detailed lithological and geochemical records, including radiogenic isotope fingerprinting, of IRD deposits from core MD01-2461, proximal to the last glacial British Ice Sheet (BIS), demonstrating persistent instability of the BIS, with significant destabilisation occurring 1.5–1.9 kyr prior to both H1 and H2, dated at 16.9 and 24.1 kyr BP, respectively, in the NE Atlantic. Paired Mg/Ca and δ¹⁸O data from the surface dwelling Globigerina bulloides and subsurface dwelling Neogloboquadrina pachyderma sinistral are used to determine late-glacial variability of temperature, salinity and stratification of the upper water column. A picture emerges that the BIS was in a continuing state of readjustment and never fully reached steady state. Increased sea surface temperatures appear to have triggered the episode of NWEIS instability preceding H1. It seems most probable that the so-called ‘precursor’ events were not linked to the H events. However, if response to a common thermal forcing is considered, an increased response time of the LIS, up to ~2 kyr longer than the NWEIS, may be inferred. Negative salinity excursions of up to 2.6 indicate significant incursions of melt water associated with peaks in NWEIS instability. Decreased surface density led to a more stable stratification of the upper water column and is associated with reduced ventilation of intermediate waters, recorded in depleted epibenthic δ¹³C ( Cibicidoides wuellerstorfi). We suggest that instability and meltwater forcing of the NWEIS temporarily weakened Atlantic Meridional Overturning Circulation, allowing transient advance of southern-sourced waters to this site, prior to H events 1 and 2.

Keywords: NW European Ice Sheet; ocean–climate linkage; Atlantic Meridional Overturning Circulation; ice rafted debris; Heinrich events; ice sheet instability

Plio-Pleistocene history of Ferrar Glacier, Antarctica: Implications for climate and ice sheet stability by J.W. Staiger; D.R. Marchant; J.M. Schaefer; P. Oberholzer; J.V. Johnson; A.R. Lewis; K.M. Swanger (pp. 489-503).

The areal distribution and elevation of glacial drifts in Vernier Valley, southern Victoria Land, are used to reconstruct the Plio-Pleistocene history of upper Ferrar Glacier.²¹Ne cosmogenic-nuclide analyses of surface cobbles on four moraines, Ferrar 1, 2, 3, and 4, provide age control. A minimum-age estimate for Ferrar Drifts calculated by assuming zero surface erosion indicates that the oldest moraine, Ferrar 4, was deposited at least ∼3400 ka. Our preferred age model, which applies a very conservative erosion rate of 5 cm Ma−¹ in age calculations, suggests that Ferrar 4 is ∼4000 ka; Ferrar 3 is ∼1200 ka; and Ferrar 2 is ∼700 ka. Based on glacial geologic data, Ferrar 1 is modern; cosmogenic ages for cobbles on this moraine suggest a value for nuclide inheritance of ∼50 ka.The Ferrar drifts are most easily interpreted in terms of a progressive reduction in the ice-surface elevation of upper Ferrar Glacier during Plio-Pleistocene time. Relative to today, the surface of upper Ferrar Glacier was ∼100 to 125 m higher during the Pliocene Climatic Optimum and ∼50 m higher during early to mid Quaternary time. Conversely, during MIS 2, the ice-surface elevation of upper Ferrar Glacier was likely no larger than today and may have stood below modern levels.The texture and sedimentology of all Ferrar drifts indicate that during ice recession from Vernier Valley the upper Ferrar Glacier lacked surface-melting ablation zones, even during the Pliocene Climatic Optimum. Results from a simple 2-D glaciological flow-band model demonstrate that upper Ferrar Glacier also lacked basal-melting zones during ice recession.We show that the development of weathering pits and desert varnish on cobbles exposed at the surface of Ferrar drifts varies in accord with cosmogenic age. The mean width and depth of the largest surface pits on boulders from Ferrar drifts increases by ∼10 mm Ma−¹ and ∼6.7 mm Ma−¹, respectively; the maximum thickness of desert varnish on surface boulders increases by ∼1.5 mm Ma−¹. These rates may be used to help calculate ages for dolerite-rich drifts elsewhere in the western Dry Valleys region.The general stability of the ice-surface elevation of upper Ferrar Glacier, and of the landscape in Vernier Valley, suggests minimal climatic amelioration in the upland region of the Dry Valleys during the last ∼4 Ma.

Keywords: Antarctica; Ferrar Glacier; Pliocene; Quaternary; cosmogenic; Dry Valleys; East Antarctic Ice Sheet

Isotope calibrated Greenland temperature record over Marine Isotope Stage 3 and its relation to CH₄ by Christof Huber; Markus Leuenberger; Renato Spahni; Jacqueline Fl�ckiger; Jakob Schwander; Thomas F. Stocker; Sigfus Johnsen; Amaelle Landais; Jean Jouzel (pp. 504-519).

Large temperature variations on millennial time scales in Greenland characterised the last ice age. Abrupt warmings, known as Dansgaard–Oeschger (DO) events, can be traced in the δ¹⁸O_ice record of Greenland ice cores. However, it has been shown that δ¹⁸O_ice is not a direct temperature proxy. Measurements of the isotopic composition of gases trapped in the ice can be used to calibrate the paleothermometer. Here we present a continuous temperature record based on high resolution δ¹⁵N measurements and firn model studies. It covers a sequence of 9 DO events (9–17) during the time period from 38 to 64 kyr BP for which temperature changes of 8 to 15 °C were estimated. The difference between the modern and the glacial δ¹⁸O_ice– T relationship can be explained by a combination of source temperature changes and changes in the annual distribution of precipitation. A detailed comparison of the temperature evolution with reconstructions of the atmospheric methane (CH₄) concentration shows that CH₄ rises lag temperature increases at the onset of DO events by 25 to 70 yr within data resolution. The strong correlation between Greenland temperature and CH₄ on millennial and submillennial time scales suggests that variations on these time scales were probably of hemispheric extent.

Keywords: Marine Isotope Stage; Dansgaard-Oeschger events; Greenland; Methane; Ice age

(U–Th)/Ne chronometry by C.E. Gautheron; L. Tassan-Got; K.A. Farley (pp. 520-535).

Production of²¹Ne from the reaction¹⁸O( α, n)²¹Ne in U and Th rich minerals such as apatite, zircon, monazite and titanite can potentially be used for chronometry. Based on a review of the available cross section data we reevaluated the production of²¹Ne from this reaction using a thin target approach and compared the results against thick target situations. The (²¹Ne/⁴He) production ratio in these minerals is about 4×10−⁸, but varies with oxygen content and Th/U ratio.²¹Ne has a stopping range of about 1 μm compared to about 20 μm for α particles; thus the (²¹Ne/⁴He) production ratio also depends on crystal size when the crystals are small enough that α ejection is important. Using a Monte Carlo model we computed the effects of ejection on the (²¹Ne/⁴He) ratio for various geometries and grain sizes. We also present measurements of the (²¹Ne/⁴He) ratio on few mg aliquots of well-dated volcanic apatites and zircons for which the duration of retention of²¹Ne and⁴He is the same. Values of (4.4±1.1)×10−⁸ and (3.5±1.2)×10−⁸ for apatite and zircon, respectively, are in agreement with the theoretical values of (4.16±0.14)×10−⁸; (3.54±0.13)×10−⁸. Based on our production rate estimates the Durango apatite and Fish Canyon Tuff zircon give Ne ages of 34.2±8.6 and 28±12 Ma, respectively, which are in agreement with independently known ages. Our results demonstrate that the²¹Ne production is well understood and can be used to measure the neon retention time for slowly cooled minerals. The⁴He and²¹Ne content of zircons from the deeply exhumed crustal section in Gold Butte, Nevada (crystallization age of 1.4 Ga) imply (U–Th)/Ne ages of 963±164 and 777±122 Ma, far older than their He ages of 16.7±1.3 and 19.1±1.5 Ma, respectively. To explain the age difference, a neon closure temperature for zircon between the values given by the thermochronometers U–Pb on apatite and Ar–Ar on muscovite is derived, probably 400±50 °C.

Keywords: (U–Th); /; Ne; (U–Th); /; He; chronometer; neon nuclear physics

Methodology of Curie discontinuity map development for regions with low thermal characteristics: An example from Israel by Lev V. Eppelbaum; Arkady N. Pilchin (pp. 536-551).

Analysis of available data indicates that different values of the Curie temperature for magnetite and titanomagnetites along with transition between ferric (Fe III) and ferrous (Fe II) iron could lead to significant errors in the Curie point depth determination using magnetic data. Based on analysis of geothermal and magnetic methods used for the Curie point depth determination in different regions it is shown that for conditions in the Eastern Mediterranean examination of the magnetic field should be used for determination of the bottom edges of magnetized bodies/layers only. The authors demonstrated that the depth of the bottom edges of magnetized bodies couldn't be greater than the depth of the Curie point for magnetite. On example of Israel and adjoining regions of the Eastern Mediterranean it is shown that in regions with a low heat flow and low vertical geothermal gradient, the depth of the Curie point is usually greater than that of the Moho discontinuity. An improved geothermal method of temperature calculation and other geothermal parameters analysis was used to determine the Curie point depth in the regions of the Eastern Mediterranean and adjoining areas. A new map of the Moho discontinuity for the region covering Israel, Jordan, Palestinian autonomy, Syria, Lebanon, and the eastern part of the Mediterranean Sea was composed. The Moho discontinuity map was utilized for the development of a first map of the Curie point depth for Israel.

Keywords: Curie point depth; rock magnetism; Moho discontinuity; gravitational field; Israel

Global reconstructions of Cenozoic seafloor ages: Implications for bathymetry and sea level by Xiqiao Xu; C. Lithgow-Bertelloni; Clinton P. Conrad (pp. 552-564).

Although accurate estimates of Cenozoic seafloor ages will serve to further our understanding of the relationship between mantle dynamics, plate tectonics, and a variety of surficial geological processes, it is difficult to estimate ages of subducted seafloor. However, given the near-constancy of surface velocities within a tectonic stage, we can estimate Cenozoic plate ages, even for subducted lithosphere. We reconstruct seafloor ages based on the Cenozoic plate reconstructions and absolute rotation poles of Gordon and Jurdy [R.G. Gordon and D.M. Jurdy, Cenozoic Global Plate Motions, J. Geophys. Res. 91 (1986) 12389–12406.]. For the western Pacific, we explore alternative models based on the reconstructions of Hall [R. Hall, Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations, J. Asian Earth Sci. 20 (2002) 353–434.]. Both reconstructions indicate an increase in average seafloor age since the early Cenozoic, resulting in an increase in the volume of ocean basins and a decreased sea level since the Early Cenozoic. These trends are more pronounced for the Gordon and Jurdy [R.G. Gordon and D.M. Jurdy, Cenozoic Global Plate Motions, J. Geophys. Res. 91 (1986) 12,389–12,406.] reconstruction because the Hall [R. Hall, Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations, J. Asian Earth Sci. 20 (2002) 353–434.] reconstruction retains older seafloor in the western Pacific, which approximately halves the predicted sea level decrease since the early Cenozoic (250 vs. 125 m compared to geologic estimates of ∼150 m). These changes in sea level occur despite decreases in oceanic lithosphere production rates of only about 20% in both models. Thus, the changing distribution of seafloor age has a larger effect on sea level than changes in spreading rates or ridge lengths. These reconstructions can also be used to estimate past heat flow, the volume of subducted buoyancy and changes in the bathymetry of the Cenozoic ocean basins.

Keywords: Cenozoic sea level; ridge volume; global seafloor age reconstructions; spreading rates; subduction rates; plate tectonic reconstructions

Late stages of exhumation constrained by structural, fluid inclusion and fission track analyses (Sesia–Lanzo unit, Western European Alps) by M.G. Malus�; P. Philippot; M. Zattin; S. Martin (pp. 565-580).

In metamorphic areas, paleogeothermal gradients are difficult to properly estimate, leading to great uncertainty in exhumation rate calculations based on fission track analysis. In this work, a new multidisciplinary approach based on fluid inclusion analysis, structural analysis and fission track dating has proven to be able to constrain the late stages of exhumation of metamorphic rocks. The study area is located in the Sesia–Lanzo unit of the Western European Alps, along the Hone shear zone. Kinematic analysis at different scales unraveled a polyphase postmetamorphic deformation history, constrained in the P–T space thanks to laboratory investigation of kinematically referenced fluid inclusions coupled with the analysis of the rheological behavior of quartz and feldspar inside syn-kinematic veins. The resulting P–T path points to an increasing paleogeothermal gradient from 18 to 30 °C km−¹ after greenschist facies metamorphic conditions, and provides a reference framework for a quantitative interpretation of fission track data in terms of exhumation rates and fault throws. According to fission track data, throws accommodated by the Hone shear zone between 33 and 30 Ma are on the order of 4.5 km, and would be underestimated by ∼25% if a “normal�? steady gradient was assumed for the whole post-metamorphic history of the area. The deepest portions of this structure were exhumed thanks to the activity of the E–W fault system that accommodated a throw of ∼4 km between 28 and 20 Ma, and a throw of ∼0.8 km during the last 20 Myr.

Keywords: exhumation; paleogeothermal gradient; fluid inclusions; fission track analysis; Hone shear zone

Adakites without slab melting: High pressure differentiation of island arc magma, Mindanao, the Philippines by Colin G. Macpherson; Scott T. Dreher; Matthew F. Thirlwall (pp. 581-593).

New geochemical data for Pleistocene magmatic rocks from the Surigao peninsula, eastern Mindanao, the Philippines, demonstrate typical adakitic traits, including elevation of Sr/Y and depletion of the heavy rare earth elements.⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios of the adakites do not support melting of the subducted Philippine Sea Plate but resemble Pliocene arc lavas generated in the same subduction zone. Excepting the heavy rare earth elements, the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge. The wide range of SiO₂ in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature. (1) Adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet-bearing assemblage. (2) Solidified basaltic rock containing garnet melted to yield adakitic magma. In either case the basaltic precursor was generated from fluid-modified mantle then differentiated within the garnet stability field. In Surigao this requires differentiation within mantle. The Surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth. This has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks.

Keywords: subduction; magmatism; differentiation; adakite; slab

A case history of paroxysmal explosion at Stromboli: Timing and dynamics of the April 5, 2003 event by M. Rosi; A. Bertagnini; A.J.L. Harris; L. Pioli; M. Pistolesi; M. Ripepe (pp. 594-606).

On April 5, 2003, Stromboli volcano (Italy) produced the most violent explosion of the past 50 years. The event was exceptionally well documented thanks to the presence on the island of several scientists and a large number of instruments deployed over the preceding months to monitor the effusive eruption that began in December 2002. Integration of visual documentation, deposit features and geophysical data allowed an accurate reconstruction of the explosive event and its dynamics. The eruption consisted of a 8-min long explosive event which evolved through four phases whose timing was precisely recorded by an infrared thermometer located about 450 m from the summit crater. Phases 2 and 3 lasted 39 and 42 s, respectively. Both had an impulsive character, were responsible for ejecting almost the entire mass of the pyroclastic products. Phases 1 and 4 represented, respectively, a short-lived precursory event and a waning tale. During Phase 2, meter-sized ballistic blocks were launched with velocities of 170 m/s to altitudes of up to 1400 m above the craters. These fell on the volcano flanks and on the village of Ginostra, about 2 km distant from the vent. A vertical jet rose above the craters which developed to feed a convective plume that reached a height of up to 4 km. The calculated mass of the Phase 2 fallout deposit and mass discharge rate were 1.1–1.4×10⁸ kg and 2.8–3.6×10⁶ kg/s, respectively. During Phase 3 a scoria flow deposit, with an estimated volume of 0.9–1.1×10⁴ m³, was erupted from the same vent that fed the ongoing sustained lava flow. The average mass discharge rate for this phase was 2.5–3.1×10⁵ kg/s.Products emitted during Phases 2 and 3 consisted of lithic and fresh magmatic material in similar proportions. The juvenile fraction consisted of a deep-originated, almost aphyric, highly vesicular pumice mingled with a shallow-derived, crystal-rich, moderately vesicular scoria.Similarities with the eruption dynamics of other historical paroxysms at Stromboli makes the April 5, 2003 explosion representative of these highly energetic events that constitute the most hazardous volcanic phenomena at Stromboli volcano.

Keywords: stromboli; Thermal monitoring; paroxysm; explosive dynamics; ballistic ejecta

Fault gouge diagenesis at shallow burial depth: Solution–precipitation reactions in well-sorted and poorly sorted powders of crushed sandstone by M.A. Kay; I.G. Main; S.C. Elphick; B.T. Ngwenya (pp. 607-614).

Using a new oedometric-type uniaxial cell, we determine rate constants for closed-system diagenesis of crushed quartz-rich aeolian sandstone powders in distilled water, heated for weeks at T ∼120°, at pore and confining pressures of ∼7 and 21 MPa. Results are presented for well-sorted distributions of fine and coarse size fractions, and for a poorly sorted mixture more representative of a fault gouge. The two well-sorted samples compact at nearly the same rate, but the poorly sorted gouge compacts much more rapidly at slow loading. The fine-size fraction reacts more quickly and fluids have a higher steady-state solute concentration than the coarse one. These results, combined with simple first order reaction rate theory, predict net dissolution followed by net precipitation for a simple two-phase mixture of particle sizes that is confirmed independently by the poorly sorted synthetic gouge. Observed super-saturation at relatively low effective pressures is consistent with Ostwald ripening. After the test the poorly sorted material is cemented sufficiently to be brought out as an intact, if friable, ‘rock’, indicating that fault sealing may be relatively rapid even under relatively closed conditions at depths of a few km.

Keywords: diagenesis; fault gouge; Ostwald ripening; pressure solution

Source characterization of the San Juan (Argentina) crustal earthquakes of 15 January 1944 (M_w 7.0) and 11 June 1952 (M_w 6.8) by Patricia Alvarado; Susan Beck (pp. 615-631).

The backarc region of the Andes in the vicinity of San Juan, Argentina, is one of the most seismically active fold and thrust belt regions in the world. Four large damaging crustal earthquakes (1894, 1944, 1952 and 1977) occurred during the last 111 yr between 30°S and 32°S. We have determined the source parameters for two of these important earthquakes, the 1944 and 1952 events, using historic seismic records. The earthquake on 15 January 1944 had an epicentral location between the eastern thin-skinned Precordillera fold and thrust belt and the thick-skinned Sierras Pampeanas basement-cored uplifts. The 11 June 1952 earthquake occurred in the eastern Precordillera about 35 km southwest of the 1944 epicenter location. The P-wave first motions, long-period teleseismic P waveform modeling, and SV/SH amplitude ratio indicate a thrust focal mechanism for the 1944 event (strike N45°E, dip 35° to the southeast, and rake 110°) with M₀=3.01×10¹⁹ N m and M_w=7.0. The 1952 earthquake focal mechanism solution indicates a more oblique mechanism (strike N40°E, dip 75° to the southeast, and rake 30°) with M₀=2.20×10¹⁹ N m and M_w=6.8. Both the 1944 and 1952 earthquakes have focal depths <12 km and simple source time functions with one pulse of moment release with durations of 10 s and 8 s, respectively. Both the shallow focal depth and the east-dipping fault plane in the focal mechanism solution for the 1944 earthquake are consistent with the parameters observed along the La Laja fault in the frontal part of the eastern Precordillera that generated a 6–8-km-long coseismic surface rupture. The 1952 earthquake focal mechanism solution and its shallow source depth suggest it is related to faults in the eastern Precordillera, but a particular fault association is difficult. The 1944 earthquake was clearly the most destructive event because its proximity to the most populated area in San Juan, large size and shallow focal depth.

Keywords: historical earthquake source; Andean backarc crust; Precordillera

Seismic characterization of mantle flow in subduction systems: Can we resolve a hydrated mantle wedge? by Teresa Mae Lassak; Matthew J. Fouch; Chad E. Hall; �douard Kaminski (pp. 632-649).

This study provides new constraints on the resolvability of mantle flow in subduction zone settings as inferred by observations of seismic anisotropy. We are motivated by the broad range of shear wave splitting observations in subduction systems that suggest complex flow geometries, changes in the deformation state of mantle minerals, or a combination of both. While shear wave splitting fast polarization directions are typically interpreted as a proxy for flow or maximum finite extension, experimental studies suggest that olivine slip systems change under higher stress and hydration states, conditions likely appropriate for subduction systems. In this study, we predict shear wave splitting as a result of mantle silicate lattice-preferred orientation development in steady-state two-dimensional mantle flow models using a textural development theory that incorporates the combined effects of intracrystalline slip and dynamic recrystallization. We utilize the resulting textures to predict shear wave splitting for populations of seismic raypaths traversing the model within the subduction zone mantle wedge. The results of our study demonstrate that combined observations of variations in fast polarization directions and splitting times make it possible to resolve a shift from anhydrous to hydrous mantle insubduction zone settings provided very good sampling of the mantle wedge. Our models are generally consistent with observed splitting variations for several subduction zones with dense data sampling, including Tonga, Japan, and Kamchatka. The implications of our work suggest that, provided adequate data sampling, shear wave splitting measurements can provide the necessary information to evaluate potential competing effects between variations in mantle flow direction and changes in the stress and hydration states of subduction zone mantle wedges.

Keywords: subduction zones; shear wave splitting; lattice-preferred orientation; mantle flow

Formation of the stratified ocean of the core by Stanislav I. Braginsky (pp. 650-656).

A mechanism of formation of the stably stratified layer of light fluid at the core–mantle boundary, which we call the stratified ocean of the core, is proposed. A turbulent diffusion in this stably stratified layer is qualitatively considered. The rate of addition of light fluid component to the layer by barodiffusion and light fluid redistribution inside the layer by turbulent diffusion are evaluated. The relevant parameters of the formation mechanism are estimated.

Keywords: Earth core; stably stratified layer; core–mantle boundary

Suppression and stimulation of seafloor hydrothermal convection by exothermic mineral hydration by Simon Emmanuel; Brian Berkowitz (pp. 657-668).

The effect of serpentinization on hydrothermal convection is explored using a dynamic 2D numerical model. Serpentinization is a highly exothermic mineral hydration process that consumes large quantities of water. The reaction is ubiquitous in the oceanic lithosphere and is generally associated with hydrothermal activity. Here, the thermal and hydration effects are incorporated into conservation equations describing fluid flow and heat transfer in hydrothermal systems. Models representing two different geological scenarios are explored. The “permeability-initiated�? case simulates rapid uplift of ultramafic basement rock accompanied by rock fracturing, while the “temperature-initiated�? scenario simulates the uplift of an ultramafic complex followed by a magmatic event at depth. In both models, simulations of convection with and without serpentinization demonstrate that mineral alteration can have an important effect on hydrothermal flow patterns and vent temperatures. Two parameters determine the impact of serpentinization on the system: (1) the basal temperature ( T_b), and (2) the dimensionless Rayleigh number ( Ra). At Ra<50 and T_b<170, peak temperatures were only slightly affected by serpentinization and convection was initially suppressed due to the downward flow of water induced by the hydration reaction. At higher Rayleigh numbers and basal temperatures, serpentinization effectively stimulated the onset of convection and significantly affected transient vent temperatures; differences in peak vent temperatures between the standard convective case and that with serpentinization were found to be as high as 170 °C. Thus, together with the geothermal heat flux, serpentinization could act to control temperatures at seafloor hydrothermal sites.

Keywords: serpentinization; reaction model; mid-ocean ridges; Lost City

Diffusive fractionation of trace elements by chaotic mixing of magmas by D. Perugini; M. Petrelli; G. Poli (pp. 669-680).

Recent research on magma mixing systems has shown that the mixing process is governed by chaotic dynamics and that this process is responsible for the generation of fractal structures that propagate within the magmatic mass from the meter to the micrometer length-scale.Laser Ablation ICP–MS trace element analyses have been performed on rock samples with evidence of chaotic mixing phenomena. Results indicate that trace elements with similar values of diffusion coefficient display good correlations in inter-elemental plots, whereas, as the difference between diffusion coefficients increases, the correlation is progressively lost. In addition, a large variability of REE patterns is observed, with the remarkable feature of the presence of positive and negative Eu anomalies occurring at short length scale, of the order of few mm.Given the chaotic nature of magma mixing structures, the mixing process has been simulated by coupling a chaotic advection and a chemical diffusion numerical scheme by considering several trace elements with variable diffusivities. Simulations indicate that such a model explains with good approximation the variable correlations among trace elements observed in natural samples. In addition, the same patterns of REE observed in natural samples, including the occurrence of positive and negative Eu anomalies at short length scale, are observed indicating that a chaotic advection/diffusion dynamic system is a suitable model to explain natural data.Results presented in this contribution indicate that at the micrometric length-scale small volumes of magmas are strongly influenced by the coupled action of chemical diffusion and chaotic flow fields and, hence, they do not represent magmas de facto present in the magmatic system because their compositions may have experienced a ‘diffusive fractionation’ process. These results may have important petrogenetic implications. For instance, if such melts were trapped as melt inclusions, they would provide misleading information about melt compositions. It is suggested that the approach of studying the degree of correlation among trace elements may be a possible method to test if melt inclusion compositions, commonly used as petrogenetic indicators, display evidence of such a ‘diffusive fractionation’ process.

Keywords: magma mixing; chaotic dynamics; diffusion; trace elements; diffusive fractionation

Deep structure and origin of the Baikal rift zone by Dapeng Zhao; Jianshe Lei; Tomofumi Inoue; Akira Yamada; Stephen S. Gao (pp. 681-691).

P-wave velocity images are determined under the Baikal rift zone in Siberia by using teleseismic tomography. Our results show prominent low-velocity anomalies in the upper mantle under the Baikal rift zone and high-velocity anomalies in the lithosphere under the Siberian craton. The low-velocity anomalies are interpreted as a mantle upwelling (plume) which has played an important role in the initiation and evolution of the Baikal rift zone. The rift formation may be also controlled by other factors such as older (prerift) linear lithosphere structures favorably positioned relative to the upwelling and favorable orientation of the far-field forces caused by the India–Asia collision.

Keywords: Baikal rift zone; teleseismic tomography; mantle upwelling; plume; India–Asia collision

The effect of heating on the microstructural evolution of La Peyratte granite deduced from acoustic velocity measurements by Thierry Reuschl�; Sevi Gbaguidi Haore; Michel Darot (pp. 692-700).

The acoustic P-waves velocity V_P was measured in a water-saturated fine-grained granite as a function of pore pressure for a series of confining pressures ranging from 10 to 75 MPa. Different crack contents were generated in the granite specimens by means of controlled thermal pre-treatment at 450, 510, 555 and 600 °C. V_P variations are interpreted as resulting from crack behaviour evolution in response to imposed pressures. Data analysis is proposed by using an effective medium approach. The elastic behaviour of the cracked rock is controlled by the crack density parameter, which varies with applied pressures due to crack closure. Two crack families corresponding to intragranular cracks and cracked grain boundaries and a decreasing exponential distribution of crack aspect ratios are introduced to take into account the heterogeneity of the rock samples and allows us to calculate the acoustic velocity V_P for various effective pressures. Comparison between theoretical and experimental values shows that the model leads to consistent velocity variations. Moreover, when applied to the four pre-treatment temperatures, the model gives informations on the effect of heating on the microstructural parameters of the samples, in particular the behaviour of both crack families when the quartz α–β transition is passed.

Keywords: granite; thermal cracking; acoustic waves velocity; effective medium

Diffusion-driven extreme lithium isotopic fractionation in country rocks of the Tin Mountain pegmatite by Fang-Zhen Teng; William F. McDonough; Roberta L. Rudnick; Richard J. Walker (pp. 701-710).

Lithium concentrations and isotopic compositions in the country rocks (amphibolites and schists) of the Tin Mountain pegmatite show systematic changes with distance to the contact. Both Li and δ⁷Li decrease dramatically along a ∼10m traverse from the pegmatite into amphibolite, with Li concentration decreasing from 471 to 68ppm and δ⁷Li decreasing from +7.6 to −19.9. Rubidium and Cs also decrease from the pegmatite contact into the country rock, but only within the first 2m of the contact, after which their concentrations remain constant. Neither mixing between pegmatite fluids and amphibolite, nor Li isotope fractionation by Rayleigh distillation during fluid infiltration is a likely explanation of these observations, due to the extremely light isotopic composition required for the amphibolite end-member in the mixing model ( δ⁷Li=−20) and the similarly extreme isotopic fractionation required in a Rayleigh distillation model. Rather, these variations are likely due to isotopic fractionation accompanying Li diffusion from the Li-rich pegmatite (Li=450 to 730ppm) into amphibolites (Li=20ppm). The fact that other alkali element concentrations vary only within 2m of the contact reflects the orders of magnitude faster diffusion of Li relative to heavier elements.Quartz mica schists in contact with the pegmatite also show large variations in both Li and δ⁷Li as a function of distance from contact (∼1wt.% to ∼70ppm and +10.8 to −18.6, respectively), but over a longer distance of >30m. Lithium concentrations of the schist decrease from ∼1wt.% adjacent to the contact to ∼70ppm 300m from the contact; the latter is a typical concentration in metapelites. The nature of the δ⁷Li variations in the schists is different than in the amphibolites. Schists within the first 2m of the contact have nearly identical δ⁷Li of +10, which mimics that of the estimated bulk pegmatite (+8 to +11). At a distance of 30m the δ⁷Li reaches the lowest value in the schists of −18.6 (similar to the lowest amphibolite measured). At a distance of 300m the δ⁷Li climbs back to +2.5, which is within the range of δ⁷Li of other schists in the region and metapelites worldwide. The behavior of Li in the schists can also be modeled by Li diffusion, with the effective diffusion coefficient in the schist being ∼10 times greater than that in the amphibolite. The effective diffusion coefficients of Li in the amphibolite and schist are >2 orders of magnitude greater that those in minerals, which implicates the importance of fluid-assisted grain-boundary diffusion over solid-state diffusion in transporting Li through these rocks.

Keywords: lithium diffusion; isotope fractionation; fluid infiltration; Tin Mountain pegmatite

Origin of Mesoproterozoic A-type granites in Laurentia: Hf isotope evidence by John W. Goodge; Jeffrey D. Vervoort (pp. 711-731).

Granitic rocks are commonly used as a means to study chemical evolution of continental crust. In particular, their isotopic compositions reflect the relative contributions of mantle and crustal sources in their genesis. In Laurentia, a distinctive belt of Mesoproterozoic A-type or “anorogenic�? granites of ∼1.4 Ga age was emplaced within composite, heterogeneous Proterozoic crust. Zircons are an ideal mineral to constrain the granite petrogenetic history because they are repositories of both age (U–Pb geochronology) and tracer (Lu–Hf isotopic) information. We measured the Hf isotope composition of zircons from 31 previously dated A-type granites intruding Proterozoic basement provinces from the southwest U.S. to the upper mid-continent. Isotopic compositions for all granites are broadly similar, with average¹⁷⁶Hf/¹⁷⁷Hf(i) ratios of 0.281871–0.282153. Averages for granites within different crustal provinces yield present-day ε_Hf values between −31.9 and −21.9. Initial ε_Hf values discriminate the granites by age of the 2.0–1.6 Ga crust which they intrude, but are independent of intrusion age, as follows (basement formation ages in parentheses): southern Granite–Rhyolite (1.5–1.3 Ga), +7.0±0.9; central Yavapai (1.8–1.7 Ga), +5.4±0.9; western Yavapai (1.8–1.7 Ga), +3.3± 1.1; Granite–Rhyolite (1.5–1.3 Ga), +1.4±0.6; Mojave (1.8–1.7 Ga), +0.2±0.8; and Penokean (1.9–1.8 Ga), −0.1±n/d. The narrow ranges of Hf isotopic signatures within these regional groupings of granites reflect the age and isotopic composition of the basement provinces they intrude. Granites in the southern Granite–Rhyolite and central Yavapai provinces have the highest initial ε_Hf, reflecting their more juvenile sources, whereas Mojave and Penokean granites show contributions from more evolved crustal sources. Simple calculations indicate that all the granites represent dominantly crustal melts; although a mantle contribution cannot be ruled out, if present it must be minor. The Hf isotope compositions of the 1.4 Ga granites appear controlled predominantly by widespread melting of heterogeneous 2.0–1.6 Ga lower crust, consistent with other geochemical indicators. In addition to constraining granite petrogenesis, the distinct age and Hf isotope signature of the zircons will prove useful as a provenance tracer in detrital zircon suites from Neoproterozoic and younger siliciclastic deposits worldwide.

Keywords: Mesoproterozoic; Hf isotopes; granite petrogenesis; Laurentia; crustal evolution

The influence of weathering process on riverine osmium isotopes in a basaltic terrain by Abdelmouhcine Gannoun; Kevin W. Burton; Nathalie Vigier; Sigurdur R. G�slason; Nick Rogers; Fatima Mokadem; Bergur Sigf�sson (pp. 732-748).

This study presents Os isotope and comprehensive major and trace element data for the dissolved load, suspended particulates and bedload for Icelandic rivers, draining predominantly basaltic catchments that range in age from historic to ca. 12 Ma. Hydrothermal waters and precipitation have also been analysed. Both Os and Re concentrations are greater in the suspended load than the bedload, while Re/Os ratios are lower, suggesting that both elements are concentrated in weathering resistant minerals. Despite this elemental fractionation the suspended particulates and bedload for each river yield indistinguishable¹⁸⁷Os/¹⁸⁸Os isotope compositions that range from 0.136 to 0.292. In contrast, the dissolved load (<0.2μm filtered) often possesses a significantly more radiogenic Os isotope composition than the corresponding suspended or bed load with¹⁸⁷Os/¹⁸⁸Os ratios ranging from 0.15 to 1.04. The isotope and elemental data for the dissolved load can be explained in terms of an unradiogenic contribution from congruent basalt weathering (and/or hydrothermal input) and a radiogenic contribution that arises from two distinct processes. For the glacier-fed rivers there is a covariation between¹⁸⁷Os/¹⁸⁸Os and the extent of glacial cover in the catchment, and this is most readily explained by the entrainment of seawater aerosols into precipitation and subsequent glacial melting. While for direct-runoff (and spring-fed rivers) there is a covariation between¹⁸⁷Os/¹⁸⁸Os and the age of the bedrock in the catchment, that cannot be explained by congruent weathering of old basalt. Calculations indicate that those direct-runoff rivers with radiogenic¹⁸⁷Os/¹⁸⁸Os values are also undersaturated with respect to the primary basalt minerals olivine, pyroxene and plagioclase, indicating that these phases are unstable and prone to preferential dissolution. Published Re–Os isotope data indicate that the same phases possess exceptionally high¹⁸⁷Re/¹⁸⁸Os ratios and thus evolve to radiogenic¹⁸⁷Os/¹⁸⁸Os compositions in very short time intervals. Taken together, these results indicate that incongruent (preferential) weathering of certain primary basalt minerals can impart a radiogenic Os isotope composition to the dissolved riverine load. Nevertheless, overall the Os isotope signal to the Oceans from Icelandic rivers is little affected because rivers with unradiogenic¹⁸⁷Os/¹⁸⁸Os values and a high discharge dominate the Os flux.

Keywords: Re–Os isotopes; basaltic rivers; Iceland; weathering susceptibility; saturation state; incongruent weathering

Germanium incorporation into sponge spicules: Development of a proxy for reconstructing inorganic germanium and silicon concentrations in seawater by Michael J. Ellwood; Michelle Kelly; William A. Maher; Patrick De Deckker (pp. 749-759).

Measurements of germanium (Ge) in deep-sea sponge skeletons are presented for sponges collected by dredge and for spicules isolated from a range of deep-sea sediment cores. Germanium to silicon (Si) ratios (Ge/Si_sp) for sponge silica ranged between 0.075 and 0.380 μmol/mol, which are much lower than the present-day seawater Ge/Si ratio of 0.7 μmol/mol. A plot of Ge/Si_sp versus estimated seawater Ge (and Si) concentration produced a linear relationship with the Ge content of spicules increasing with seawater Ge concentration. Plots of Ge/Si_sp versus depth for both dredged sponges and sediment-bound spicules produced oceanic profiles similar to those of dissolved Ge (and Si) concentration. To explain the fractionation seen in the sponge Ge/Si_sp data two models are presented. The first model used to interpret the data assumes that sponges only respond to the Ge content of the ambient seawater, implying that, Ge incorporation into sponge silica is independent of the seawater Ge/Si ratio up to a Si concentration of about 100 μmol/L. This model is consistent with the⁶⁸Ge uptake results of Davie et al. [E.I. Davie, T.L. Simpson, R. Garonne, Experimental germanium incorporation into siliceous sponge spicules, Biol. Cell 48 (1983) 191–202] for cultured spicules. Their results showed that the incorporation of Ge in sponge silica is only dependent on the Ge concentration of the water in which a sponge is growing. The second model used to explain the data assumes that Ge/Si_sp fractionation results from subtle differences in the uptake kinetics of Ge and Si. While the assumptions used by each model to describe the data are different, it is possible to use sponge Ge/Si_sp data to reconstruct palaeo-Ge concentrations using model I, and to reconstruct palaeo-Si concentrations using both models. Palaeo-Si concentrations estimated using both models are in good agreement.

Keywords: sponge; germanium; silicon; Ge; /; Si fractionation; Southern Ocean

Germanium sequestration by soil: Targeting the roles of secondary clays and Fe-oxyhydroxides by Allison M. Scribner; Andrew C. Kurtz; Oliver A. Chadwick (pp. 760-770).

Germanium is a trace element that behaves similarly to silicon and shows promise as a tracer of silicate weathering processes. Sequestration of Ge by clay minerals is thought to be the dominant mechanism responsible for Ge/Si fractionation during weathering. In some environments, Ge may be sequestered by iron-oxyhydroxides. We examine Ge behavior in soils along a well-characterized climate gradient on the Hawaiian island of Maui to clarify the relationship between Fe geochemistry and soil Ge/Si ratios. All of the sites are developed on basaltic lavas and tephras approximately 400ky old, are highly Si-depleted, and have Ge/Si ratios higher than parent material. Sites experiencing less than ∼3300mm rain/yr host oxidized, highly weathered basaltic soils, in which Fe is present as secondary Fe-oxyhydroxide minerals. In contrast, the wetter sites (>3340mm/yr) show extensive Fe reduction, mobilization, and loss. We show that Ge sequestration is independent of Fe redox behavior along the gradient, and thus precipitation of Fe-oxyhydroxides is not a major factor contributing to Ge/Si fractionation during weathering. Instead, elevated soil Ge/Si ratios reflect partitioning of Ge into secondary clay minerals, plus additional retention of Ge by refractory non-silicates.

Keywords: weathering; trace elements; germanium; silicon; iron; Hawaii; soil

Flow and fracturing of viscoelastic media under diffusion-driven bubble growth: An analogue experiment for eruptive volcanic conduits by J. Taddeucci; O. Spieler; M. Ichihara; D.B. Dingwell; P. Scarlato (pp. 771-785).

To visualize the behavior of erupting magma in volcanic conduits, we performed shock tube experiments on the ductile–brittle response of a viscoelastic medium to diffusion-driven bubble expansion. A sample of shear-thinning magma analogue is saturated by gas Ar under high pressure. On rapid decompression, Ar supersaturation causes bubbles to nucleate, grow, and coalesce in the sample, forcing it to expand, flow, and fracture. Experimental variables include saturation pressure and duration, and shape and lubrication of the flow path. Bubble growth in the experiments controls both flow and fracturing, and is consistent with physical models of magma vesiculation. Two types of fractures are observed: i) sharp fractures along the uppermost rim of the sample, and ii) fractures pervasively diffused throughout the sample. Rim fractures open when shear stress accumulates and strain rate is highest at the margin of the flow (a process already inferred from observations and models to occur in magma). Pervasive fractures originate when wall-friction retards expansion of the sample, causing pressure to build-up in the bubbles. When bubble pressure overcomes wall-friction and the tensile strength of the porous sample, fractures open with a range of morphologies. Both types of fracture open normally to flow direction, and both may heal as the flow proceeds. These experiments also illustrate how the development of pervasive fractures allows exsolving gas to escape from the sample before the generation of a permeable network via other processes, e.g., bubble coalescence. This is an observation that potentially impact the degassing of magma and the transition between explosive and effusive eruptions.

Keywords: volcanic conduit; analogue experiment; vesiculation; fragmentation; degassing

Use of vegetable oil and silica powder for scale modelling of magmatic intrusion in a deforming brittle crust by Olivier Galland; Peter R. Cobbold; Erwan Hallot; Jean de Bremond d'Ars; Gatien Delavaud (pp. 786-804).

In the upper crust, intrusive bodies adopt different configurations, depending on the tectonic setting. This paper describes a new technique for analogue modelling of such intrusions, specifically of low-viscosity magma into a deforming brittle crust. Proper dynamic scaling is an important consideration. If the crust has a Coulomb failure envelope, the model material should have a cohesion C of 40 Pa to 7500 Pa. If the intruding magma is highly mobile, the model fluid should have a viscosity of 4×10−⁹ Pa s to 75 Pa s.For the model crust, we have used crystalline silica powder (SI-CRYSTAL), siliceous microspheres (SI-SPHERE) of grain size <30 μm, and a mixture (SI-MIX) of both materials. The mechanical properties of these powders have been obtained by shear and tension–shear tests. SI-CRYSTAL powder is cohesive ( C≈300 Pa, angle of internal friction ϕ≈45°) and represents competent rock. SI-SPHERE powder is much less cohesive ( C≈10 Pa, ϕ≈25°) and represents incompetent rock. SI-MIX has intermediate properties. The model magma is a vegetable oil, which solidifies at room temperature. Its viscosity when molten is η=2×10−² Pa s at 50 °C.Using these materials, we have done some preliminary experiments, to investigate the intrusion of low-viscosity magma into sedimentary basins. In non-deformed settings, intrusions were saucer-shaped cone sheets. In horizontal extension, they were steep dykes. Finally, in horizontal shortening, intrusions were basal sills that branched into thrust faults.

Keywords: physical modelling; magma emplacement; brittle tectonics; vegetable oil; silica powder

Magnetism of biotite crystals by David J. Dunlop; �zden �zdemir; Denis G. Rancourt (pp. 805-819).

We report Mössbauer spectra, magnetic hysteresis, acquisition and demagnetization of isothermal remanent magnetization (IRM), and low-temperature IRM warming curves of biotite crystals, as well as a summary of domain observations on magnetic extracts. The biotites are from 1.0 to 1.2 Ga gneisses and 1.85 Ga plutons of the Grenville, Churchill and Bear Provinces (Canada). Most have paramagnetic susceptibilities in the range (0.45–1.2)×10−³ SI, similar to reported values for iron-rich biotites. Magnetite, identified by its 120 K Verwey transition, is an inclusion in all the biotites. Sizes and domain states of inclusions correlate with magnetite content as measured by saturation magnetization M_s. The higher M_s is, the lower are the domain state indicators M_rs/ M_s ( M_rs is saturation IRM) and coercive force H_c. Added to a base population (0.003–0.1%) of pseudo-single-domain (PSD) grains with M_rs/ M_s from 0.1 to 0.3 and H_c from 7 to 20 mT is ≤1% (in two exceptional cases, 7% and 14%) of coarse-grained multidomain (MD) magnetite with much softer hysteresis properties. The underlying PSD magnetite fraction in biotites potentially retains useful paleomagnetic remanence. In the more magnetic micas, this stable fraction may be swamped by MD magnetite with less stable behavior.

Keywords: biotite; magnetic properties; hysteresis properties; M�ssbauer spectra; susceptibility

Magnetite-like nanocrystals formed by laser-driven shocks in siderite by A. Isambert; T. De Resseguier; A. Gloter; B. Reynard; F. Guyot; J.-P. Valet (pp. 820-827).

Laser-driven shock experiments were carried out on Mn and Mg-bearing natural crystalline siderite under vacuum. Raman spectroscopy and transmission electron microscopy were performed on samples recovered from shock pressures between 8.4GPa and 25.9GPa. Two different iron oxides were observed in the laser impact crater: hematite – already present in the starting sample – and a spinel-structured phase, both associated with iron carbonates. This nanometer-sized Mn-bearing magnetite-like phase results from shock-induced decarbonation of siderite. High-pressure shocks, such as meteorite impacts, are a plausible mechanism for generating nanocrystals of magnetite from Fe-carbonate-bearing terrestrial and extraterrestrial materials.

Keywords: nanometer-sized magnetite; Mn-rich siderite; martian meteorites; laser shocks

Why dynamos are prone to reversals by F. Stefani; G. Gerbeth; U. G�nther; M. Xu (pp. 828-840).

In a recent paper [F. Stefani, G. Gerbeth. Asymmetric polarity reversals, bimodal field distribution, and coherence resonance in a spherically symmetric mean-field dynamo model. Phys. Rev. Lett. 94 (2005) 184506] it was shown that a simple mean-field dynamo model with a spherically symmetric helical turbulence parameter α can exhibit a number of features which are typical for Earth's magnetic field reversals. In particular, the model produces asymmetric reversals (with a slow decay of the dipole of one polarity and a fast recreation of the dipole with opposite polarity), a positive correlation of field strength and interval length, and a bimodal field distribution. All these features are attributable to the magnetic field dynamics in the vicinity of an exceptional point of the spectrum of the non-selfadjoint dynamo operator where two real eigenvalues coalesce and continue as a complex conjugated pair of eigenvalues. Usually, this exceptional point is associated with a nearby local maximum of the growth rate dependence on the magnetic Reynolds number. The negative slope of this curve between the local maximum and the exceptional point makes the system unstable and drives it to the exceptional point and beyond into the oscillatory branch where the sign change happens. A weakness of this reversal model is the apparent necessity to fine-tune the magnetic Reynolds number and/or the radial profile of α in order to adjust the operator spectrum in an appropriate way. In the present paper, it is shown that this fine-tuning is not necessary in the case of higher supercriticality of the dynamo. Numerical examples and physical arguments are compiled to show that, with increasing magnetic Reynolds number, there is strong tendency for the exceptional point and the associated local maximum to move close to the zero growth rate line where the indicated reversal scenario can be actualized. Although exemplified again by the spherically symmetric α² dynamo model, the main idea of this “self-tuning�? mechanism of saturated dynamos into a reversal-prone state seems well transferable to other dynamos. As a consequence, reversing dynamos might be much more typical and may occur much more frequently in nature than what could be expected from a purely kinematic perspective.

Keywords: geomagnetic field; reversals; superchrons; superplumes

Discussion of “Magnetostratigraphic confirmation of a much faster tempo for sea-level change for the Middle Triassic Latemar platform carbonates�? by D.V. Kent, G. Muttoni and P. Brack [Earth Planet. Sci. Lett. 228 (2004), 369–377] by Linda A. Hinnov (pp. 841-846).

Kent et al. report on new magnetostratigraphic data obtained from the Middle Triassic Latemar carbonate platform (Dolomites, Italy). The result is important because it addresses the so-called ‘Latemar controversy,' and appears to corroborate radioisotope-dated ash beds in the Latemar platform indicating that the buildup must have taken place in 2–4 million years, but not the 9–12 million years of Milankovitch forcing inferred from cyclostratigraphic analyses. Unfortunately, Kent et al. omit basic information that runs contrary to the conclusion that the Latemar carbonates have yielded a primary paleomagnetic signal. Here, the missing details are supplied by “zooming in" on the chronostratigraphic interval that was investigated. In sum, Kent et al.'s results do not confirm a “faster tempo for sea level change" for the Latemar as much as raise questions about the magnetization of these carbonate rocks. There are also shortcomings in Kent et al.'s reappraisal of the cyclic content of the Cimon del Latemar (CDL) series that need clarification. Finally, the Latemar controversy is examined in the context of the distribution of time in Middle Triassic stratigraphy of the Dolomites.

Keywords: Triassic; Dolomites; Latemar; magnetostratigraphy; cyclostratigraphy

Reply to ‘Discussion of “Magnetostratigraphic confirmation of a much faster tempo for sea-level change for the Middle Triassic Latemar platform carbonates�? by D. V. Kent, G. Muttoni and P. Brack [Earth Planet. Sci. Lett. 228 (2004), 369-377]’ by L. Hinnov by Dennis V. Kent; Giovanni Muttoni; Peter Brack (pp. 847-850).

Erratum to “Hydrothermal convection in carbonaceous chondrite parent bodies�? [Earth Planet. Sci. Lett. 240 (2005) 234–250] by Bryan J. Travis; Gerald Schubert (pp. 851-851).

Author Index Volume 243 (pp. 852-858).

Contents Volume 243 (pp. 859-862).

Sections

Personal tools

Earth and Planetary Science Letters (v.243, #3-4)

Sections

Personal tools

Local resources

Earth and Planetary Science Letters (v.243, #3-4)