Earth and Planetary Science Letters (v.240, #2)

Editorial Board (pp. co2).

Recent advances in understanding ice sheet dynamics by Shawn J. Marshall (pp. 191-204).

Glaciers and ice sheets play a dynamic role in Earth's climate system, influencing regional- and global-scale climate and responding to climate change on time scales from years to millennia. They are also an integral part of Earth's landscape in alpine and polar regions, where they are an active agent in isostatic, tectonic, and Earth surface processes. This review paper summarizes recent progress in understanding and modelling ice sheet dynamics, from the microphysical processes of ice deformation in glaciers to continental-scale processes that influence ice dynamics. Based on recent insights and research directions, it can be expected that a new generation of ice sheet models will soon replace the current standard. Improvements that can be foreseen in the near future include: (i) the addition of internally-consistent evolutionary equations for ice crystal fabric (anisotropic flow laws), (ii) more generalized flow laws that include different deformation mechanisms under different stress regimes, (iii) explicit incorporation of the effects of chemical impurities and grain size (dynamic recrystallization) on ice deformation, (iv) higher-order stress solutions to the momentum balance (Stokes' equation) that governs ice sheet flow, and (v) the continued merger of ice sheet models with increasingly complex Earth systems models, which include fully-coupled subglacial hydrological and geological processes. Examples from the Greenland Ice Sheet and Vatnaj�kull Ice Cap, Iceland are used to illustrate several of these new directions and their importance to glacier dynamics.

Keywords: ice sheet modelling; ice rheology; glacier dynamics; subglacial hydrology; Greenland; Vatnaj�kull

What are the climate controls on δD in precipitation in the Zongo Valley (Bolivia)? Implications for the Illimani ice core interpretation by Fran�oise Vimeux; Robert Gallaire; Sandrine Bony; Georg Hoffmann; John C.H. Chiang (pp. 205-220).

Controversy has surrounded the interpretation of the water isotopic composition ( δD or δ¹⁸O) in tropical and subtropical ice cores in South America. Although recent modeling studies using AGCM have provided useful constraints at interannual time scales, no direct calibration based on modern observations has been achieved. In the context of the recent ice core drilling at Nevado Illimani (16°39′S–67°47′W) in Bolivia, we examine the climatic controls on the modern isotopic composition of precipitation in the Zongo Valley, located on the northeast side of the Cordillera Real, at about 55 km from Nevado Illimani. Monthly precipitation samples were collected from September 1999 to August 2004 at various altitudes along this valley. First we examine the local and regional controls on the common δD signal measured along this valley. We show that (1) local temperature has definitely no control on δD variations, and (2) local rainout is a poor factor to explain δD variations. We thus seek regional controls upstream the Valley potentially affecting air masses distillation. Based on backtrajectory calculations and using satellite data (TRMM precipitation, NOAA OLR) and direct observations of precipitation (IAEA/GNIP), we show that moisture transport history and the degree of rainout upstream are more important factors explaining seasonal δD variations. Analysis of a 92-yr simulation from the ECHAM-4 model (T30 version) implemented with water stable isotopes confirms our observations at seasonal time scale and emphasize the role of air masses distillation upstream as a prominent factor controlling interannual δD variations. Lastly, we focus on the isotopic depletion along the valley when air masses are lifted up. Our results suggest that, if the temperature gradient between the base and the top of the Andes was higher by a few degrees during the Last Glacial Maximum (LGM), less than 10% of the glacial to interglacial isotopic variation recorded in the Illimani ice core could be accounted for by this temperature change. It implies that the rest of the variation would originate from wetter conditions along air masses trajectory during LGM.

Keywords: South America; Andes; water stable isotopes; calibration; ice cores; glacial–interglacial transition

The age of SNC meteorites and the antiquity of the Martian surface by Audrey Bouvier; Janne Blichert-Toft; Jeffrey D. Vervoort; Francis Albar�de (pp. 221-233).

We report new Sm–Nd, Lu–Hf, and Pb–Pb mineral and whole-rock isotope data for the basaltic shergottite Zagami, as well as Pb–Pb whole-rock isotope data for the basaltic shergottite Los Angeles, the lherzolitic shergottite Dar-al-Gani 476 (DaG 476), and the clinopyroxenite Nakhla. In agreement with previous findings, our new Sm–Nd and Lu–Hf mineral ages on the Martian meteorite Zagami are young (155 and 185 Ma, respectively). The²⁰⁷Pb/²⁰⁶Pb–²⁰⁴Pb/²⁰⁶Pb compositions of the insoluble fractions of shergottites (Zagami, Los Angeles, and literature data for Shergotty and EETA79001) form an excellent alignment indicative of a 4.0 Ga crystallization age. The range of Pb isotope compositions observed in the leachates of these samples attests to negligible contamination of the shergottites by terrestrial Pb and argues against mixing relationships. The age of 4.048±0.017 Ga (MSWD=1.5) provided by the Pb isotope compositions of the Zagami whole-rock and residues is therefore taken to date the crystallization of this rock, which, so far, was believed to be only ∼180 Ma old. Based on this result, we argue that the lithosphere of Mars is extremely old and that most mineral ages were reset recently by acidic aqueous solutions percolating through the Martian surface. This interpretation is consistent with photographic interpretations of erosional features on Mars. It also relieves the constraint imposed by the presence of anomalies of¹⁴²Nd and¹⁸²W (both products of extinct radioactive nuclides) that the Martian mantle should have preserved primordial isotopic heterogeneities, thus allowing for the planet interior to be actively convecting.

Keywords: Mars; Zagami; SNC meteorites; Pb isotopes; Hf isotopes; Nd isotopes; chronology; aqueous alteration

Hydrothermal convection in carbonaceous chondrite parent bodies by Bryan J. Travis; Gerald Schubert (pp. 234-250).

Numerical simulations of the thermal evolution of carbonaceous chondrite parent bodies indicate a period of several million years during which convective motion of water could occur. The present study considers variations of permeability, radiogenic heating, and body radius. The model accounts for the radial variation of gravity within the body, melting of ice, freezing of liquid water, and variable water properties and incorporates a realistic equation of state for water. Some regions of a parent body experience no pore water flow while other regions experience hundreds of pore volumes of liquid water flow. The spatial heterogeneity in pore water flow reflects the convective pattern in the body. Hydrothermal alteration of minerals inside the parent body will accordingly vary greatly with location in the body, and one parent body could be the source of chemically diverse meteorites.

Keywords: carbonaceous chondrite meteorites; hydrothermal convection

Iron isotope fractionation during planetary differentiation by Stefan Weyer; Ariel D. Anbar; Gerhard P. Brey; Carsten M�nker; Klaus Mezger; Alan B. Woodland (pp. 251-264).

The Fe isotope composition of samples from the Moon, Mars (SNC meteorites), HED parent body (eucrites), pallasites (metal and silicate) and the Earth's mantle were measured using high mass resolution MC-ICP-MS. These high precision measurements ( δ⁵⁶Fe≈±0.04‰, 2 S.D.) place tight constraints on Fe isotope fractionation during planetary differentiation.Fractionation during planetary core formation is confined to <0.1‰ for δ⁵⁶Fe by the indistinguishable Fe isotope composition of pallasite bulk metal (including sulfides and phosphides) and olivine separates. However, large isotopic variations (≈0.5‰) were observed among pallasite metal separates, varying systematically with the amounts of troilite, schreibersite, kamacite and taenite. Troilite generally has the lightest ( δ⁵⁶Fe≈−0.25‰) and schreibersite the heaviest ( δ⁵⁶Fe≈+0.2‰) Fe isotope composition. Taenite is heavier then kamacite. Therefore, these variations probably reflect Fe isotope fractionation during the late stage evolution and differentiation of the S- and P-rich metal melts, and during low-temperature kamacite exsolution, rather than fractionation during silicate–metal separation.Differentiation of the silicate portion of planets also seems to fractionate Fe isotopes. Notably, magmatic rocks (partial melts) are systematically isotopically heavier than their mantle protoliths. This is indicated by the mean of 11 terrestrial peridotite samples from different tectonic settings ( δ⁵⁶Fe=+0.015±0.018‰), which is significantly lighter than the mean of terrestrial basalts ( δ⁵⁶Fe=+0.076±0.029‰). We consider the peridotite mean to be the best estimate for the Fe isotope composition of the bulk silicate Earth, and probably also of bulk Earth. The terrestrial basaltic mean is in good agreement with the mean of the lunar samples ( δ⁵⁶Fe=+0.073±0.019‰), excluding the high-Ti basalts. The high-Ti basalts display the heaviest Fe isotope composition of all rocks measured here ( δ⁵⁶Fe≈+0.2‰). This is interpreted as a fingerprint of the lunar magma ocean, which produced a very heterogeneous mantle, including the ilmenite-rich source regions of these basalts.Within uncertainties, samples from Mars (SNC meteorites), HED (eucrites) and the pallasites (average olivine+metal) have the same Fe isotope compositions as the Earth's mantle. This indicates that the solar system is very homogeneous in Fe isotopes. Its average δ⁵⁶Fe is very close to that of the IRMM-014 standard.

Keywords: iron isotopes; solar system; terrestrial planets; Moon; core formation; magma ocean

Absence of extraterrestrial³He in Permian–Triassic age sedimentary rocks by K.A. Farley; P. Ward; G. Garrison; S. Mukhopadhyay (pp. 265-275).

Helium concentration and isotopic composition were measured in a suite of samples across the Permian–Triassic boundary at Opal Creek, Canada, to determine whether high extraterrestrial helium concentrations are associated with a possible extinction-inducing impact event at this time. No extraterrestrial³He was detected, implying that neither fullerene-hosted nor IDP-hosted He is present at or near the boundary. This observation is consistent with similar studies of some Permian–Triassic sections, but contrasts sharply with reports of both fullerene- and IDP-hosted extraterrestrial³He at other sections.Step-heat experiments indicate rapid diffusion of extraterrestrial helium from sediments heated to temperatures above ∼70 °C. Given the likelihood of burial and associated heating in Permian–Triassic age rocks, the initially unexpected absence of IDP-hosted³He likely indicates thermally induced diffusive loss. Indeed one of the key sections (Graphite Peak, Antarctica) from which extraterrestrial³He has been reported at and near the Permian–Triassic boundary has been sufficiently heated that the reported preservation of extraterrestrial helium, in both IDPs and fullerenes, is inexplicable. Recent contamination provides a plausible explanation for extraterrestrial³He in these samples.While no extraterrestrial³He was detected at Opal Creek, there is a sharp increase in nucleogenic³He very close to or at the Permian–Triassic boundary. This presumably arises from the major lithologic change at this time, from cherts in the Permian to shales and siltstones in the Triassic. Increased nucleogenic³He is associated with increases in both lithium and organic carbon content into the Triassic. Either the production rate or the retention of this³He is higher in the shales and siltstones than in the cherts. Care must be taken to eliminate such artifacts before interpreting changes in³He concentration in terms of fluctuations in the delivery of³He from space.

Keywords: extraterrestrial helium; cosmic dust; Permian-Triassic boundary; fullerene

Volcanic resurfacing and the early terrestrial crust: Zircon U–Pb and REE constraints from the Isua Greenstone Belt, southern West Greenland by Balz Samuel Kamber; Martin J. Whitehouse; Robert Bolhar; Stephen Moorbath (pp. 276-290).

The oldest known bona fide succession of clastic metasediments occurs in the Isua Greenstone Belt, SW Greenland and consists of a variety of mica schists and rare metaconglomerates. The metasediments are in direct contact with a felsic metavolcanic lithology that has previously been dated to 3.71 Ga. Based on trace element geochemical data for >30 metasediments, we selected the six samples with highest Zr concentrations for zircon extraction. These samples all yielded very few or no zircon. Those extracted from mica schists yielded ion probe U/Pb ages between 3.70 and 3.71 Ga. One metaconglomerate sample yielded just a single zircon of 3.74 Ga age.The mica schist hosted zircons have U/Pb ages, Th/U ratios, REE patterns and Eu anomalies indistinguishable from zircon in the adjacent 3.71 Ga felsic metavolcanic unit. Trace element modelling requires the bulk of material in the metasediments to be derived from variably weathered mafic lithologies but some metasediments contain substantial contribution from more evolved source lithologies. The paucity of zircon in the mica schists is thus explained by incorporation of material from largely zircon-free volcanic lithologies. The absence of older zircon in the mica schists and the preponderance of mafic source material imply intense, mainly basaltic resurfacing of the early Earth. The implications of this process are discussed.Thermal considerations suggest that horizontal growth of Hadean crust by addition of mafic–ultramafic lavas must have triggered self-reorganisation of the protocrust by remelting. Reworking of Hadean crust may have been aided by burial of hydrated (weathered) metabasalt due to semi-continuous addition of new voluminous basalt outpourings. This process causes a bias towards eruption of Zr-saturated partial melts at the surface with O-isotope compositions potentially different from the mantle. The oldest zircons hosted in sediments would have been buried to substantial depth or formed in plutons that crystallised at some depth, from which it took hundreds of millions of years for them to be exhumed and incorporated into much younger sediments.

Keywords: early Archaean; Hadean; basaltic resurfacing; zircon dating

Small inflation sources producing seismic and infrasonic pulses during the 2000 eruptions of Miyake-jima, Japan by Tomokazu Kobayashi; Yoshiaki Ida; Takao Ohminato (pp. 291-301).

During the 2000 activity of Miyake-jima volcano, Japan, we detected long period seismic signals with initial pulse widths of 1–2 s, accompanied by infrasonic pulses with almost the same pulse widths. The seismic signals were observed from 13 July 2000, a day before the second summit eruption. The occurrences of the seismic signals were intermittent with a gradual increase in their magnitudes and numbers building toward a significant explosive eruption on 18 August. After the eruption, the seismic and infrasonic events ceased. The results of a waveform inversion show that the initial motions were excited by an isotropic inflation source beneath the south edge of the caldera at a depth of 1.4 km. On the other hand, the sources of the infrasonic pulses were located in the summit caldera area. The times at which the infrasonic pulses were emitted at the surface were delayed by about 3 s from the origin times of the seismic events. It is suggested that small isotropic inflations excited seismic waves in the crust and simultaneously caused acoustic waves that traveled in the conduit and produced infrasonic pulses at the crater bottom. Considering the observed time differences and gas temperatures emitted from the vent, the conduit should have been filled with vapor mixed with SO₂ gas and volcanic ash. The change of the time differences between the seismic and infrasonic signals suggests that the seismic source became shallower within half a day before the August 18 explosive eruption. We interpret the source process as a fragmentation process of magma in which gas bubbles burst and quickly released part of the pressure that had been sustained by the tensional strength of magma.

Keywords: Miyake-jima; Long period seismic event; Infrasonic pulse; Fragmentation process

Are harmonic tremors self-excited thermoacoustic oscillations? by F.H. Busse; Peter A. Monkewitz; M. Hellweg (pp. 302-306).

The thermoacoustic instability is proposed as a possible cause of volcanic harmonic tremors. Properties of this instability are discussed in relationship to conditions in active volcanoes, and directions of possible future research are outlined.

Keywords: volcanic tremors; thermoacoustic instability; Sondhauss tube; thermoacoustic engine

Seasonal seismicity at western United States volcanic centers by L.B. Christiansen; S. Hurwitz; M.O. Saar; S.E. Ingebritsen; P.A. Hsieh (pp. 307-321).

We examine 20-yr data sets of seismic activity from 10 volcanic areas in the western United States for annual periodic signals (seasonality), focusing on large calderas (Long Valley caldera and Yellowstone) and stratovolcanoes (Cascade Range). We apply several statistical methods to test for seasonality in the seismic catalogs. In 4 of the 10 regions, statistically significant seasonal modulation of seismicity (>90% probability) occurs, such that there is an increase in the monthly seismicity during a given portion of the year. In five regions, seasonal seismicity is significant in the upper 3 km of the crust. Peak seismicity occurs in the summer and autumn in Mt. St. Helens, Hebgen Lake/Madison Valley, Yellowstone Lake, and Mammoth Mountain. In the eastern south moat of Long Valley caldera (LVC) peak seismicity occurs in the winter and spring. We quantify the possible external forcing mechanisms that could modulate seasonal seismicity. Both snow unloading and groundwater recharge can generate large stress changes of >5 kPa at seismogenic depths and may thus contribute to seasonality.

Keywords: seasonality; earthquake triggering; pore-fluid pressure; seismic activity; volcano

GIA-induced secular variations in the Earth's long wavelength gravity field: Influence of 3-D viscosity variations by Konstantin Latychev; Jerry X. Mitrovica; Mark E. Tamisiea; Jeroen Tromp; Christina C. Christara; Robert Moucha (pp. 322-327).

Predictions of present day secular variations in the Earth's long wavelength geopotential driven by glacial isostatic adjustment (GIA) have previously been analyzed to infer the radial profile of mantle viscosity and to constrain ongoing cryospheric mass balance. These predictions have been based on spherically symmetric Earth models. We explore the impact of lateral variations in mantle viscosity using a new finite-volume formulation for computing the response of 3-D Maxwell viscoelastic Earth models. The geometry of the viscosity field is constrained from seismic-to-mographic images of mantle structure, while the amplitude of the lateral viscosity variations is tuned by a free parameter in the modeling. We focus on the zonal J˙_ℓ harmonics for degrees ℓ=2,…,8 and demonstrate that large-scale lateral viscosity variations of two to three orders of magnitude have a modest, 5–10%, impact on predictions of J˙₂. In contrast, predictions of higher degree harmonics show a much greater sensitivity to lateral variation in viscosity structure. We conclude that future analyses of secular trends (for degree ℓ>2) estimated from ongoing (GRACE, CHAMP) satellite missions must incorporate GIA predictions based on 3-D viscoelastic Earth models.

Keywords: geopotential harmonics; glacial isostatic adjustment; 3-D structure

Modeling hydrothermal fluid circulation and gravity signals at the Phlegraean Fields caldera by Micol Todesco; Giovanna Berrino (pp. 328-338).

The Phlegraean Fields caldera is an active volcanic system where episodes of ground deformation are accompanied by significant changes in geochemical and geophysical parameters monitored at the surface. These changes derive from a complex interaction between magmatic system and hydrothermal fluid circulation. We calculate the gravity changes associated with the variable density of hydrothermal fluids. We simulate the multi-phase and multi-component fluid circulation triggered by a pulsating magma degassing, periodically increasing the discharge of CO₂-enriched fluids into the shallow hydrothermal system. The simulated evolution of the hydrothermal system successfully reproduces the observed composition of gas discharged at the surface. At the same time, results indicate that changes in average fluid density generate a detectable gravity signal that is of the same order of magnitude of the observed changes. This contribution to gravity changes can explain the peculiar behavior of gravity data collected at Solfatara, where surface hydrothermal phenomena are present. Simultaneous fitting of two independent sets of monitoring data (gas composition and gravity changes) confirms the conceptual model proposed for the hydrothermal system at Solfatara, and it provides new insights for the interpretation of gravity data.

Keywords: Phlegraean Fields; Gas composition; Gravity; Hydrothermal circulation; Monitoring; Models

Raman spectroscopic carbonaceous material thermometry of low-grade metamorphic rocks: Calibration and application to tectonic exhumation in Crete, Greece by Jeffrey M. Rahl; Kristin M. Anderson; Mark T. Brandon; Charalambos Fassoulas (pp. 339-354).

We present new Raman spectra data of carbonaceous material (CM) to extend the range of the Raman spectra of CM thermometer (RSCM) to temperatures as low as 100 °C. Previous work has demonstrated that Raman spectroscopy is an excellent tool to describe the degree of graphitization of CM, a process that is independent of pressure but strongly dependent on metamorphic temperature. A linear relationship between temperature and the Raman parameter R2 (derived from the area of the defect band relative to the ordered graphite band) forms the basis of a previous thermometer. Because R2 shows little variability in low-temperature samples, 330 °C serves as a lower limit on the existing thermometer. Herein, we present Raman spectra from a suite of low-temperature (100 to 300 °C) samples from the Olympics Mountains and describe other aspects of the Raman spectra of CM that vary over this range. In particular, the Raman parameter R1 (the ratio of heights of the disordered peak to ordered peak) varies regularly between 100 and 350 °C. These data, together with published results from higher-temperature rocks, are used to calibrate a modified RSCM thermometer, applicable from 100 to 700 °C. Application to low-grade metasediments in the Otago region in the South Island of New Zealand gives temperatures consistent with previous estimates, demonstrating the reliability of the modified RSCM thermometer.We apply the modified RSCM thermometer to 53 samples from Crete to evaluate the role of the Cretan detachment fault in exhuming Miocene high pressure/low-temperature metamorphic rocks exposed there. The metamorphic rocks below the detachment (the Plattenkalk and Phyllite–Quartzite units) give metamorphic temperatures that range from 250 to 400 °C, consistent with previous petrologic estimates. We also demonstrate that the Tripolitza unit, which lies directly above the detachment, gives an average metamorphic temperature of about 260 °C. The modest break in metamorphic temperature in central Crete indicates that the Cretan detachment accounts for only 5 to 7 km of exhumation of the underlying HP–LT metamorphic rocks, which were initially accreted at ∼35 km. We argue that the bulk of the exhumation (∼28 km out of 35 km total) occurred by pervasive brittle stretching and erosion of structural units above the detachment.

Keywords: Crete; graphitization; Raman spectroscopy; geothermometry

Trace element characteristics of the fluid liberated from amphibolite-facies slab: Inference from the metamorphic sole beneath the Oman ophiolite and implication for boninite genesis by Tsuyoshi Ishikawa; Shiori Fujisawa; Kazuya Nagaishi; Toshiaki Masuda (pp. 355-377).

Major and trace element compositions of amphibolites and quartzose rocks in the 230-m-thick metamorphic sole underlying the mantle section of the Oman ophiolite in Wadi Tayin area were determined to investigate the chemical characteristics of the hydrous fluid released from subducted amphiboltie-facies slab. The fluid-immobile element compositions indicate that protoliths of these rocks are mid-ocean ridge basalt-like tholeiite and deep-sea chert, which is consistent with the idea that these rocks represent Tethyan oceanic crust overridden during the early, intraoceanic thrusting stage of the Oman ophiolite emplacement. The rare-earth element (REE) and high field-strength element concentrations of the amphibolites show limited variations, within a factor of two except for a few evolved samples, throughout transect of the sole. On the other hand, concentrations of fluid-mobile elements, especially B, Rb, K and Ba, in amphibolites are highly elevated in upper 30 m of the sole (>600 °C in peak metamorphic temperature), suggesting the equilibration with evolved, B–Rb–K–Ba-rich fluids during prograde metamorphism. The comparison with amphibolites in the lower 150 m (500 to 550 °C) demonstrates that the trace element spectra of the fluids equilibrated with the high-level amphibolites may vary as a function of metamorphic temperature. The fluids are characterized by striking enrichments of B, Rb, K and Ba and moderate to minor enrichments of Sr, Li, Be and Pb. At higher temperature (up to 700 °C), the fluids become considerably enriched in light REE and Nb in addition to the above elements. The estimated trace element spectra of the fluids do not coincide with the compositions of basalts from matured intra-oceanic arcs, but satisfactorily explain the characteristics of the low-Pb andesites and boninites found in the Oman ophiolite. Compositional similarity between the boninites of Oman and other localities suggests that the fluids estimated here well represent the amphibolite-derived fluids involved in the magmatism of immatured, hot, shallow subduction zones.

Keywords: trace elements; fluids; amphibolites; boninites; subduction zones; Oman ophiolite

Metamorphic effect on zircon Lu–Hf and U–Pb isotope systems in ultrahigh-pressure eclogite-facies metagranite and metabasite by Yong-Fei Zheng; Yuan-Bao Wu; Zi-Fu Zhao; Shao-Bing Zhang; Ping Xu; Fu-Yuan Wu (pp. 378-400).

The Hf isotope composition of original igneous or detrital zircons in high-grade metamorphic rocks can be used to trace protolith origin, but metamorphic effect on the Hf isotope composition of newly grown domains remains to evaluate. We report a detailed in situ combined study of intragrain U–Pb and Lu–Hf isotopes in zircons from granitic gneiss and eclogite in the Dabie orogen of China that experienced ultrahigh-pressure eclogite-facies metamorphism. The results show correlations in²⁰⁶Pb/²³⁸U age, initial Hf isotope composition, and Th/U and Lu/Hf ratios between the domains of different origins. The metamorphic domains are characterized by low Th/U and Lu/Hf ratios but high ɛ_Hf( t) values relative to the igneous core and mantle of pre-metamorphic ages. Positive correlations are observed between Th/U and Lu/Hf ratios, pointing to the similar effect of metamorphism on both U–Th–Pb and Lu–Hf isotope systems. Thus the metamorphic domains are distinguished from the igneous core and mantle by their low Lu/Hf ratios that are less than 0.001 for the granitic gneiss and less than 0.0001 for the eclogite. Despite differences in both protolith age and geochemical source between granitic gneiss and eclogite, rim ɛ_Hf( t) values are variably 3.1 to 13.5 greater than core ɛ_Hf( t) values when calculated at timing of protolith formation. This indicates that the zircon overgrowth was associated with a metamorphic medium that has high¹⁷⁶Hf/¹⁷⁷Hf but low¹⁷⁶Lu/¹⁷⁷Hf ratios. While the metamorphic domains contain more radiogenic Hf isotopes than the original igneous core and mantle, their Lu/Hf ratios are significantly lower than those of core and mantle. Therefore, the metamorphic zircons acquired their initial Hf isotope ratios from metamorphic fluids that have high¹⁷⁶Hf/¹⁷⁷Hf ratios but low Lu/Hf ratios with sound variability depending on the Lu–Hf isotope compositions of pre-existing and co-precipitating phases.

Keywords: zircon; metamorphism; fluid; protolith; recrystallization; new growth

Inviscid behaviour of fines-rich pyroclastic flows inferred from experiments on gas–particle mixtures by O. Roche; M.A. Gilbertson; J.C. Phillips; R.S.J. Sparks (pp. 401-414).

Experiments were carried out on granular flows generated by instantaneous release of gas-fluidised, bidisperse mixtures and propagating into a horizontal channel. The mixture consists of fine (<100 μm) and coarse (>100 μm) particles of same density, with corresponding grain size ratios of ∼2 to 9. Initial fluidisation of the mixture destroys the interparticle frictional contacts, and the flow behaviour then depends on the initial bed packing and on the timescale required to re-establish strong frictional contacts. At a fines mass fraction ( α) below that of optimal packing (∼40%), the initial mixtures consist of a continuous network of coarse particles with fines in interstitial voids. Strong frictional contacts between the coarse particles are probably rapidly re-established and the flows steadily decelerate. Some internal friction reduction appears to occur as α and the grain size ratio increases, possibly due to particle rolling and the lower roughness of internal shear surfaces. Segregation only occurs at large grain size ratio due to dynamical sieving with fines concentrated at the flow base. In contrast, at α above that for optimal packing, the initial mixtures consist of coarse particles embedded in a matrix of fines. Flow velocities and run-outs are similar to that of the monodisperse fine end-member, thus showing that the coarse particles are transported passively within the matrix whatever their amount and grain size are. These flows propagate at constant height and velocity as inviscid fluid gravity currents, thus suggesting negligible interparticle friction. We have determined a Froude number of 2.61±0.08 consistent with the dam-break model for fluid flows, and with no significant variation as a function of α, the grain size ratio, and the initial bed expansion. Very little segregation occurs, which suggests low intensity particle interactions during flow propagation and that active fluidisation is not taking place. Strong frictional contacts are only re-established in the final stages of emplacement and stop the flow motion. We infer that fines-rich (i.e. matrix-supported) pyroclastic flows propagate as inviscid fluid gravity currents for most of their emplacement, and this is consistent with some field data.

Keywords: Volcanology; pyroclastic flows; analogue experiments; fluidisation; granular flows; fluid gravity currents

Rates of hydrothermal cooling of new oceanic upper crust derived from lithium-geospeedometry by Laurence A. Coogan; Simone A. Kasemann; Sumit Chakraborty (pp. 415-424).

Episodic emplacement and cooling of lavas and dikes at mid-ocean ridges leads to large fluctuations in hydrothermal fluxes and biological activity. However, the processes operating beneath the seafloor during these transient events such as permeability creation and dike cooling are poorly understood. We have developed a new approach to determine the cooling rate of the sheeted dike complex based on the extent of diffusion of lithium from plagioclase into clinopyroxene during cooling. We have calibrated this Li-geospeedometer using new high-temperature experiments to determine both the temperature dependence of the partitioning of Li between plagioclase and clinopyroxene and the diffusion coefficient for Li in clinopyroxene. Application of this method to lavas and dikes from ODP Hole 504B shows that cooling rates vary dramatically with depth in the upper oceanic crust. Extremely rapid cooling rates (>450 °C hr−¹) in the upper part of the sheeted dike complex are sufficient to power hydrothermal megaplume formation within the overlying water column.

Keywords: geospeedometry; hydrothermal megaplumes; ODP Hole 504B; lithium diffusion; sheeted dike complex

Did the Altyn Tagh fault extend beyond the Tibetan Plateau? by Brian J. Darby; Bradley D. Ritts; Yongjun Yue; Qingren Meng (pp. 425-435).

The pre-Miocene northeastern termination of Altyn Tagh fault is a critical outstanding problem for understanding the mechanics of Cenozoic deformation resultant from the Indo-Asian collision and mechanisms of Tibetan Plateau formation. Structures beyond the widely accepted NE end of the Altyn Tagh fault, near the town of Yumen, are needed in order to accommodate strike-slip deformation related to plate-like lateral extrusion tectonics, but structures with the necessary slip magnitudes and histories have not been identified. We report on a series of newly recognized and documented E to ENE-striking faults within the Alxa block, NE of the Tibetan Plateau, that are visible on remotely sensed images and confirmed by field studies. These structures are demonstrably left-lateral faults based on offset geology and kinematic indicators such as striae and s–c fabrics in fault gouge. The faults have post-Cretaceous offsets of at least tens to possibly >150 km, but limited post-Miocene displacement, constrained by offset sedimentary basins. These characteristics suggest that strike-slip faults of the Alxa region have a similar structural history as the central-eastern Altyn Tagh fault and can provide a mechanism for accommodating Oligocene–Early Miocene extrusion along the Altyn Tagh fault.

Keywords: Altyn Tagh fault; Tibet; China; Alxa; strike-slip; tectonics

Active intracontinental transpressional mountain building in the Mongolian Altai: Defining a new class of orogen by Dickson Cunningham (pp. 436-444).

Mountain ranges that are actively forming around the western and northern perimeter of the Indo-Eurasia collisional deformational field, such as the Mongolian Altai, comprise a unique class of intracontinental intraplate transpressional orogen with structural and basinal elements that are distinct from contractional and extensional orogens. Late Cenozoic uplift and mountain building in the Mongolian Altai is dominated by regional-scale dextral strike–slip faults that link with thrust and oblique–slip faults within a 300-km-wide deforming belt sandwiched between the more rigid Junggar Basin block and Hangay Precambrian craton. Dominant orogenic elements in the Mongolian Altai include double restraining bends, terminal restraining bends, partial restraining bends, single thrust ridges, thrust ridges linked by strike–slip faults, and triangular block uplifts in areas of conjugate strike–slip faults. The overall pattern is similar to a regional strike–slip duplex array; however, the significant amount of contractional and oblique–slip displacement within the range and large number of historical oblique–slip seismic events renders the term “transpressional duplex�? more accurate. Intramontane and range flanking basins can be classified as ramp basins, half-ramp basins, open-sided thrust basins, pull-apart basins, and strike–slip basins. Neither a classic fold-and-thrust orogenic wedge geometry nor a bounding foredeep exists. The manner in which upper crustal transpressional deformation is balanced in the lower crust is unknown; however, crustal thickening by lower crustal inflation and northward outflow of lower crustal material are consistent with existing geological and geodetic data and could account for late Cenozoic regional epeirogenic uplift in the Russian Altai and Sayan regions.

Keywords: intracontinental mountain building; intraplate processes; transpression; Altai; Central Asia; active tectonics

Alteration induced changes of magnetic fabric as exemplified by dykes of the Koolau volcanic range by David Kr�sa; Emilio Herrero-Bervera (pp. 445-453).

We studied 93 samples from 8 basaltic dykes of the Koolau volcanic range on the island of Oahu,Hawaii,USA,to determine the influence of hydrothermal alteration on the magnetic fabric as determined by anisotropy of magnetic susceptibility (AMS) measurements. Rock magnetic as well as microscopic investigations show that only ≈25% of the samples have retained their original magnetomineralogical composition of unaltered Ti-poor titanomagnetite. The remaining samples have undergone hydrothermal alteration which transformed the primary magnetic phase into a granular intergrowth of titanomagnetite, titanomaghemite and hematite. In both sample groups,this magnetic phase occurs in coarse (tens of microns),irregularly shaped particles as well as interstitial clusters of smaller (<5 μm) grains. Our investigations show that hydrothermal alteration does change the bulk susceptibility and the degree of anisotropy but not the directions of principal axes of the AMS ellipsoid which are predominantly corresponding to normal magnetic fabric. The stability of AMS directions, regardless of the degree of alteration, points towards the model of distribution anisotropy as the controlling factor for the observed magnetic fabric.

Keywords: petrofabrics; magnetic fabrics; rock magnetism; magnetic mineralogy; hydrothermal alteration; dykes

Nd and Sr isotopic variations in Chinese eolian deposits during the past 8 Ma: Implications for provenance change by Jimin Sun (pp. 454-466).

A record of changes in Nd and Sr isotopic composition of the eolian deposits from the central Loess Plateau has been determined for the past 8 Ma. The isotopic records of the silicate fraction of the Quaternary and Tertiary eolian deposits allow interpreting the interplay between the Sr isotopic variations in the eolian deposits and the late Cenozoic tectonic and climatic changes. The results indicate that the temporal variations of Nd and Sr isotopes show remarkable changes around the beginning of the Quaternary. The lower values of the¹⁴³Nd/¹⁴⁴Nd and the decreasing trend of the⁸⁷Sr/⁸⁶Sr ratios after 2.58 Ma ago are attributed to the additions of relatively younger crust materials in response to the climatic cooling and the late Cenozoic uplift induced glacial grinding in the high orogenic belts in central Asia. In this context, the substantial changes in climate and tectonics have modified dust sources significantly, and the Quaternary loess forming processes are preferentially sampling relatively younger and high relief crust materials than that of the Tertiary Red Clay.

Keywords: Sm–Nd isotope; Rb–Sr isotope; Eolian deposits; Provenance; Loess Plateau

Refining the pre-industrial atmospheric Pb isotope evolution curve in Europe using an 8000 year old peat core from NW Spain by M.E. Kylander; D.J. Weiss; A. Mart�nez Cort�zas; B. Spiro; R. Garcia-Sanchez; B.J. Coles (pp. 467-485).

Pb pollution has existed for several millennia and remains relevant today. By using peat cores as environmental archives it is possible to reconstruct this long history on a regional scale. This is a significant contribution to the findings from ice core records, the only other archive recording purely atmospheric additions. Without information that allows linking and comparison between sites regionally, within Europe and elsewhere, our ability to make coherent global models of the natural Pb cycle, and anthropogenic forcing of this cycle, is limited. In this respect, the characteristics of the Pb pre-pollution aerosol (PPA) are important to define globally. We characterize for the first time a PPA in Southern Europe with [Pb]=0.78±0.86 μg g−¹, net Pb accumulation rates of 0.032±0.030 mg m−² y−¹ and a²⁰⁶Pb/²⁰⁷Pb signature of 1.25470±0.02575. This PPA Pb isotope signature is more radiogenic than that found thus far in Western and Northern Europe. Spain is a historically important mining site. Using three-isotope plots and a pool of potential Pb isotope signatures, a detailed source appointment of both natural and anthropogenic Pb sources was made. We found evidence of Saharan aridification and its termination ∼4400 BP and/or agricultural signals and strong local control (from rock and soil) of the Pb PPA. Human impact is first recorded at 3210 BP but does not exceed 50% of deposited Pb until 3005 BP. Mines in SE Spain dominate early Pb pollution history at this site. During the rise of Roman rule, contributions come from mines in N, NW and SW Spain with no strong indication of other European mining activities. In Medieval and Industrial times local contributions to the peat bog are reduced.

Keywords: Pb isotopes; pollution; ombrotrophic peat; Saharan aridification; Spain; climate; atmospheric deposition

Biomass effects on stalagmite growth and isotope ratios: A 20th century analogue from Wiltshire, England by J.U.L. Baldini; F. McDermott; A. Baker; L.M. Baldini; D.P. Mattey; L. Bruce Railsback (pp. 486-494).

Increases in calcite deposition rates combined with decreases in δ¹³C and δ¹⁸O in three modern stalagmites from Brown's Folly Mine, Wiltshire, England, are correlative with a well-documented re-vegetation above the mine. Increased soil PCO_₂ resulted in greater amounts of dissolved CaCO₃ in the drip waters, which consequently increased annual calcite deposition rates. The absence of deposition prior to 1916 (28 years after the mine was closed) indicates that vegetation had not yet sufficiently developed to allow higher PCO_₂ values to form in the soil. Lower δ¹³C values through time may reflect the increased input of isotopically light biogenic carbon to the total dissolved inorganic carbon (DIC). δ¹⁸O decreased synchronously with δ¹³C, reflecting the increased importance of isotopically light winter recharge due to greater biomass-induced summer evapotranspiration. This is the first empirical demonstration that vegetation density can control stalagmite growth rates, δ¹³C, and δ¹⁸O, contributing critical insights into the interpretation of these climate proxies in ancient stalagmites.

Keywords: stalagmite; carbon isotopes; oxygen isotopes; fractionation; vegetation; growth rate

The Upper Valanginian (Early Cretaceous) positive carbon–isotope event recorded in terrestrial plants by Darren R. Gr�cke; Gregory D. Price; Stuart A. Robinson; Evgenij Y. Baraboshkin; J�rg Mutterlose; Alastair H. Ruffell (pp. 495-509).

Our understanding of the ancient ocean-atmosphere system has focused on oceanic proxies. However, the study of terrestrial proxies is equally necessary to constrain our understanding of ancient climates and linkages between the terrestrial and oceanic carbon reservoirs. We have analyzed carbon–isotope ratios from fossil plant material through the Valanginian and Lower Hauterivian from a shallow-marine, ammonite-constrained succession in the Crimean Peninsula of the southern Ukraine in order to determine if the Upper Valanginian positive carbon–isotope excursion is expressed in the atmosphere. δ¹³C_plant values fluctuate around −23‰ to −22‰ for the Valanginian–Hauterivian, except during the Upper Valanginian where δ¹³C_plant values record a positive excursion to ∼−18‰. Based upon ammonite biostratigraphy from Crimea, and in conjunction with a composite Tethyan marine δ¹³C_carb curve, several conclusions can be drawn: (1) the δ¹³C_plant record indicates that the atmospheric carbon reservoir was affected; (2) the defined ammonite correlations between Europe and Crimea are synchronous; and (3) a change in photosynthetic carbon–isotope fractionation, caused by a decrease in atmospheric pCO₂, occurred during the Upper Valanginian positive δ¹³C excursion. Our new data, combined with other paleoenvironmental and paleoclimatic information, indicate that the Upper Valanginian was a cool period (icehouse) and highlights that the Cretaceous period was interrupted by periods of cooling and was not an equable climate as previously thought.

Keywords: carbon isotopes; plants; CO; ₂; icehouse; OAE; Valanginian; Early Cretaceous

A quantitative analysis of the desiccation and re-filling of the Mediterranean during the Messinian Salinity Crisis by P.Th. Meijer; W. Krijgsman (pp. 510-520).

Notwithstanding the great deal of attention that the Messinian evaporites of the Mediterranean region have received from an observational point of view, there is, to date, no consensus as to their mechanism of formation. We aim to contribute to the investigation through a quantitative analysis of the processes of desiccation and re-filling. These processes are thought to have played a role in particular during the deposition of the upper part of the evaporite sequence. We calculate the evolution of sea level and average salinity based on both the present-day geometry and a paleogeographic reconstruction and assess the sensitivity to variations in the freshwater budget. Our results support previous inferences that desiccation and re-filling are fast; desiccation occurs on a time scale of 3–8 kyr, re-filling probably even faster. Equilibrium sea levels imply that most water has gone from the western basin while a significant water column remains in the eastern basin. Whether or not the eastern basin reaches the level of halite saturation depends critically on, in particular, the freshwater budget. The fast rate of desiccation and re-filling imply that temporal differences in the onset of salt precipitation between western and eastern basin and between marginal basins and basin centres are below the resolution of (astronomical) dating. Also, when Atlantic sea level periodically varied from below to above the level of the intervening sill, the Mediterranean basin will have responded with repeated desiccation and re-filling. Fast re-filling is found to require only a small connection to the Atlantic Ocean. This, in combination with the previous results, suggests the Mediterranean is unlikely to attain stable intermediate water levels.

Keywords: Mediterranean Sea; Messinian; modelling; evaporite

Radiation-induced decomposition of U(VI) phases to nanocrystals of UO₂ by Satoshi Utsunomiya; Rodney C. Ewing; Lu-Min Wang (pp. 521-528).

U⁶⁺-phases are common alteration products, under oxidizing conditions, of uraninite and the UO₂ in spent nuclear fuel. These U⁶⁺-phases are subjected to a radiation field caused by the α-decay of U, or in the case of spent nuclear fuel, incorporated actinides, such as²³⁹Pu and²³⁷Np. In order to evaluate the effects of α-decay events on the stability of the U⁶⁺-phases, we report, for the first time, the results of ion beam irradiations (1.0 MeV Kr²⁺) of U⁶⁺-phases. The heavy-particle irradiations are used to simulate the ballistic interactions of the recoil-nucleus of an α-decay event with the surrounding structure. The Kr²⁺-irradiation decomposed the U⁶⁺-phases to UO₂ nanocrystals at doses as low as 0.006 displacements per atom (dpa). U⁶⁺-phases accumulate substantial radiation doses (∼1.0 displacement per atom) within 100,000 yr if the concentration of incorporated²³⁹Pu is as high as 1 wt.%. Similar nanocrystals of UO₂ were observed in samples from the natural fission reactors at Oklo, Gabon. Multiple cycles of radiation-induced decomposition to UO₂ followed by alteration to U⁶⁺-phases provide a mechanism for the remobilization of incorporated radionuclides.

Keywords: U(VI)-phase; ion irradiation; uraninite; nanocrystals; Oklo

Elasticity of (Mg, Fe)(Si, Al)O₃-perovskite at high pressure by Li Li; John P. Brodholt; Stephen Stackhouse; Donald J. Weidner; Maria Alfredsson; G. David Price (pp. 529-536).

The most abundant mineral on Earth has a perovskite crystal structure and a chemistry that is dominated by MgSiO₃ with the next most abundant cations probably being aluminum and ferric iron. The dearth of experimental elasticity data for this chemically complex mineral limits our ability to calculate model seismic velocities for the lower mantle. We have calculated the single crystal elastic moduli ( c_ij) for (Mg, Fe^3 +)(Si, Al)O₃ perovskite using density functional theory in order to investigate the effect of chemical variations and spin state transitions of the Fe³⁺ ions. Considering the favored coupled substitution of Mg²⁺–Si^4 + by Fe³⁺–Al³⁺, we find that the effect of ferric iron on seismic properties is comparable with the same amount of ferrous iron. Ferric iron lowers the elastic moduli relative to the Al charge-coupled substitution. Substitution of Fe³⁺ for Al³⁺, giving rise to an Fe/Mg ratio of 6%, causes 1.8% lower longitudinal velocity and 2.5% lower shear velocity at ambient pressure and 1.1% lower longitudinal velocity and 1.8% lower shear velocity at 142 GPa. The spin state of the iron for this composition has a relatively small effect (< 0.5% variation) on both bulk modulus and shear modulus.

Keywords: elasticity; perovskite; ferric iron; density functional theory

Erratum to “Room-temperature magnetic properties of ferrihydrite: A potential magnetic remanence carrier?�?[Earth Planet. Sci. Lett. 236 (2005) 856–870] by Johari Pannalal; Sean A. Crowe; Maria T. Cioppa; David T.A. Symons; Arne Sturm; David A. Fowle (pp. 537-537).

Sections

Personal tools

Earth and Planetary Science Letters (v.240, #2)

Sections

Personal tools

Local resources

Earth and Planetary Science Letters (v.240, #2)