Helium isotopes in ferromanganese crusts from the central Pacific Ocean

Helium isotopes have been measured in samples of two ferromanganese crusts (VA13/2 and CD29-2) from the central Pacific Ocean. With the exception of the deepest part of crust CD29-2 the data can be explained by a mixture of implanted solar- and galactic cosmic ray-produced (GCR) He, in extraterrestrial grains, and radiogenic He in wind-borne continental dust grains. ⁴He concentrations are invariant and require retention of less than 12% of the in situ He produced since crust formation. Loss has occurred by recoil and diffusion. High ⁴He in CD29-2 samples older than 42 Ma are correlated with phosphatization and can be explained by retention of up to 12% of the in situ-produced ⁴He. ³He/⁴He of VA13/2 samples varies from 18.5 to 1852 R_a due almost entirely to variation in the extraterrestrial He contribution. The highest ³He/⁴He is comparable to the highest values measured in interplanetary dust particles (IDPs) and micrometeorites (MMs). Helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in-falling terrestrial and extraterrestrial grains of Fe-Mn crusts. The extraterrestrial ³He concentration of the crusts rules out whole, undegassed 4-40 μm diameter IDPs as the host. Instead it requires that the extraterrestrial He inventory is carried by numerous particles with significantly lower He concentrations, and occasional high concentration GCR-He-bearing particles.     

A single asteroidal source for extraterrestrial Ordovician chromite grains from Sweden and China: High-precision oxygen three-isotope SIMS analysis

We examined oxygen three-isotope ratios of 48 extraterrestrial chromite (EC) grains extracted from mid-Ordovician sediments from two different locations in Sweden, and one location in south-central China. The ages of the sediments (∼470 Ma) coincide with the breakup event of the L chondrite parent asteroid. Elemental compositions of the chromite grains are generally consistent with their origin from L or LL chondrite parent bodies. The average Δ¹⁷O (‰-deviation from the terrestrial mass-fractionation line, measured in situ from 15 μm spots by secondary ion mass spectrometry; SIMS) of EC grains extracted from fossil meteorites from Thorsberg and Brunflo are 1.17 ± 0.09‰ (2σ) and 1.25 ± 0.16‰, respectively, and those of fossil micrometeorites from Thorsberg and Puxi River are 1.10 ± 0.09‰, and 1.11 ± 0.12‰, respectively. Within uncertainty these values are all the same and consistent with the L chondrite group average Δ¹⁷O = 1.07 ± 0.18‰, but also with the LL chondrite group average Δ¹⁷O = 1.26 ± 0.24‰ (Clayton et al., 1991). We conclude that the studied EC grains from correlated sediments from Sweden and China are related, and most likely originated in the same event, the L chondrite parent body breakup. We also analyzed chromites of modern H, L and LL chondrites and show that their Δ¹⁷O values coincide with averages of Δ¹⁷O of bulk analyses of H, L and LL chondrites. This study demonstrates that in situ oxygen isotope data measured by SIMS are accurate and precise if carefully standardized, and can be used to classify individual extraterrestrial chromite grains found in sediments.     

Extreme Enrichment of Tellurium in Deep-Sea Sediments

Tellurium is a sort of scattered rare element on the earth. Its concentration is very low in earth＇s crust, only 1.0 ng/g. However, it has extremely high abundance in Co-rich crusts, marine polymetallic nodules, deep-sea sediments and aerolites. To find out the origin of tellurium enrichment in deep-sea sediments, we analyzed and compared tellurium concentrations and helium isotope compositions in the magnetic parts and those in the bulk parts of deep-sea sediments. The result indicates that the helium content, 3He/4He ratio and tellurium concentration are obviously higher in the magnetic parts than those in the bulk parts. The 3He abundance varies synchronously with the tellurium concentration. 3He and Te have a distinct positive correlation with each other. It is the first time that the paper brings forward that the extreme enrichment of tellurium in deep-sea sediments, like helium isotope anomalies, probably results from the input of interplanetary dust particles （IDPs）. Similarly, the extreme enrichment of tellurium in marine polymetallic nodules and Co-rich crusts is possibly related to IDPs.     

Evaluation of osmium isotopes and iridium as paleoflux tracers in pelagic carbonates

New osmium (Os) isotope and platinum group element (PGE) concentration data are used in conjunction with published ³He and Th isotope data to determine the relative proportions of lithogenic, extraterrestrial and hydrogenous iridium (Ir) in a Pacific pelagic carbonate sequence from the Ocean Drilling Program (ODP) Site 806 on the Ontong Java Plateau (OJP). These calculations demonstrate that lithogenic and extraterrestrial contributions to sedimentary Ir budget are minor, while hydrogenous Ir accounts for roughly 85% of the total Ir. Application of analogous partitioning calculations to previously reported data from a North Pacific red clay sequence (LL44-GPC3) yields very similar results. Total Ir burial fluxes at Site 806 and LL44-GPC3 are also similar, 45 and 30 pg cm⁻² kyr⁻¹, respectively. Average Ir/³He and Ir/xs²³⁰Th_initial ratios calculated from the entire Site 806 data set are similar to those reported earlier for Pacific sites. In general, down-core profiles of Ir, ³He and xs²³⁰Th_initial, are not well correlated with one another. However, all three data sets show similar variance and yield sediment mass accumulation rate estimates that agree within a factor of two. While these results indicate that Ir concentration has potential as a point-paleoflux tracer in pelagic carbonates, Ir-based paleoflux estimates are likely subject to uncertainties that are similar to those associated with Co-based paleoflux estimates. Consequently, local calibration of Ir flux in space and time will be required to fully assess the potential of Ir as a point paleoflux tracer. Measured ¹⁸⁷Os/¹⁸⁸Os of the OJP sediments are systematically lower than the inferred ¹⁸⁷Os/¹⁸⁸Os of contemporaneous seawater and a clear glacial-interglacial ¹⁸⁷Os/¹⁸⁸Os variation is lacking. Mixing calculations suggest Os contributions from lithogenic sources are insufficient to explain the observed ¹⁸⁷Os/¹⁸⁸Os variations. The difference between the ¹⁸⁷Os/¹⁸⁸Os of bulk sediment and that of seawater is interpreted in terms of subtle contributions of unradiogenic Os carried by particulate extraterrestrial material. Down-core variations of ¹⁸⁷Os/¹⁸⁸Os with Pt/Ir and Os/Ir also point to contributions from extraterrestrial particles. Mixing calculations for each set of several triplicate analyses suggest that the unradiogenic Os end member cannot be characterized by primary extraterrestrial particles of chondritic composition. It is noteworthy that in efforts aimed at determining the effect of extraterrestrial contributions, ¹⁸⁷Os/¹⁸⁸Os of pelagic carbonates has greater potential compared to abundances of PGE. An attempt has been made for the first time to estimate sediment mass accumulation rates based on amount of extraterrestrial Os in the OJP samples and previously reported extraterrestrial Os flux. Throughout most of the OJP record, Os isotope-based paleoflux estimates are within a factor of two of those derived using other constant flux tracers. Meaningful flux estimates cannot be made during glacial maxima because the OJP sediments do not record the low ¹⁸⁷Os/¹⁸⁸Os reported previously. We speculate that this discrepancy may be related to focusing of extraterrestrial particles at the OJP, as has been suggested to explain down-core ³He variations.     

Noble gas isotope sites and mobility in mafic rocks and olivine

Important He and Ar isotope studies on rocks and minerals, relevant to the geochemical and degassing history of the Earth, are often hampered by insufficient knowledge of the retentivity of different types of sites in minerals (inclusions, matrix) for these species, and of the relative importance of radiogenic and trapped components and possible differences in their behavior.To identify sites of noble gas isotopes, shed some light on their origin and estimate their residence times in olivine, which is a mineral considered as a good natural sampler, we investigated 2.5 Ga old ultramafic rocks from the Monche Pluton (Kola Peninsula, north-east part of the Baltic shield) using several extraction methods: crushing, fusion, slow step-wise and rapid incremental heating. Previous studies indicated that these rocks contain mainly trapped noble gases; however, to constrain the possible contribution of in-situ generated radiogenic helium, U and Th concentrations were also measured in the samples.The helium release pattern obtained by relatively fast (∼1.5 h long) incremental heating of olivine includes three distinct release peaks for helium: a low-temperature (600 °C) l-peak, a middle (800-1100 °C) m-peak and a high-temperature (∼1400 °C) h-peak. However, helium extraction from a powdered aliquot of the same olivine yields mainly the middle m-peak indicating that gases released in the l- and h-peaks occupy gas-liquid inclusions opened in the course of crushing and grinding. Moreover, slow step-wise heating (14 h) also results in a broad He release peak but in two well-separated l- and h-peaks of non-atmospheric ⁴⁰Ar∗. This feature implies helium migration from l- and h-vesicles into the matrix m during long step-wise heating experiments, whereas less movable Ar remains in inclusions at even relatively high almost-magmatic temperatures.Using a simple phenomenological model envisaging the three different residence sites for noble gases, both fast- and slow-heating release patterns for ⁴⁰Ar∗ and He, including those for the crushed sample, could be reproduced. The diffusion parameters inferred from the modeling of olivine (D₀ = 2.4 × 10⁻² cm² s⁻¹ and Ea = 133 kJ mol⁻¹) are similar to those published by Shuster et al. (2003) and Blard et al. (2008). The high matrix/fluid solubility coefficient for helium, H_He ∼ 0.01, exceeds estimates reported by Trull and Kurz (1993); however, the product D_He(T) × H_He, the “permeability” (that governs He migration in vesicles + matrix composed materials), is very similar to their value. Extrapolation to the ambient temperature (0 °C) gives long and similar helium residence times in l- and h-vesicles, exceeding 10¹⁰ yrs, and even longer time scales ∼10¹⁶ yrs are obtained for the helium residence in the matrix. Therefore, at low temperatures our samples may be considered as excellent samplers of trapped volatile species, including helium.     

An extraterrestrial He-based timescale for the Paleocene-Eocene thermal maximum (PETM) from Walvis Ridge, IODP Site 1266

In the deep-sea, the Paleocene-Eocene Thermal Maximum (PETM) is often marked by clay-rich condensed intervals caused by dissolution of carbonate sediments, capped by a carbonate-rich interval. Constraining the duration of both the dissolution and subsequent cap-carbonate intervals is essential to computing marine carbon fluxes and thus testing hypotheses for the origin of this event. To this end, we provide new high-resolution helium isotope records spanning the Paleocene-Eocene boundary at ODP Site 1266 in the South Atlantic. The extraterrestrial ³He, ³He_ET, concentrations replicate trends observed at ODP Site 690 by Farley and Eltgroth (2003). By assuming a constant flux of ³He_ET we constrain relative changes in accumulation rates of sediment across the PETM and construct a new age model for the event. In this new chronology the zero carbonate layer represents 35 kyr, some of which reflects clay produced by dissolution of Paleocene (pre-PETM) sediments. Above this layer, carbonate concentrations increase for ∼165 kyr and remain higher than in the latest Paleocene until 234 ⁺⁴⁸/₋₃₄ kyr above the base of the clay. The new chronology indicates that minimum δ¹³C values persisted for a maximum of 134 ⁺²⁷/₋₁₉ kyr and the inflection point previously chosen to designate the end of the CIE recovery occurs at 217 ⁺⁴⁴/₋₃₁ kyr. This allocation of time differs from that of the cycle-based age model of Röhl et al. (2007) in that it assigns more time to the clay layer followed by a more gradual recovery of carbonate-rich sedimentation. The new model also suggests a longer sustained δ¹³C excursion followed by a more rapid recovery to pre-PETM δ¹³C values. These differences have important implications for constraining the source(s) of carbon and mechanisms for its subsequent sequestration, favoring models that include a sustained release of carbon after an initial pulse.     

Atmospheric helium isotope ratio: Possible temporal and spatial variations

The atmospheric ³He/⁴He ratio has been considered to be constant on a global scale, because the residence time of helium is significantly longer than the mixing time in the atmosphere. However, this ratio may be decreasing with time owing to the anthropogenic release of crustal helium from oil and natural gas wells, although this observation has been disputed. Here, we present the ³He/⁴He ratios of old air trapped in historical slags in Japan and of modern surface air samples collected at various sites around the world, measured with a newly developed analytical system. In air helium extracted from metallurgical slag found at refineries in operation between AD 1603 and 1907 in Japan, we determined a mean ³He/⁴He ratio of (5106 ± 108) × 10^-5 R_HESJ (where R_HESJ is the ³He/⁴He ratio of the Helium Standard of Japan), which is consistent with the previously reported value of (5077 ± 59) × 10^-5 R_HESJ for historical slags in France and United Arab Emirates and about 4% higher than that of average modern air, (4901 ± 4) × 10^-5 R_HESJ. This result implies that the air ³He/⁴He ratio has decreased with time as expected by anthropogenic causes. Our modern surface air samples revealed that the ³He/⁴He ratio increases from north to south at a rate of (0.16 ± 0.08) × 10^-5 R_HESJ/degree of latitude, suggesting that the low ³He/⁴He ratio originates in high-latitude regions of the northern hemisphere, which is consistent with the fact that most fossil fuel is extracted and consumed in the northern hemisphere.     

The contribution of sulphur dioxide from ablating micrometeorites to the atmospheres of Earth and Mars

Atmospheric composition is a key control on climate and the habitability of planetary surfaces. Ablation of infalling micrometeorites has been recognised as one way in which atmospheric chemistry can be changed, especially at times in solar system history when the infall rates of exogenous material were high. Despite its potential to influence climate and habitability, extraterrestrial sulphur dioxide is currently an unquantified contribution to the atmospheres of the terrestrial planets. We have used flash pyrolysis to simulate the atmospheric entry of micrometeorites and Fourier-transform infrared spectroscopy to identify and quantify the sulphur dioxide produced from the carbonaceous meteorites Orgueil (CI1), ALH 88045 (CM1), Cold Bokkeveld (CM2), Murchison (CM2) and Mokoia (CV3). We have used this approach to understand the introduction of sulphur dioxide to the atmospheres of Earth and Mars from infalling micrometeorites. Sulphates, present in carbonaceous chondrites at a few wt.%, are resistant to thermal decomposition, limiting the yields of sulphur dioxide from unmelted micrometeorites. Infalling micrometeorites are a minor source of present-day sulphur dioxide on Earth and Mars, calculated to be up to around 2400 tonnes and about 350 tonnes, respectively. During the Late Heavy Bombardment (LHB), the much greater infall rates of micrometeoritic dust are calculated to be associated with average production rates of sulphur dioxide of around 20 Mt yr⁻¹ for the early Earth and 0.5 Mt yr⁻¹ for early Mars, for a LHB of 100 Myr. These rates of delivery of sulphur dioxide at high altitudes would have reduced the solar energy reaching the surfaces of these planets, via scattering of sunlight by stratospheric sulphate aerosols, and may have had detrimental effects on developing biospheres by promoting cooler climates and reducing the probability of liquid water on planetary surfaces.     

Constraints on the loss of matrix-sited helium during vacuum crushing of mafic phenocrysts

Vacuum crushing is an efficient technique to selectively release the mantle-derived helium component trapped within olivine and pyroxene phenocrysts. However, contrary to previous assumptions, recent studies have shown that this method may liberate significant matrix-sited cosmogenic ³He (³He_c) or radiogenic ⁴He (⁴He^∗). Because this loss may bias both the determination of magmatic ³He/⁴He ratios and the accuracy of ³He_c measurements, it is essential to understand what mechanism is responsible and under what conditions matrix helium loss is manifest. To address this question, olivines and pyroxenes with various amounts of matrix-sited ³He (from 10⁷ to 10¹¹ at. g⁻¹) were crushed in air or in vacuum using several crushing devices. Sample temperature was controlled during each crushing experiment, and ranged from 25 to 325 °C. The resulting powders were then sieved to obtain several homogeneous grain fractions ranging between <10 and >300 μm. The ³He_c concentrations measured in each fraction clearly show that significant ³He_c loss (>20%) affects only the finest fraction (<10 μm) and, importantly, only under hot conditions (here T ?300 °C). Even the smallest fractions (<10 μm) quantitatively retain matrix-sited ³He_c when crushed under cold conditions (T ?25 °C), regardless of the duration and energy of crushing. These results invalidate the model previously proposed by (Yokochi R., Marty B., Pik R. and Burnard P. (2005) High ³He/⁴He ratios in peridotite xenoliths from SW Japan revisited: evidence for cosmogenic ³He released by vacuum crushing. Geochem. Geophys. Geosyst.6, doi:10.1029/2004GC000836) that involved spallation tracks and implied that the magnitude of loss was mainly controlled by the grain size. Moreover, new diffusion experiments were carried out to constrain the diffusivity of matrix-sited helium in crushed olivines. When used to model diffusive ³He_c loss as a function of grain size during crushing, these new data predict the observed release fairly well. Therefore, we conclude that temperature-enhanced volume diffusion is one of the main mechanisms controlling the release of ³He_c during crushing. For future applications, special attention should thus be paid to control the grain size, the crushing duration, and the temperature of the sample.     

事件沉积及其周期性的氦同位素指示

物理、化学或生物事件所造成的事件沉积一般都会在沉积层中留下痕迹.事件沉积的突发性使得不同物源的物质比如稀有气体He在尚未达到平衡时就被保存在沉积地层中.海底沉积物中地球4He的丰度受古大陆尘所支配,海底沉积物中的He主要源于3He/4He比率不足0.1Ra的大陆物质(Ra = 1.40×10-6,是空气的3He/4He比率)和3He/4He比率大于100Ra的地球外星际尘粒.计算表明:亚洲风成尘粒总的质量累积速率(mass accumulation rate,MAR)与4He通量之间有一个显著的相关性.由氧同位素变化所记录的冰期-间冰期循环的100 kyr周期的起始点与地壳4He通量中100 kyr周期的起始点清晰地匹配.在大洋钻探计划中,用海底沉积物中地球外3He的通量来考查星际尘粒的全球生长速率发现:3He埋藏通量也与100 kyr冰期-间冰期循环周期相关.对白垩纪-第三纪界面(KTB)粘土中较高的地球外3He( [3He]Et)计算,发现该界面事件的短期性.来自中国梅山和日本西南部Sasayama的二叠纪-三叠纪界面(PTB)富勒烯的He同位素限定了这种富勒烯形成于地球外环境.在整个宇宙体系中,各物源之间的He及其同位素组成存在着很大差异,这为解析沉积事件提供了新的高灵敏度的手段.     

Oxygen isotope measurements of individual unmelted Antarctic micrometeorites

We present oxygen isotope measurements of 28 unmelted Antarctic micrometeorites measuring 150-250 μm (long axis) collected in the South Pole water well. The micrometeorites were all unmelted and classified as either fine-grained, scoriaceous, coarse-grained or composite (a mix of two other classes). Spot analyses were made of each micrometeorite type using an ion microprobe. The oxygen isotope values were measured relative to standard mean ocean water (SMOW) and range from δ¹⁸O = 3‰ to 60‰ and δ¹⁷O = −1‰ to 32‰, falling along the terrestrial fractionation line (TFL) within 2σ errors. Several analytical spots (comprising multiple phases) were made on each particle. Variability in the oxygen isotope ratios was observed among micrometeorite types, between micrometeorites of the same type and between analytical spots on a single micrometeorite indicating that micrometeorites are isotopically heterogeneous. In general, the lowest isotope values are associated with the coarse-grained micrometeorites whereas most of the fine-grained and scoriaceous micrometeorites have an average δ¹⁸O ? 22‰, suggesting that the matrix in micrometeorites is isotopically heavier than the anhydrous silicate phases. The oxygen isotope values for the coarse-grained micrometeorites, composed mainly of anhydrous phases, do not lie along the carbonaceous chondrite anhydrous mineral (CCAM) line, as observed for olivines, pyroxenes and some kinds of chondrules in carbonaceous chondrites, suggesting that coarse-grained MMs are not related to chondrules, as previously thought. Our measurements span the same range as values found for melted micrometeorites in other studies. Although four of the micrometeorites have oxygen isotope values lying along the TFL, close to the region where the bulk CI carbonaceous chondrites are found, 21 particles have very enriched ¹⁷O and ¹⁸O values that have not been reported in previous analyses of chondrite matrix material, suggesting that they could be a new type of Solar System object. The parent bodies of the micrometeorites with higher ¹⁸O values may be thermal metamorphosed carbonaceous asteroids that have not been found as meteorites either because they are friable asteroids that produce small particles rather than rocks upon collision with other bodies, or because the rocks they produce are too friable to survive atmospheric entry.     

Factors controlling compositions of cosmic spinels: application to atmospheric entry conditions of meteoritic materials

During their deceleration through the Earth's atmosphere, meteoritic materials, i.e., interplanetary dust particles, micrometeorites and meteorites, experience thermal shocks which may alter their pristine mineralogy, texture or chemical characteristics. Among these changes, one of the most ubiquitous is the formation of spinels resulting from partial melting and subsequent crystallization of the meteoritic material. These “cosmic spinels” differ from terrestrial spinels by their high Ni and Fe³⁺ contents and show large variations in composition. In order to better understand the factors controlling their chemistry, pulse-heating experiments simulating atmospheric entry of extraterrestrial objects were carried out using Orgueil samples as proxies of meteoritic material. Covering a large range of experimental conditions (temperature 500°C < T <1500°C, duration: 5 s < t < 120 s, and oxygen fugacity: −0.68 < log fO₂ < −8), this work shows (1) that the whole range of composition of cosmic spinels analyzed so far at the micrometer scale in fine-grained and scoriaceous micrometeorites, in cosmic spherules or in the fusion crust of several stony meteorites can be reproduced, and (2) that these compositional changes can be expressed as a function of temperature, time and oxygen fugacity.We also show that, due to their fast crystallization kinetics, cosmic spinels can record through their composition, i.e., Al₂O₃ contents and FeO/Fe₂O₃ ratio, the diverse conditions of the atmosphere crossed by the extraterrestrial object during its fall towards the Earth's surface. Chemistry of cosmic spinels is thus a powerful tool for constraining the entry conditions in the Earth's atmosphere of any extraterrestrial object, including altitude of deceleration, entry angle and incident velocity. These in turn, may provide valuable information on the origin of the extraterrestrial material.     

The influence of artificial radiation damage and thermal annealing on helium diffusion kinetics in apatite

Recent work [Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett.249(3-4), 148-161] revealing a correlation between radiogenic ⁴He concentration and He diffusivity in natural apatites suggests that helium migration is retarded by radiation-induced damage to the crystal structure. If so, the He diffusion kinetics of an apatite is an evolving function of time and the effective uranium concentration in a cooling sample, a fact which must be considered when interpreting apatite (U-Th)/He ages. Here we report the results of experiments designed to investigate and quantify this phenomenon by determining He diffusivities in apatites after systematically adding or removing radiation damage.Radiation damage was added to a suite of synthetic and natural apatites by exposure to between 1 and 100 h of neutron irradiation in a nuclear reactor. The samples were then irradiated with a 220 MeV proton beam and the resulting spallogenic ³He used as a diffusant in step-heating diffusion experiments. In every sample, irradiation increased the activation energy (E_a) and the frequency factor (D_o/a²) of diffusion and yielded a higher He closure temperature (T_c) than the starting material. For example, 100 h in the reactor caused the He closure temperature to increase by as much as 36 °C. For a given neutron fluence the magnitude of increase in closure temperature scales negatively with the initial closure temperature. This is consistent with a logarithmic response in which the neutron damage is additive to the initial damage present. In detail, the irradiations introduce correlated increases in E_a and ln(D_o/a²) that lie on the same array as found in natural apatites. This strongly suggests that neutron-induced damage mimics the damage produced by U and Th decay in natural apatites.To investigate the potential consequences of annealing of radiation damage, samples of Durango apatite were heated in vacuum to temperatures up to 550 °C for between 1 and 350 h. After this treatment the samples were step-heated using the remaining natural ⁴He as the diffusant. At temperatures above 290 °C a systematic change in T_c was observed, with values becoming lower with increasing temperature and time. For example, reduction of T_c from the starting value of 71 to ∼52 °C occurred in 1 h at 375 °C or 10 h at 330 °C. The observed variations in T_c are strongly correlated with the fission track length reduction predicted from the initial holding time and temperature. Furthermore, like the neutron irradiated apatites, these samples plot on the same E_a − ln(D_o/a²) array as natural samples, suggesting that damage annealing is simply undoing the consequences of damage accumulation in terms of He diffusivity.Taken together these data provide unequivocal evidence that at these levels, radiation damage acts to retard He diffusion in apatite, and that thermal annealing reverses the process. The data provide support for the previously described radiation damage trapping kinetic model of Shuster et al. (2006) and can be used to define a model which fully accommodates damage production and annealing.     

Subduction of solar-type noble gases from extraterrestrial dust: constraints from high-pressure low-temperature metamorphic deep-sea sediments

Solar-type helium (He) and neon (Ne) in the Earths mantle were suggested to be the result of solar-wind loaded extraterrestrial dust that accumulated in deep-sea sediments and was subducted into the Earths mantle. To obtain additional constraints on this hypothesis, we analysed He, Ne and argon (Ar) in high pressure–low temperature metamorphic rocks representing equivalents of former pelagic clays and cherts from Andros (Cyclades, Greece) and Laytonville (California, USA). While the metasediments contain significant amounts of ⁴He, ²¹Ne and ⁴⁰Ar due to U, Th and K decay, no solar-type primordial noble gases were observed. Most of these were obviously lost during metamorphism preceding 30 km subduction depth. We also analysed magnetic fines from two Pacific ODP drillcore samples, which contain solar-type He and Ne dominated by solar energetic particles (SEP). The existing noble gas isotope data of deep-sea floor magnetic fines and interplanetary dust particles demonstrate that a considerable fraction of the extraterrestrial dust reaching the Earth has lost solar wind (SW) ions implanted at low energies, leading to a preferential occurrence of deeply implanted SEP He and Ne, fractionated He/Ne ratios and measurable traces of spallogenic isotopes. This effect is most probably caused by larger particles, as these suffer more severe atmospheric entry heating and surface ablation. Only sufficiently fine-grained dust may retain the original unfractionated solar composition that is characteristic for the Earths mantle He and Ne. Hence, in addition to the problem of metamorphic loss of solar noble gases during subduction, the isotopic and elemental fractionation during atmospheric entry heating is a further restriction for possible subduction hypotheses.     

Investigation of an early Pleistocene marine osmium isotope record from the eastern equatorial Pacific

Osmium isotope composition (¹⁸⁷Os/¹⁸⁸Os) and concentrations of Os, Ir and Pt are reported for an early Pleistocene section from the ODP Site 849 in the eastern equatorial Pacific. Using the data obtained in this study, the contributions from detrital and extraterrestrial particulate matter to Os concentration and ¹⁸⁷Os/¹⁸⁸Os of sediment are estimated. Our calculations show that detrital contributions to sedimentary Os are too small (<2%) to significantly shift measured bulk sediment ¹⁸⁷Os/¹⁸⁸Os away from seawater values. A moderate but significant negative correlation between ¹⁸⁷Os/¹⁸⁸Os and ³He/¹⁸⁸Os indicate that the average particulate extraterrestrial Os flux to this site is 1.21 ± 0.47 pg cm⁻² kyr⁻¹, which constitutes ?3% of total Os burial flux. The estimates of detrital and extraterrestrial Os are used to calculate the seawater ¹⁸⁷Os/¹⁸⁸Os in the early Pleistocene. The most notable features of this early Pleistocene ¹⁸⁷Os/¹⁸⁸Os record are: (1) glacial-interglacial ¹⁸⁷Os/¹⁸⁸Os differences are insignificant within errors of estimates, (2) glacial ¹⁸⁷Os/¹⁸⁸Os values are higher compared to those reported for the late Pleistocene glacials. Comparison of ¹⁸⁷Os/¹⁸⁸Os values at Site 849 to the late Pleistocene records suggests that average seawater ¹⁸⁷Os/¹⁸⁸Os change has been modest (∼5%) since the early Pleistocene. Assuming that ¹⁸⁷Os/¹⁸⁸Os difference between the glacial periods of the late and the early Pleistocene results solely from temperature dependence of weathering rates, it has been calculated that average surface temperature during the late Pleistocene glacials was 0.8 ± 0.2 °C lower than glacials in the early Pleistocene. This inference is consistent with temperature estimates based on a recent study of pCO₂ reconstruction in the Pleistocene. This observation based on limited studies of marine ¹⁸⁷Os/¹⁸⁸Os records seems to suggest that temperature played an important role in influencing chemical weathering during the Pleistocene glacials. However, more studies are needed to confirm if this temperature-weathering feedback was operational throughout the Pleistocene. A significant down core Ir-³He co-variation coupled with similar burial fluxes of Ir at Site 849 and at LL44 GPC-3 in the north Pacific point to the utility of Ir concentration as a point paleoflux tracer. However, a twofold difference in Ir burial fluxes between the eastern and the western equatorial Pacific suggests that calibration in space and time is required to use Ir concentration as a robust indicator of paleoflux through time. Significant co-variation of concentrations of Os and total alkenone during the glacials coupled with lighter δ¹³C of benthic foraminifera indicates that productivity and carbon burial played a dominant control on scavenging of Os at Site 849. In a broader context, this data set encourages future investigation of response of PGE behavior to paleoceanographic processes.     

Apparent decoupling of the He and Ne isotope systematics of the Icelandic mantle: The role of He depletion, melt mixing, degassing fractionation and air interaction

We present new He-Ne data for geothermal fluids and He-Ne-Ar data for basalts from throughout the Icelandic neovolcanic zones and older parts of the Icelandic crust. Geothermal fluids, subglacial glasses, and mafic phenocrysts are characterized by a wide range in helium isotope ratios (³He/⁴He) encompassing typical MORB-like ratios through values as high as 36.8 R_A (where R_A = air ³He/⁴He). Although neon in geothermal fluids is dominated by an atmospheric component, samples from the northwest peninsula show a small excess of nucleogenic ²¹Ne, likely produced in-situ and released to circulating fluids. In contrast, geothermal fluids from the neovolcanic zones show evidence of a contribution of mantle-derived neon, as indicated by ²⁰Ne enrichments up to 3% compared to air. The neon isotope composition of subglacial glasses reveals that mantle neon is derived from both depleted MORB-mantle and a primordial, ‘solar’ mantle component. However, binary mixing between these two endmembers can account for the He-Ne isotope characteristics of the basalts only if the ³He/²²Ne ratio of the primordial mantle endmember is lower than in the MORB component. Indeed, the helium to neon elemental ratios (⁴He/²¹Ne∗ and ³He/²²Ne_s where ²¹Ne∗ = nucleogenic ²¹Ne and ²²Ne_s = ‘solar’-derived ²²Ne) of the majority of Icelandic subglacial glasses are lower than theoretical values for Earth’s mantle, as observed previously for other OIB samples. Helium may be depleted relative to neon in high-³He/⁴He ratio parental melts due to either more compatible behavior during low-degree partial melting or more extensive diffusive loss relative to the heavier noble gases. However, Icelandic glasses show higher ⁴He/⁴⁰Ar∗ (⁴⁰Ar∗ = radiogenic Ar) values for a given ⁴He/²¹Ne∗ value compared to the majority of other OIB samples: this observation is consistent with extensive open-system equilibrium degassing, likely promoted by lower confining pressures during subglacial eruptions of Icelandic lavas. Taken together, the He-Ne-Ar systematics of Icelandic subglacial glasses are imprinted with the overlapping effects of helium depletion in the high-³He/⁴He ratio parental melt, binary mixing of two distinct mantle components, degassing fractionation and interaction with atmospheric noble gases. However, it is still possible to discern differences in the noble gas characteristics of the Icelandic mantle source beneath the neovolcanic zones, with MORB-like He-Ne isotope features prevalent in the Northern Rift Zone and a sharp transition to more primitive ‘solar-like’ characteristics in central and southern Iceland.     

Chemical composition of magnetite in Martian meteorite ALH 84001: Revised appraisal from thermochemistry of phases in Fe-Mg-C-O

Martian meteorite Allan Hills (ALH) 84001 contains sub-micron magnetite grains, suggested to be of biogenic origin, in its globules of Fe-Mg carbonate mineral. There is disagreement on whether the low Mg content of the magnetite could only arise from biological metabolism ( [Treiman, 2003] and [Thomas-Keprta et al., 2009]). However, constraints on the magnetite’s biogenicity are far less certain than had been inferred. The thermochemical bases for the equilibrium calculations are reviewed in detail; there are inconsistencies and gaps in fundamental data for siderite, macromolecular carbons, and magnesioferrite. The calculations of Treiman (2003), assuming formation of magnetite via “siderite = magnetite + CO₂ + CO”, are incorrect because of a flaw in the computer code used. The corrected location of this equilibrium (Thomas-Keprta et al., 2009) is no longer crucial, because of recent finds that the magnetite grains are associated with macromolecular carbon; this implies that the dominant magnetite-forming reaction was “siderite = magnetite + CO₂ + C”. From the location of this equilibrium, using the corrected computer code and best available thermochemical data, the Mg-poor magnetite grains (and macromolecular carbon) in carbonates in ALH 84001 could have formed by decomposition of the carbonates at geologically reasonable pressures and temperatures. The low-Mg compositions of the magnetite grains remain consistent with an abiotic origin within the known geological history of ALH 84001.     

Sources of radiogenic helium in a clay till aquitard and its use to evaluate the timing of geologic events

Concentrations of dissolved gases (He, Ne, Ar and N₂) were measured in pore waters collected from a Pleistocene (0-80 m depth) and Cretaceous (80-156 m depth) aged, clay-rich, aquitard system using both a conventional copper-tube method and in situ diffusion samplers. Both methods yielded similar results. He data revealed a well-defined increase in concentration with depth, from 1.7 × 10⁻⁸ cm³ He STP g⁻¹ water near the water table (at 2.3 m below ground surface) to 6.9 × 10⁻⁶ cm³ He STP g⁻¹ water at ∼80 m below ground surface (B.G.) The concentration at depth greatly exceeded values for atmospheric solubility at in situ temperatures and could not be attributed to steady-state production from α-decay of U and Th within the aquitard system. Trends in He isotope ratios and concentrations suggest accumulation as a result of both release of residual ⁴He from within the Pleistocene clay-rich till (ca. 87%) and an ascending radiogenic helium flux from underlying Cretaceous-age sediments (ca. 13%). One-dimensional transport modeling of the He data suggests that between 15 ka and 25 ka were required to produce the observed profile. This time frame agrees with studies utilizing isotope tracers (i.e., D/H and ¹⁴C-DIC) to estimate the timing of deposition for the till at this site. Our results demonstrate the utility of using helium, a nonreactive tracer, for delineating hydrogeologic processes in slow permeability geologic strata, and also support the contention that thick clay-rich surficial aquitards can provide long-term containment of contaminants, including hazardous wastes.     

Evidence of microbially mediated arsenic mobilization from sediments of the Aquia aquifer, Maryland, USA

Sediments from the Aquia aquifer in coastal Maryland were collected as part of a larger study of As in the Aquia groundwater flow system where As concentration are reported to reach levels as high as 1072 nmol kg⁻¹, (i.e., ∼80 μg/L). To test whether As release is microbially mediated by reductive dissolution of Fe(III) oxides/oxyhydroxides within the aquifer sediments, the Aquia aquifer sediment samples were employed in a series of microcosm experiments. The microcosm experiments consisted of sterilized serum bottles prepared with aquifer sediments and sterilized (i.e., autoclaved), artificial groundwater using four experimental conditions and one control condition. The four experimental conditions included the following scenarios: (1) aerobic; (2) anaerobic; (3) anaerobic + acetate; and (4) anaerobic + acetate + AQDS (anthraquinone-2,6-disulfonic acid). AQDS acts as an electron shuttle. The control condition contained sterilized aquifer sediments kept under anaerobic conditions with an addition of AQDS. Over the course of the 27 day microcosm experiments, dissolved As in the unamended (aerobic and anaerobic) microcosms remained constant at around ∼28 nmol kg⁻¹ (2 μg/L). With the addition of acetate, the amount of As released to the solution approximately doubled reaching ∼51 nmol kg⁻¹ (3.8 μg/L). For microcosm experiments amended with acetate and AQDS, the dissolved As concentrations exceeded 75 nmol kg⁻¹ (5.6 μg/L). The As concentrations in the acetate and acetate + AQDS amended microcosms are of similar orders of magnitude to As concentrations in groundwaters from the aquifer sediment sampling site (127-170 nmol kg⁻¹). Arsenic concentrations in the sterilized control experiments were generally less than 15 nmol kg⁻¹ (1.1 μg/L), which is interpreted to be the amount of As released from Aquia aquifer sediments owing to abiotic, surface exchange processes. Iron concentrations released to solution in each of the microcosm experiments were higher and more variable than the As concentrations, but generally exhibited similar trends to the As concentrations. Specifically, the acetate and acetate + AQDS amended microcosm typically exhibited the highest Fe concentrations (up to 1725 and 6566 nmol kg⁻¹, respectively). The increase in both As and Fe in the artificial groundwater solutions in these amended microcosm experiments strongly suggests that microbes within the Aquia aquifer sediments mobilize As from the sediment substrate to the groundwaters via Fe(III) reduction.     

Apatite (U-Th)/He thermochronometry using a radiation damage accumulation and annealing model

Helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal, a quantity that varies over the lifetime of the apatite. Using recently published laboratory data we develop and investigate a new kinetic model, the radiation damage accumulation and annealing model (RDAAM), that adopts the effective fission-track density as a proxy for accumulated radiation damage. This proxy incorporates creation of crystal damage proportional to α-production from U and Th decay, and the elimination of that damage governed by the kinetics of fission-track annealing. The RDAAM is a version of the helium trapping model (HeTM; Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett.249, 148-161), calibrated by helium diffusion data in natural and partially annealed apatites. The chief limitation of the HeTM, now addressed by RDAAM, is its use of He concentration as the radiation damage proxy for circumstances in which radiation damage and He are not accumulated and lost proportionately from the crystal.By incorporating the RDAAM into the HeFTy computer program, we explore its implications for apatite (U-Th)/He thermochronometry. We show how (U-Th)/He dates predicted from the model are sensitive to both effective U concentration (eU) and details of the temperature history. The RDAAM predicts an effective He closure temperature of 62 °C for a 28 ppm eU apatite of 60 μm radius that experienced a 10 °C/Ma monotonic cooling rate; this is 8 °C lower than the 70 °C effective closure temperature predicted using commonly assumed Durango diffusion kinetics. Use of the RDAAM is most important for accurate interpretation of (U-Th)/He data for apatite suites that experienced moderate to slow monotonic cooling (1-0.1 °C/Ma), prolonged residence in the helium partial retention zone, or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss. Under common circumstances the RDAAM predicts (U-Th)/He dates that are older, sometimes much older, than corresponding fission-track dates. Nonlinear positive correlations between apatite (U-Th)/He date and eU in apatites subjected to the same temperature history are a diagnostic signature of the RDAAM for many but not all thermal histories.Observed date-eU correlations in four different localities can be explained with the RDAAM using geologically reasonable thermal histories consistent with independent fission-track datasets. The existence of date-eU correlations not only supports a radiation damage based kinetic model, but can significantly limit the range of acceptable time-temperature paths that account for the data. In contrast, these datasets are inexplicable using the Durango diffusion model. The RDAAM helps reconcile enigmatic data in which apatite (U-Th)/He dates are older than expected using the Durango model when compared with thermal histories based on apatite fission-track data or other geological constraints. It also has the potential to explain at least some cases in which (U-Th)/He dates are actually older than the corresponding fission-track dates.