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Microbial methane oxidation - or methanotrophy - is a key control of the global methane budget on Earth, and perhaps in other planetary systems. Here, we explore the potential role of mass-18 isotopologues of methane, expressed as Δ13CH 3 D and Δ12CH 2 D 2 values, in tracking both aerobic and anaerobic methanotrophy in nature. We examine two well documented methanotrophic environments: the Lake Pavin (France) water column, where methane degradation is dominated by aerobic methanotrophy (AeOM), and the Black Sea sediments (offshore Romania), dominated by anaerobic methanotrophy (AOM) coupled to sulfate-reduction. In both settings, lighter isotopologues are preferentially consumed, generating elevated 13CH 4 /12CH 4 , 12CH 3 D/12CH 4 , 13CH 3 D/12CH 4 and 12CH 2 D 2 /12CH 4 ratios. This results in increasing of δ13C and δD values in the residual methane for both settings, as observed commonly in systems dominated by methanotrophy. As a result, AeOM and AOM cannot be easily distinguished by the development of δ13C and δD. In contrast, the Δ13CH 3 D and Δ12CH 2 D 2 (departure from stochastic) values have opposite trajectories, with minimal decreases in the case of the AeOM-dominated system, but dramatic increases in the case of AOM, with Δ13CH 3 D and Δ12CH 2 D 2 reaching values as high as 15.7 ‰ and 76.6 ‰, respectively. This contrasting behavior of clumped isotopologues signatures illustrates fundamental distinction between the two processes and the way they segregate methane isotopologues. These data demonstrate that both AeOM and AOM have distinctive kinetic isotope effects in natural settings, consistent with preliminary laboratory work. In particular, we find that γ-values (which measure the deviation to the product of 'normal' bulk isotope fractionation factors) are close to unity in the case of AeOM (i.e. a negligible clumped isotope effect), but significantly below unity in the case of AOM (i.e. strong clumped isotope effect). In addition, our data also illustrate how AOM under low-sulfate conditions may promote methane isotopologue equilibration. Taken together, we suggest these data and apparent isotopologue fractionation factors extrapolated from these two environments may help refine the potential bio-signatures of methane affected by methanotrophy. [ABSTRACT FROM AUTHOR]

This study applies a combined isotope and doubly-substituted isotopologue ('clumped') methane approach to samples collected over a 9-year long-term experiment at the Kidd Creek scientific observatory located 2.4 and 2.9 km depth below surface, combined with previously published data from 2.1 km below surface. The observatory is located in a fractured rock system within Kidd Creek Mine in Timmins, Ontario, Canada, situated within a 2.7 Ga Volcanogenic Massive Sulphide (VMS) deposit on the Canadian Shield. Isotope and isotopologue methane data suggest a temporal variation in the various sources of methane within the fracture fluids system between 2.1 and 2.9 km below surface. Predominantly abiogenic methane is identified in samples collected from the deepest level of the mine (2.9 km). Comparing new data from the 2.4 km level with previous data from 2.1 km suggests addition of a small component of microbially-generated methane to the fracture water systems at 2.4 km. The temporal evolution of the methane isotopologue signatures suggest an additional process is occurring within these waters. Specifically, methane in samples from 2.4 km (and some from 2.9 km) approach low-temperature thermodynamic equilibrium in clumped isotopologue space, which is not consistent with kinetically-controlled methane production (either microbial or abiogenic). Anaerobic Oxidation of Methane (AOM) during microbial methanotrophy is shown to be the most likely process to drive such re-equilibration via isotopic bond re-ordering. This study provides an unprecedented high-resolution temporal record over more than a decade for methane in a deep subsurface crystalline environment and demonstrates the advantages of clumped isotopologue studies to identify multiple processes controlling the methane cycle in these systems including both abiotic and biotic methane production and methanotrophy. [ABSTRACT FROM AUTHOR]

Abstract Stable isotope compositions of methane (δ13C and δD) and of short-chain alkanes are commonly used to trace the origin and fate of carbon in the continental crust. In continental sedimentary systems, methane is typically produced through thermogenic cracking of organic matter and/or through microbial methanogenesis. However, secondary processes such as mixing, migration or biodegradation can alter the original isotopic and composition of the gas, making the identification and the quantification of primary sources challenging. The recently resolved methane 'clumped' isotopologues Δ13CH 3 D and Δ12CH 2 D 2 are unique indicators of whether methane is at thermodynamic isotopic equilibrium or not, thereby providing insights into formation temperatures and/or into kinetic processes controlling methane generation processes, including microbial methanogenesis. In this study, we report the first systematic use of methane Δ13CH 3 D and Δ12CH 2 D 2 in the context of continental sedimentary basins. We investigated sedimentary formations from the Southwest Ontario and Michigan Basins, where the presence of both microbial and thermogenic methane was previously proposed. Methane from the Silurian strata coexist with highly saline brines, and clumped isotopologues exhibit large offsets from thermodynamic equilibrium, with Δ12CH 2 D 2 values as low as −23‰. Together with conventional δ13C and δD values, the variability in Δ13CH 3 D and Δ12CH 2 D 2 to first order reflects a mixing relationship between near-equilibrated thermogenic methane similar to gases from deeper Cambrian and Middle Ordovician units, and a source characterized by a substantial departure from equilibrium that could be associated with microbial methanogenesis. In contrast, methane from the Devonian-age Antrim Shale, associated with less saline porewaters, reveals Δ13CH 3 D and Δ12CH 2 D 2 values that are approaching low temperature thermodynamic equilibrium. While microbial methanogenesis remains an important contributor to the methane budget in the Antrim Shale, it is suggested that Anaerobic Oxidation of Methane (AOM) could contribute to reprocessing methane isotopologues, yielding Δ13CH 3 D and Δ12CH 2 D 2 signatures approaching thermodynamic equilibrium. [ABSTRACT FROM AUTHOR]

• Isotope fractionation of melts is controlled by terminal gas velocities. • Theoretical analysis allows for extrapolation to a variety of astrophysical environments. • Isotope fractionation of pebbles feeding planet growth may be controlled by terminal velocity. • Chemical and isotopic compositions of planets may depend on pebble terminal velocities. We use new experiments and a theoretical analysis of the results to show that the isotopic fractionation associated with laser-heating aerodynamic levitation experiments is consistent with the velocity of flowing gas as the primary control on the fractionation. The new Fe and Mg isotope data are well explained where the gas is treated as a low-viscosity fluid that flows around the molten spheres with high Reynolds numbers and minimal drag. A relationship between the ratio of headwind velocity to thermal velocity and saturation is obtained on the basis of this analysis. The recognition that it is the ratio of flow velocity to thermal velocity that controls fractionation allows for extrapolation to other environments in which molten rock encounters gas with appreciable headwinds. In this way, in some circumstances, the degree of isotope fractionation attending evaporation is as much a velocimeter as it is a barometer. [ABSTRACT FROM AUTHOR]

We report a temperature calibration for Δ 47 from bivalve carbonate that lies within error of theoretical predictions (Schauble et al., 2006; Guo et al., 2009). The temperature sensitivity of this calibration is lower than several different earlier calibrations determined using either inorganic calcite, corals, foraminiferans and coccoliths, or brachiopods and bivalved molluscs, but it agrees with more recent Δ 47 -temperature relationships determined from measurements of clumped-isotopes in mollusc and brachiopod shells. We demonstrate that mollusc shell temperature calibrations originating from different laboratories that have not been corrected for instrument backgrounds may differ by as much as ∼0.07‰ in Δ 47 over the ∼0 to 30 °C temperature range even where dΔ 47 /d T agree. Because recent calibrations for Δ 47 vs. T from several different laboratories agree for bivalved mollusc shells, yet differ from an early calibration for bivalved molluscs, we suggest it is unlikely that temperature-Δ 47 variability is attributable to phylum-specific vital effects, and instead conclude that differences in calibration slope between phyla and/or inorganic calcite are more easily explained by variability in measurements made in different laboratories. Discrepancies in both calibration slopes and/or intercepts indicate that Δ 47 values measured in natural materials may be more significantly influenced by instrument-specific effects, as well as effects from sample preparation and handling and purification of CO 2 than current techniques are able to correct for, and therefore, temperatures obtained by comparing measurements of Δ 47 to independently determined calcite calibrations may err by a far greater amount than acknowledged in previous studies. [ABSTRACT FROM AUTHOR]

Abstract: We modeled the Δ17O of atmospheric O2 using 27 ordinary differential equations comprising a box model composed of the stratosphere, troposphere, geosphere, hydrosphere and biosphere. Results show that 57% of the deficit in 17O in O2 relative to a reference water fractionation line is the result of kinetic isotope fractionation attending the Dole effect, 33% balances the positive Δ17O of O(1D) in the stratosphere, and 10% is from evapotranspiration. The predicted Δ′17O O2 relative to waters is −0.410‰ as measured at the δ18O of air. The value for Δ′17O O2 varies at fixed δ18O with the concentration of atmospheric CO2, gross primary production, and net primary production as well as with reaction rates in the stratosphere. Our model prediction is consistent with our measurements of the oxygen isotopic composition of air O2 compared with rocks if rocks define a fractionation line with an intercept in δ′17O=103ln(δ17O/103 +1) vs. δ′18O=103ln(δ18O/103 +1) space less than SMOW but more positive than some recent measurements imply. The predicted Δ17O is less negative than that obtained from recent measurements of O2 directly against SMOW. Underestimation of Δ′17O O2 can only be ameliorated if the integrated (bulk) Δ′17O for stratospheric CO2 is significantly greater than measurements currently allow. Our results underscore the need for high-precision comparisons of the 17O/16O and 18O/16O ratios of atmospheric O2, VSMOW, and rocks. [Copyright &y& Elsevier]

Magnesium isotopes are potentially powerful tracers for high-temperature geochemical processes if relevant fractionation factors are known. However, experimental data for Mg isotope fractionation are lacking at high temperatures. We performed experiments at 600, 700, and 800°C and 1GPa to establish the equilibrium magnesium isotope partitioning between forsterite (Mg2SiO4) and magnesite (MgCO3) and between spinel (MgAl2O4) and magnesite, making use of the carbonate as an isotope exchange medium to overcome sluggish diffusion-limited magnesium isotope exchange between spinel and forsterite. Using the three-isotope method, the magnitudes of exchange between forsterite and magnesite, and between spinel and magnesite, were determined at the three temperatures for varying lengths of time, allowing equilibrium isotope partitioning to be established. Results are as follows: =0.04±0.04‰ at 800°C, 0.11±0.10‰ at 700°C, and 0.44±0.10‰ at 600°C; and =0.90±0.28‰ at 800°C, 1.10±0.27‰ at 700°C, and 1.73±0.38‰ at 600°C. From these experimentally determined equilibrium fractionation values, we derive the temperature-dependent equilibrium fractionation between spinel and forsterite by difference, yielding values of 0.86±0.29‰ at 800°C, 0.99±0.29‰ at 700°C, and 1.29±0.39‰ at 600°C. These data agree well with first-principles estimates of equilibrium magnesium isotope fractionation between spinel and forsterite at high temperatures. The data allow the calculation of an experimentally determined equation for the temperature dependence of 26Mg/24Mg fractionation between spinel and olivine: . This first high-T experimental calibration of Mg isotope fractionation of mantle minerals is consistent with expectations based on the crystal chemical environment of Mg in these phases. [ABSTRACT FROM AUTHOR]

Abstract: The geochemical implications of mass-dependent fractionation of stable Ni isotopes in natural materials are difficult to assess because the inter-mineral equilibrium fractionation factors for Ni are unknown. We report experimentally determined equilibrium fractionation of Ni isotopes between Ni-metal and Ni-talc from 500°C to 950°C and 8 to 13 kbar using the three-isotope method. While significant isotope exchange was observed between metal and talc, little or no exchange occurred between Ni-metal and Ni-oxide under similar conditions. These results demonstrate the importance of chemical potential differences between the initial and final phase assemblages in facilitating isotope exchange in the experiments. Metal had greater 62Ni/58Ni than talc in all experiments and the equilibrium temperature dependence may be described by the relation (±2se). Kinetic fractionation resulting from diffusion appeared to perturb isotopic equilibration at long time scales and/or high temperatures. These experiments place constraints on Ni isotope variations observed in natural systems. For example, Ni isotope fractionation between alloy and chondrule in the Ausson meteorite is larger than predicted for an equilibrium process at the canonical metamorphic temperatures of L5 chondrites. At 2500K, the results imply a difference of 0.04‰ between equilibrated bulk silicate Earth and chondrites, indicating that Ni isotopes are not likely to be useful as tracers of planetary differentiation, unless fractionation occurred by a Rayleigh distillation process involving a well-mixed silicate reservoir. [Copyright &y& Elsevier]

Abstract: S. Simon et al. incorrectly suggest that in earlier work we claimed there was no Ti3+ in Wark-Lovering rim pyroxenes. In neither the paper by nor the subsequent paper by did we assert that there was no Ti3+ in rim pyroxenes. Rather, we found that many pyroxenes have Ti3+ below detection while others have lower Ti3+/Ti4+ than is typical of CAI interiors, indicating rim formation in a relatively oxidizing environment. showed through exhaustive testing that the suggestion by that EMPA data in the paper by were flawed is incorrect. Here we consider each point raised in the comment by S. Simon et al. and reiterate that our electron microprobe data and the XANES data of agree and demonstrate a statistically significant (∼2σ) or greater difference between rim and interior pyroxene Ti3+/Ti4+. We show that the oxidation states of Ti in Wark-Lovering rim pyroxenes, the chemistry of rim pyroxenes, and the modal abundances of rim minerals are best explained by reaction between the CAI and gas that was orders of magnitude more oxidizing than the solar-like gas from which the CAIs originally formed. [Copyright &y& Elsevier]

Abstract: The difference between the precise MC-ICPMS analyses of bulk calcium–aluminum-rich inclusion (CAI) fragments (e.g., Jacobsen et al., 2008) and supra-canonical values obtained by micro-analytical techniques, e.g., laser ablation MC-ICPMS (Young et al., 2005) and SIMS (Taylor et al., 2005), at face value seems to be problematic and therefore leads many to dismiss claims of the solar system (26Al/27Al)0 greater than ~5×10−5 as spurious. Here we use mass balance calculations to quantify the importance of open system isotopic exchange during CAI evolution and show that in situ supra-canonical, in-situ canonical, and bulk canonical measurements can all exist for an individual CAI. The calculations describe mechanisms of isotopic exchange that may have occurred early (100''ska) and late (~1.5Ma) in the solar nebula and much later (>10''sMa) on parent body planetesimals. A range of possible modal mineralogies is modeled in order to populate the compositional space defined by in situ and bulk CAI measurements. In support of these simulated data we describe in situ measurements of 27Al/24Mg, 25Mg/24Mg, and 26Mg/24Mg obtained by LA-MC-ICPMS comprising core-to-rim traverses across three CV3 CAIs. The CAIs exhibit distinctive Mg isotopic zoning profiles and varying abundances of daughter products of the short-lived nuclide 26Al that are consistent with varying amounts of open system isotope exchange. [Copyright &y& Elsevier]

Abstract: Variations in the stable isotope abundances of transition metals have been observed in the geologic record and trying to understand and reconstruct the physical/environmental conditions that produced these signatures is an area of active research. It is clear that changes in oxidation state lead to large fractionations of the stable isotopes of many transition metals such as iron, suggesting that transition metal stable isotope signatures could be used as a paleo-redox proxy. However, the factors contributing to these observed stable isotope variations are poorly understood. Here we investigate how the kinetics of iron redox electrochemistry generates isotope fractionation. Through a combination of electrodeposition experiments and modeling of electrochemical processes including mass-transport, we show that electron transfer reactions are the cause of a large isotope separation, while mass transport-limited supply of reactant to the electrode attenuates the observed isotopic fractionation. Furthermore, the stable isotope composition of electroplated transition metals can be tuned in the laboratory by controlling parameters such as solution chemistry, reaction overpotential, and solution convection. These methods are potentially useful for generating isotopically-marked metal surfaces for tracking and forensic purposes. In addition, our studies will help interpret stable isotope data in terms of identifying underlying electron transfer processes in laboratory and natural samples. [Copyright &y& Elsevier]

Abstract: The magnesium isotopic composition of Calcium-, Aluminium-rich Inclusions (CAIs) and chondrules from the CBb chondrites HH237 and QUE94411 was measured using MC-ICPMS coupled with a laser ablation system. CAIs from CBb chondrites exhibit limited mass-dependent fractionation (δ 25Mg′ (DSM3) <1.3‰) and formed with undetectable 26Al (26Al/27Al <4.6×10−6). Petrographic observations suggest that CBb CAIs are igneous. The magnesium isotopic composition of CBb igneous CAIs contrast with that of CV3 igneous CAIs which are usually mass fractionated and formed with an elevated initial abundance of 26Al. We contend that the absence of 26Al in CAIs is due either to a late formation in the case of a stellar origin of 26Al, or to a lack of exposure to impulsive flares in the case of an irradiation origin of 26Al. In both cases, it implies that a protoplanetary disk was present ∼4563 Ma ago, when CBb chondrites agglomerated. Chondrules have δ 25Mg′ (DSM3) varying from −0.80 to 0.95‰. A rough negative correlation is observed between the δ 25Mg′ of chondrules and their 24Mg/27Al ratio. This correlation is attributed to evaporation rather than mixing. Contrary to CAIs, chondrules from CBb chondrites have a magnesium isotopic composition similar to that of CV3 chondrules. This last result is surprising as CBb chondrules are significantly different from CV3 chondrules in mineralogy and chemistry. If chondrules from CBb chondrites formed in an impact-related vapour plume as proposed by Krot et al. [A.N. Krot, Y. Amelin, P. Cassen and A. Meibom, Young chondrules in CB chondrites formed by a giant impact in the early Solar System, Nature 436 (2005) 989–992], our data show that physical conditions in the vapour plume were similar to those of the solar accretion disk at the time and location of the formation of CV chondrules. We note that the oxygen isotopic composition of CAIs is incompatible with their remelting in the putative impact vapour plume. Alternatively, it is possible that CBb chondrules formed in a protoplanetary disk as the differences between these and “normal” CV3 chondrules can also be explained in term of spatial and temporal variations of the protoplanetary disk. We show that their young Pb–Pb age is not an argument in favour of an impact origin as protoplanetary disks can last as long as 10 Myr around protostars. If CBb chondrites formed in the solar accretion disk, we speculate they might be the last formed chondrite group. Such a hypothesis might shed light on the unique properties of CBb chondrites. [Copyright &y& Elsevier]

Abstract: Most rocky objects in the solar system, including the primitive chondrites and the terrestrial planets themselves, formed at oxygen fugacities (f O2) near that of the Iron–Wüstite (IW) f O2 buffer. Conversely, the most ancient rocky objects of the solar system, the calcium aluminum-rich inclusions (CAIs), formed at f O2 values 5 orders of magnitude lower than the IW buffer in an environment more closely resembling a solar gas. High-resolution Mg isotope data and estimates for f O2 for rims on CAIs show that this shift from ∼solar to protoplanetary (chondritic) f O2 occurred in 100,000 to 300,000 yr for these objects. Magnesium isotopes show further that the rise in f O2 was accompanied by a rise in the partial pressure of Mg. These results establish that CAIs entered a region resembling where planet progenitors formed within 3×105 yr of their formation in the solar nebula. [Copyright &y& Elsevier]

The mass-dependent fractionation laws that describe the partitioning of isotopes are different for kinetic and equilibrium reactions. These laws are characterized by the exponent relating the fractionation factors for two isotope ratios such that α2/1 = α3/1β. The exponent β for equilibrium exchange is (1/m1 − 1/m2)/(1/m1 − 1/m3), where mi are the atomic masses and m1 < m2 < m3. For kinetic fractionation, the masses used to evaluate β depend upon the isotopic species in motion. Reduced masses apply for breaking bonds whereas molecular or atomic masses apply for transport processes. In each case the functional form of the kinetic β is ln(M1/M2)/ln(M1/M3), where Mi are the reduced, molecular, or atomic masses. New high-precision Mg isotope ratio data confirm that the distinct equilibrium and kinetic fractionation laws can be resolved for changes in isotope ratios of only 3‰ per amu. The variability in mass-dependent fractionation laws is sufficient to explain the negative Δ17O of tropospheric O2 relative to rocks and differences in Δ17O between carbonate, hydroxyl, and anhydrous silicate in Martian meteorites. (For simplicity, we use integer amu values for masses when evaluating β throughout this paper.) [Copyright &y& Elsevier]

First results from a new UV laser ablation MC-ICPMS method for measuring Mg isotope ratios in situ in meteoritical materials show that there are mass-dependent variations in δ25Mg and δ26Mg up to 1.5 ‰ per amu in chondrules and 0.3‰ per amu in a CAI from the Allende meteorite. In both cases the mass-dependent fractionation is associated with alteration. Comparisons with laser ablation O isotope data indicate that incorporation of pre-existing grains of forsterite with distinct Mg and O isotopic compositions and post-formation alteration both contributed to the variability in Mg isotope ratios in the chondrules, resulting in a correlation between high δ25Mg and low Δ17O. The laser ablation analyses of the CAI show that high-precision determinations of both δ25Mg and δ26Mg can be used to discriminate features of the 26Al-26Mg isotope system that are relevant to chronology from those that result from element mobility. [Copyright &y& Elsevier]

Aerobic methane oxidation (AeOM) is an important biological sink of methane on Earth. Stable isotopes are critical tools in tracking the sources and sinks of Earth's surface methane budget. However, the major factors that influence the two multiply-substituted (clumped) isotope signatures of AeOM, Δ 13 CH 3 D and Δ 12 CH 2 D 2 , are not well known. Here we quantify the influence of kinetics as a function of temperature and different methane monooxygenase (MMO) enzymes (modulated by copper) on the isotopologue concentrations of residual methane by the obligate aerobic methanotroph, Methylococcus capsulatus (Bath). We observe deviations from traditional closed-system distillation (Rayleigh) fractionation during exponential growth at high oxidation rates. We model this as a reservoir effect controlled by the ratio of oxidation rate in the cells to transport rate of methane into the cells, where environmental temperature affects both rates. We also test whether clumped isotope fractionation values vary for the particulate versus soluble MMOs, but the results show minimal differences. We further determine that the back reaction (re-equilibration) of methane with medium water is unlikely. Together, the observations and model demonstrate that at low oxidation-to-transport ratios, the clumped isotope signatures follow canonical Rayleigh fractionation, whereas at high ratios, more positive Δ 12 CH 2 D 2 values result, deviating from simple Rayleigh-like trajectories. This study shows that the methane oxidation-to-transport ratio is a critical influence on clumped isotope signatures of AeOM that should be considered when interpreting the isotopic data of natural methane samples in both open and closed systems. [ABSTRACT FROM AUTHOR]

Microbial anaerobic oxidation of methane (AOM) substantially mitigates atmospheric methane emissions on Earth and is a process to consider for astrobiological targets where methane has been detected. The measurement of doubly substituted, or "clumped", methane isotopes has proven useful in tracing processes of methane formation and oxidation. Both near-equilibrium and extreme disequilibrium methane clumped isotope signatures can be attributed to AOM, but, to date, understanding the mechanistic and environmental controls on those signatures has been lacking. We report measurements of methane clumped isotope compositions of residual methane in AOM-active microbial incubations using sediment slurries from Svalbard and Santa Barbara Channel methane seeps. Incubation experiments of Svalbard sediment slurries resulted in residual methane with very high Δ13CH 3 D and Δ12CH 2 D 2 values up to 19.5‰ and 65.1‰, respectively. We found similarly high Δ13CH 3 D and Δ12CH 2 D 2 values in fluid samples from the Chamorro Seamount, a serpentinite mud volcano in the Mariana forearc, suggesting that minimal reversibility of AOM intracellular reactions leads to kinetic fractionation of clumped isotopologues. When conditions were consistent with a low thermodynamic drive for AOM, however, methane isotopologues approached intra-species quasi-equilibrium. This was clearly observed in isotope exchange experiments with methyl-coenzyme M reductase (Mcr) and in microbial incubations of the Santa Barbara Channel sediment slurries. Using an isotopologue fractionation model, we highlight the critical role of reversibility in controlling the trajectory of gases in Δ13CH 3 D vs. Δ12CH 2 D 2 space during AOM. The near-equilibrium methane isotopologue signatures are generalized as a result of the Mcr-catalyzed intracellular isotope exchange operating under near-threshold free energy conditions, as shown in the deep-biosphere incubations. Our results show that the reversibility of the Mcr-catalyzed reaction is central to understanding the meaning of methane isotopologue ratios affected by microbial production and oxidation. [ABSTRACT FROM AUTHOR]

Silicon stable isotopes are fractionated by a host of low-temperature aqueous processes, making them potentially useful as a weathering proxy. Here we characterize the silicon isotope signature of surficial chemical weathering of glassy basaltic lava flows at Kilauea Volcano, Hawaii. Fresh basalt flow surfaces (<40 years old) frequently feature opaque amorphous silica surface coatings up to 80 μm thick. These silica coatings and associated silica cements are enriched in the heavier isotopes of Si (δ 30 Si NBS-28 = +0.92‰ to +1.36‰) relative to their basaltic substrate (δ 30 Si NBS-28 = −0.3‰ to −0.2‰). Secondary clays and opals are typically depleted in 30 Si relative to the dissolved reservoirs from which they precipitated, so this sense of isotopic fractionation is unusual. Mechanisms capable of producing isotopically heavy secondary minerals were explored by conducting batch alteration experiments on fresh basaltic glass. Batch acidic alteration of basalt glass in HCl, H 2 SO 4 , and HF produced silica-rich surface layers resembling the Hawaiian surface coatings. Differences in fluid chemical composition affected the direction and magnitude of Si isotope fractionation. Basalt leaching in HCl or H 2 SO 4 produced 30 Si-enriched fluids (1000 ln α precip-Si(aq) ≅ −0.8‰) and 30 Si-depleted secondary silica layers. In contrast, HF-bearing experiments produced highly 30 Si-depleted fluid compositions (1000 ln α precip-Si(aq) up to +8‰). Larger isotopic fractionations were observed in experiments with lower fluid–rock ratios. In Hawaii, where altering fluids contain H 2 SO 4 and HCl but minimal HF, high δ 30 Si values for the silica coatings were likely achieved by Rayleigh fractionation. Aqueous 30 Si-enriched silica was released during incongruent basalt dissolution then subsequently transported and deposited from an evaporating solution at the flow surface. Our results indicate that (1) altering fluid chemistry and fluid–rock ratio impact the Si isotope signature of chemical weathering and (2) δ 30 Si of solids produced by low temperature aqueous alteration may diverge sharply from watershed- or landscape-scale weathering trends. [ABSTRACT FROM AUTHOR]

The origin of variations in iron isotope compositions of mantle minerals is uncertain, and predictions of equilibrium inter-mineral iron isotope fractionation conflict. This hinders interpretation of the petrologic and geochemical implications of Fe isotope data from mantle lithologies. To address this, we present a revised ionic model for predicting equilibrium iron isotope fractionation between mantle minerals and use it to interpret measured inter-mineral iron isotopic fractionation from five distinct mantle xenolith lithologies from San Carlos, Arizona. The samples represent a broad range of modal abundances and include lherzolite, harzburgite, dunite, clinopyroxenite, and websterite. The xenoliths exhibit Fe-isotopic variation between minerals in a single sample, and between samples. In all cases where spinel and olivine coexist, the 57 Fe/ 54 Fe of spinel is greater than that of the corresponding olivine, agreeing with expectations for equilibrium fractionation from theory (ionic model), but disagreeing with predictions based on Mössbauer data. The 57 Fe/ 54 Fe values of clinopyroxenes from the xenoliths show no clear systematic differences. We interpret this to be a result of varying degrees of metasomatism, perhaps involving interaction with a melt. The spinel peridotite samples (lherzolite, harzburgite, and dunite) are partially melted residual mantle that exhibit a decrease in whole-rock 57 Fe/ 54 Fe with increasing olivine abundance. This is consistent with progressive extraction of a 57 Fe-rich partial melt. The clinopyroxenite has the highest whole-rock 57 Fe/ 54 Fe, consistent with its origin as a cumulate from an unrelated magma possessing elevated 57 Fe/ 54 Fe. The websterite sample is transitional to Group II type xenoliths, has the lowest whole-rock 57 Fe/ 54 Fe of the investigated samples, and likely experienced a more complex metasomatic history. This study demonstrates that the Fe isotope compositions of San Carlos xenoliths and their component minerals record the complex petrologic history and local heterogeneity of the subcontinental mantle lithosphere. [ABSTRACT FROM AUTHOR]

Aerobic oxidation of methane (MOx) is an important biologically mediated process that consumes methane in a wide range of environments. Here we report results of culture experiments with the aerobic methane-oxidizing bacterium Methylosinus trichosporium (OB3b) that are used to characterize the mass-18 isotopologue (Δ 13 CH 3 D and Δ 12 CH 2 D 2) signatures of MOx in residual methane gases. MOx activity was confirmed by simultaneous decrease of methane and oxygen in the bulk gas headspace. Bulk carbon (13C/12C) and hydrogen (D/H) isotope ratios of the methane gas increased while both Δ 13 CH 3 D and Δ 12 CH 2 D 2 decreased as the oxidation proceeded. The corresponding fractionation factors (α) calculated from our experimental results are 0.98485 ± 0.00006 for 13C/12C, 0.7265 ± 0.0010 for D/H, 0.7141 ± 0.0011 for 13CH 3 D/12CH 4 , and 0.4757 ± 0.0023 for 12CH 2 D 2 /12CH 4. Deviations of the mass-18 fractionation factors from the Rule of the Geometric Mean (RGM) expressed as γ values are 0.9981 ± 0.0017 for 13CH 3 D/12CH 4 and 0.9013 ± 0.0045 for 12CH 2 D 2 /12CH 4. Our α and γ values suggest that while MOx fractionates 13CH 3 D within error of the RGM, the Δ 13 CH 3 D and Δ 12 CH 2 D 2 trajectories are very sensitive to even small deviations in 13CH 3 D/12CH 4 from the RGM. Fractionation of 12CH 2 D 2 deviates considerably from RGM, and this causes dramatic and robust effects on the trajectories of residual methane in Δ 13 CH 3 D vs. Δ 12 CH 2 D 2 space. Our models suggest that Δ 13 CH 3 D and Δ 12 CH 2 D 2 could potentially mimic microbial methanogenesis signatures in an environment that exhibits a strong Rayleigh Distillation process with little to no replenishment of methane during oxidation. However, in closed or open systems where oxidation is attended by simultaneous methane production, we find that modest increases in Δ 13 CH 3 D and dramatic increases in Δ 12 CH 2 D 2 are to be expected, thus resulting in isotopologue signatures distinct from microbial methanogenesis. The overall trend in these conditions suggest that methane altered by MOx is distinguishable from other methane sources in Δ 13 CH 3 D and Δ 12 CH 2 D 2 space. • Δ 12 CH 2 D 2 and Δ 13 CH 3 D decrease in a closed system by MOx. • Δ 13 CH 3 D and Δ 12 CH 2 D 2 trajectories are sensitive to deviations from RGM. • Minor deviations in 13CH 3 D from RGM can drastically change the mass-18 trajectory. • Major deviations in 12CH 2 D 2 from RGM during MOx, increases Δ 12 CH 2 D 2 signatures. • Methane altered by MOx is distinguishable in Δ 13 CH 3 D and Δ 12 CH 2 D 2 space. [ABSTRACT FROM AUTHOR]

Calcium-aluminum-rich inclusions (CAIs) are highly refractory objects found in different chondrite groups and represent some of the oldest known solids of the Solar System. As such, CAIs provide key information regarding the conditions prevailing in the solar protoplanetary disk as well as subsequent mixing and transport processes. Many studies have investigated CAIs for their isotopic compositions and reported nucleosynthetic isotope anomalies in numerous elements, which are typically explained by the variable incorporation of isotopically highly anomalous presolar phases. However, with the exception of 54Cr-enriched nanospinels, the exact presolar phases responsible for the isotopic heterogeneities are yet to be identified. To address this issue, we here present in-situ Ti isotopic analyses obtained on a diverse set of CAIs from various CV3 chondrites. The in-situ measurements were performed by targeting individual mineral phases of 15 CAIs with laser-ablation mass spectrometry and indicate significant inter- and intra-CAI isotopic heterogeneity in the neutron-rich isotope 50Ti. This is particularly pronounced for primitive fine-grained CAIs, whereas coarse-grained CAIs, which have been subject to melting, exhibit smaller degrees of Ti isotopic heterogeneity. To further investigate this Ti isotopic heterogeneity, we additionally obtained Ti isotopic compositions of sequential acid leachates from two fine-grained and two coarse-grained CAIs derived from CV3 chondrites. In contrast to potential expectations from the first part of the study, we do not observe any significant intra-CAI Ti isotopic heterogeneity between the different leaching steps. The lack of intra-CAI Ti isotopic heterogeneity in the acid leachate samples of this study likely reflects that the leaching procedure is unable to efficiently separate the carriers of isotopically anomalous Ti in CAIs. By comparing the bulk CAI Ti isotope compositions with Ti isotope data for hibonite-rich objects from the literature, we find that the range of Ti isotope compositions recorded by CAIs from various chondrite groups can be accounted for by the averaging of hibonite grains. In turn, the variable Ti isotope compositions of hibonite grains can be explained by the averaging of isotopically diverse presolar grains present in the Sun's parental molecular cloud. This effect of averaging is statistically supported by the central limit theorem, and the concept has the potential to be useful for other isotopic systems. [ABSTRACT FROM AUTHOR]

Abstract: Eucrites, a subgroup of basaltic achondrites, represent the remnants of the earliest magmatic stage of the Solar System formation, possibly on the asteroid 4 Vesta. The precise determination of their crystallization age is therefore important for constraining the evolutionary history of the asteroid, including melting and differentiation processes of the parent body. Zircons would be the best mineral for such purpose. However, extra-terrestrial zircons are rare, and those found in eucrites are typically small (<5μm). Using a Cameca IMS-1280 high-resolution ion microprobe, precise uranium–lead (U–Pb) and Pb–Pb ages of the non-cumulate basaltic eucrite zircons in Béréba, Cachari, Caldera, Camel Donga, and Juvinas were determined successfully at scale of<5μm. We obtained a weighted average 207Pb/206Pb age of 4541±11Ma and U–Pb concordia age of 4525±24Ma for the five eucrite samples studied. A synthesis of available age data of howardites, eucrites and diogenites indicate a protracted magmatic history on Vesta, with a most pronounced core–mantle differentiation event marked at 4564±2Ma, followed by a peak of basaltic magmatism at 4552±7Ma, which gradually diminished over the next 50Ma. Thermal modeling of Vesta suggests that partial melt fraction of greater than 10%, with temperature well exceeding 1500K could be maintained over this period throughout its interior. This is consistent with the chronological records of eucrites suggesting persistent magmatism on Vesta during the first 50Ma in the early Solar System. [Copyright &y& Elsevier]

Abstract: The Si stable isotope fractionation between metal and silicate has been investigated experimentally at 1800, 2000, and 2200°C. We find that there is a significant silicon stable isotope fractionation at high temperature between metal and silicate in agreement with . Further we find that this fractionation is insensitive to the structure and composition of the silicate as the fractionation between silicate melt and olivine is insignificant within the error of the analyses. The temperature-dependent silicon isotope fractionation is Δ30Sisilicate–metal =7.45±0.41×106/T 2. We also demonstrate the viability of using laser ablation MC-ICPMS as a tool for measuring silicon isotope ratios in high pressure and temperature experiments. [Copyright &y& Elsevier]

Abstract: reported low Ti3+/Ti4+ values in Ti-rich pyroxenes in the Wark–Lovering rim (WL) of a Leoville CAI (144A) as compared to the interior of the inclusion. These electron microprobe analyses were interpreted as evidence that the growth of the WL rim is the manifestation of an evolution to a more oxidizing environment. Further work by used XANES analyses to argue for higher Ti3+ abundances and interpreted the data of as the result of X-ray contamination by neighboring phases, specifically spinel. Late-stage alteration was also included as a possible explanation. To investigate further the oxidation state of Ti in WL rims, we re-analyzed Leoville 144A to obtain a more complete data set of Ti3+/Ti4+ values in the Wark–Lovering rims. We conducted experiments on spinel-mixing to determine whether this was a plausible explanation for the observed paucity of Ti3+ in WL rims. While we found a wider range of Ti3+/Ti4+ in these WL rim data than in our original study, our new data show that the original conclusion that rims are lower in Ti3+/Ti4+ than interiors remains valid. We conclusively rule out spinel-mixing as an explanation for our data, and we see no clear inconsistency between our electron microprobe data and the XANES data. The WL rim of CAI Ef3 was also analyzed by EMPA and compared to the results of Leoville 144A. To predict compositional consequences of this hypothesis, we constructed a reaction space between Ti-rich pyroxene in the WL rim, perovskite, Mg(g), Ca(g), O2(g), and SiO(g). We find the oxidation of Ti3+, coupled with Ti loss via perovskite formation, explains many features of WL rim EMPA analyses. We maintain that the WL rim pyroxenes are compositionally distinct from those in the interior, and are evidence of a more oxidizing environment during WL rim formation. [ABSTRACT FROM AUTHOR]

Abstract: The range in 56Fe/54Fe isotopic compositions measured in naturally occurring iron-bearing species is greater than 5‰. Both theoretical modeling and experimental studies of equilibrium isotopic fractionation among iron-bearing species have shown that significant fractionations can be caused by differences in oxidation state (i.e., redox effects in the environment) as well as by bond partner and coordination number (i.e., nonredox effects due to speciation). To test the relative effects of redox vs. nonredox attributes on total Fe equilibrium isotopic fractionation, we measured changes, both experimentally and theoretically, in the isotopic composition of an Fe2+–Fe3+–Cl–H2O solution as the chlorinity was varied. We made use of the unique solubility of FeCl4 − in immiscible diethyl ether to create a separate spectator phase against which changes in the aqueous phase could be quantified. Our experiments showed a reduction in the redox isotopic fractionation between Fe2+- and Fe3+-bearing species from 3.4‰ at [Cl−]=1.5M to 2.4‰ at [Cl−]=5.0M, due to changes in speciation in the Fe–Cl solution. This experimental design was also used to demonstrate the attainment of isotopic equilibrium between the two phases, using a 54Fe spike. To better understand speciation effects on redox fractionation, we created four new sets of ab initio models of the ferrous chloride complexes used in the experiments. These were combined with corresponding ab initio models for the ferric chloride complexes from previous work. At 20°C, 1000 ln β (β = 56Fe/54Fe reduced partition function ratio relative to a dissociated Fe atom) values range from 6.39‰ to 5.42‰ for Fe(H2O)6 2+, 5.98‰ to 5.34‰ for FeCl(H2O)5 +, and 5.91‰ to 4.86‰ for FeCl2(H2O)4, depending on the model. The theoretical models predict ferric–ferrous fractionation about half as large (depending on model) as the experimental results. Our results show (1) oxidation state is likely to be the dominant factor controlling equilibrium Fe isotope fractionation in solution and (2) nonredox attributes (such as ligands present in the aqueous solution, speciation and relative abundances, and ionic strength of the solution) can also have significant effects. Changes in the isotopic composition of an Fe-bearing solution will influence the resultant Fe isotopic signature of any precipitates. [Copyright &y& Elsevier]

Abstract: Transition metal stable isotope signatures can be useful for tracing both natural and anthropogenic signals in the environment, but only if the mechanisms responsible for fractionation are understood. To investigate isotope fractionations due to electrochemistry (or redox processes), we examine the stable isotope behavior of iron and zinc during the reduction reaction +2e− =Mmetal as a function of electrochemical driving force, temperature, and time. In all cases light isotopes are preferentially electroplated, following a mass-dependent law. Generally, the extent of fractionation is larger for higher temperatures and lower driving forces, and is roughly insensitive to amount of charge delivered. The maximum fractionations are δ56/54Fe=−4.0‰ and δ66/64Zn=−5.5‰, larger than observed fractionations in the natural environment and larger than those predicted due to changes in speciation. All the observed fractionation trends are interpreted in terms of three distinct processes that occur during an electrochemical reaction: mass transport to the electrode, chemical speciation changes adjacent to the electrode, and electron transfer at the electrode. We show that a large isotope effect adjacent the electrode surface arises from the charge-transfer kinetics, but this effect is attenuated in cases where diffusion of ions to the electrode surface becomes the rate-limiting step. Thus while a general increase in fractionation is observed with increasing temperature, this appears to be a result of thermally enhanced mass transport to the reacting interface rather than an isotope effect associated with the charge-transfer kinetics. This study demonstrates that laboratory experiments can successfully distinguish isotopic signatures arising from mass transport, chemical speciation, and electron transfer. Understanding how these processes fractionate metal isotopes under laboratory conditions is the first step towards discovering what role these processes play in fractionating metal isotopes in natural systems. [Copyright &y& Elsevier]

Abstract: Literature data demonstrate that on a global, asteroid-wide scale (plausibly on the order of 100km), ordinary chondrites (OC) have heterogeneous oxidation states and O-isotopic compositions (represented, respectively, by the mean olivine Fa and bulk Δ17O compositions of equilibrated samples). Samples analyzed here include: (a) two H5 chondrite Antarctic finds (ALHA79046 and TIL 82415) that have the same cosmic-ray exposure age (7.6Ma) and were probably within ∼1km of each other when they were excavated from the H-chondrite parent body, (b) different individual stones from the Holbrook L/LL6 fall that were probably within ∼1m of each other when their parent meteoroid penetrated the Earth’s atmosphere, and (c) drill cores from a large slab of the Estacado H6 find located within a few tens of centimeters of each other. Our results indicate that OC are heterogeneous in their bulk oxidation state and O-isotopic composition on 100-km-size scales, but homogeneous on meter-, decimeter- and centimeter-size scales. (On kilometer size scales, oxidation state is heterogeneous, but O isotopes appear to be homogeneous.) The asteroid-wide heterogeneity in oxidation state and O-isotopic composition was inherited from the solar nebula. The homogeneity on small size scales was probably caused in part by fluid-assisted metamorphism and mainly by impact-gardening processes (which are most effective at mixing target materials on scales of ⩽1m). [Copyright &y& Elsevier]

We report the results of spinel-magnesite Mg isotope exchange experiments at 600, 700, and 800 °C and 1 GPa to establish the equilibrium Mg isotope partitioning between magnesite (MgCO 3) and spinel as a function of Cr substitution for Al in the spinel phase. We used the three-isotope method to obtain equilibrium fractionation factors between MgAlCrO 4 and magnesite and MgCr 2 O 4 and magnesite. The experimentally-determined temperature-dependent Mg isotope fractionations are Δ 26 Mg MgAlCrO 4 - Mgs = 0.96 ± 0.22 × 106/T2 and Δ 26 Mg MgCr 2 O 4 - Mgs = 0.55 ± 0.08 × 106/T2 (2 s.e.). When combined with the previous experimentally determined fractionation between forsterite and magnesite (Macris et al., 2013), the corresponding Mg isotope fractionations between these two Cr-bearing spinel compositions and forsterite are Δ 26 Mg MgAlCrO 4 - Fo = 0.84 ± 0.23 × 106/T2 and Δ 26 Mg MgCr 2 O 4 - Fo = 0.43 ± 0.10 × 106/T2. The experimentally determined isotopic fractionation relationship between magnesiochromite and magnesite agrees with theoretical predictions based on the crystal chemical environment of Mg in these minerals. By combining these new results with the existing experimental calibration of equilibrium Mg isotope exchange between pure spinel and forsterite, we arrive at the temperature-dependent Mg isotope fractionation between spinel and forsterite as a function of Cr concentration in the spinel: Δ 26 M g Mg Al 1 - x Cr x 2 O 4 - Fo = - 0.67 ± 0.26 x + 1.10 ± 0.23 10 6 / T 2 When applied to natural samples, the combination of measured Mg isotope fractionation between spinel and forsterite, the Cr concentrations of the spinels, and estimates of temperature from independent geothermometers can be used to identify differences in closure temperatures between Mg isotope exchange and cation exchange, or the presence of disequilibrium in these systems. [ABSTRACT FROM AUTHOR]

Methane generated by microorganisms is most often depleted in the doubly substituted isotopologue 12 CH 2 D 2 relative to the stochastic reference distribution. To constrain the controls on depleted Δ 12 CH 2 D 2 values, we experimentally isolated the root cause with microorganisms that produce methane from methylphosphonate via the C-P lyase pathway. This mechanism of methane production preserves the three hydrogens from methylphosphonate and adds one hydrogen from water. When maintaining the same methylphosphonate source, but varying the D/H composition of growth medium water, we observed significant shifts in methane Δ 12 CH 2 D 2 values, but little to no change in Δ 13 CH 3 D values. We reproduced these observations with a model that considers only the combinatorial isotope effect. The variation in Δ 12 CH 2 D 2 values of product methane resulted from the differences in D/H between reactants water and methylphosphonate. This work validates the hypothesis that combinatorial effects can strongly influence methane Δ 12 CH 2 D 2 values, and must be considered for low temperature, abiotic or biotic systems where methane hydrogen is derived from multiple reservoirs. [ABSTRACT FROM AUTHOR]

Abstract We determined the Fe isotope fractionation between the metal and silicate phases of two aubrite meteorites, Norton County and Mount Egerton. We find that the metallic phase is high in 57Fe/54Fe with respect to the silicate phase, with Δ57Fe metal-silicate = 0.08‰ ± 0.04 for Mount Egerton and 0.09 ± 0.02‰ for Norton County. These data, combined with new measurements of 57Fe/54Fe of IIIAB iron meteorites, are used to constrain the origins of the high 57Fe/54Fe exhibited by all classes of iron meteorites. We find that if the parent bodies of the iron meteorites had chondritic bulk 57Fe/54Fe values, their cores must have been unusually small (≤8% by mass). Relaxing the constraint that the bodies were chondritic in their bulk iron isotope ratios allows for larger core mass fractions commensurate with usual expectations. In this case, the elevated 57Fe/54Fe values of iron meteorites are due in part to evaporation of melt during the accretion stages of the parent bodies and not solely the result of metal-silicate differentiation. [ABSTRACT FROM AUTHOR]