The measurement of methane clumped isotopologues (Delta(CH3D)-C-13 and Delta(CH2D2)-C-12) allows exploring isotope bond ordering within methane molecules, and may reveal equilibrium temperatures. Whether such temperature reflects the formation or re-equilibration temperature of the methane is not well understood, but would have critical implications for the use of methane clumped isotopologues as geo-thermometers. Here we investigate gas bubbles from vigorous emissions at cold seeps (n = 14) in the Sea of Marmara, Turkey. These cold seeps are sourced from deeper sedimentary reservoirs. Conventional geochemical tracers such as carbon and hydrogen bulk isotopic ratios (C-13/C-12 and D/H) or n-alkane molecular ratios, suggest these gases reflect various degrees of mixing between thermogenic and microbial sources. Some samples would generally be considered purely microbial in origin (C-1/C2+ > 1500; delta C-13 < -60 parts per thousand) We report measurements of Delta(CH3D)-C-13 and Delta(CH2D2)-C-12 showing that a fraction of those gases are in internal thermodynamic equilibrium, with the abundances of the two mass-18 isotopologues indicating concordant temperatures of similar to 90 degrees C and similar to 130 degrees C. These concordant temperatures are recorded by gases of putative microbial and thermogenic origin; the temperatures of equilibration are irrespective of the formation mechanism of the gases. We conclude that the two high-temperatures recorded by Delta(CH3D)-C-13 and Delta(CH2D2)-C-12 are best explained by non-enzymatic re-equilibration at two local subsurface temperatures. First principles suggest that unequal rates of exchange are possible. Disequilibrium signatures where the two isotopologues yield discordant apparent temperatures are exhibited by other samples. In those cases the data define a trend of variable Delta(CH3D)-C-13 at nearly constant Delta(CH2D2)-C-12. These signatures are enigmatic, and we investigate and reject multiple possible explanations including mixing, diffusion or Anaerobic Oxidation of Methane. Different rates of re-equilibration between the two rare isotopologues are implied, although lacks experimental foundation at present. In general, all of these data point towards re-equilibration of the mass-18 methane isotopologues as an important process. (C) 2020 Elsevier B.V. All rights reserved.

The development of high-resolution gas source mass spectrometry has permitted entirely new types of measurements of multiply-substituted isotopologues in gas species of geochemical significance. Here, we present recent advances afforded by measurements of 15N15N in natural samples, together with 14N14N and 15N14N. We show that the abundance of the doubly-substituted 15N15N isotopologue in hydrothermal gases, often mixtures of volatiles of widely different origins, allows tracing the provenance of nitrogen. The approach is based on the recent finding that atmospheric N2 has a substantial enrichment in 15N15N of nearly 20‰ relative to any other source of N2. This is particularly useful for the study of hydrothermal gases, where characterizing the isotopic composition and provenance of volcanic N2 is important for a wide range of applications in high-temperature geochemistry, but where air-derived N2 is unavoidable. In this review, we summarize the evidence that 15N15N is an unambiguous tracer of air contamination. We compare two sets of published 15N15N data acquired on gases from plume and arc volcanoes. We show how different sources of volcanic N2 may be in plume versus arc environments, and discuss the first-order constraints on the deep N cycle that are provided by the new 15N15N data. Important findings include that the δ15N tracer, used alone or in conjunction with N2/Ar and N2/He ratios, can be surprisingly deceiving. Isotope fractionation of atmospheric nitrogen occurs within hydrothermal systems, resulting in negative δ15N values similar to estimates for mantle values, yet with 15N15N values that preclude a mantle origin. The 15N15N data show that the true δ15N of volcanic components is positive in arcs but near-zero at the Yellowstone plume. In other words, atmospheric N2 can mimic mantle δ15N, and mantle δ15N can look like the value of air. Without 15N15N, the apportioning of mantle and atmospheric N2 in mixed gases can easily be wrong. With 15N15N, we also determine the true N2/3He and N2/36Ar ratios of volcanic components in hydrothermal systems. Results inform our understanding of the deep nitrogen cycle. Plume and arc volcanic endmembers show distinct isotope and elemental ratios, consistent with sub-arc sources being overwhelmed by near-quantitative slab devolatilization, while the Yellowstone plume source is not reflecting volatile subduction.

Deep-sea hydrothermal fluids are often enriched in carbon dioxide, methane, and hydrogen. Methane effuses from metal-rich black smokers such as the Rainbow hydrothermal field, at temperatures higher than 200 degrees C. At the Lost City field, CH4 emanates from alkaline fluids at <100 degrees C. The abundance of the rare, mass-18 CH4 isotopologues, (CH3D)-C-13 and (CH2D2)-C-12, can mitigate degeneracies in the conventional isotopic signatures of methane. We studied the isotopologue compositions of methane from the Rainbow, Lucky Strike, Von Damm, and Lost City hydrothermal fields. At Rainbow, where the vented fluids are at similar to 360 degrees C, our coupled Delta(CH2D2)-C-12 - Delta(CH3D)-C-13 data establish that methane is in internal equilibrium at 343(-35)(+41)degrees C. This may track the formation temperature of abiotic methane, or it may be the result of equilibration of methane isotopologues within the carrier fluid. Lucky Strike and Von Damm have fluid temperatures <300 degrees C and although Delta(CH3D)-C-13 values are indistinguishable from those at Rainbow, (CH2D2)-C-12 abundances are marginally higher. At Lost City, Delta(CH3D)-C-13 data show a range of values, which at face value correspond to apparent temperatures of between 265(-24)(+28) degrees C and 158(-14)(+16) degrees C, far hotter than fluid temperatures. A unique aspect of the Lost City data is the range of large (CH2D2)-C-12 excesses. The Delta(CH2D2)-C-12 data correspond to temperatures of between 101(-8)(+9) degrees C and 69(-4)(+4) degrees C, showing a near-perfect match with fluid temperatures. We find that mixing scenarios involving microbial methane may not account for all of the isotope data. We suggest that Delta(CH2D2)-C-12 values, unlike Delta(CH3D)-C-13 values, are prone to near-complete re-equilibration at host fluid temperatures. We suggest that Delta(CH3D)-C-13 isotopologue data are consistent with abiotic methane being synthesized at similar to 350 degrees C. On the other hand, (CH2D2)-C-12 isotopologue ordering records post formation residence temperatures. We explore a possible mechanism decoupling the reequilibration systematics of the doubly-substituted isotopologues. (C) 2020 Elsevier Ltd. All rights reserved.

Almahata Sitta (AhS) 202 from the UoK collection represents a clast from the polymict breccia asteroid 2008 TC3. AhS 202 was recognized as a unique carbonaceous chondrite (CC) with a high magnetite content. Here we report that it also contains a significant amount of amphibole, a mineral that is very rare in chondrites and has not previously been reported in significant abundance in a CC. We present new petrographic, oxygen isotope, and micro-FTIR data. We discuss petrogenesis and possible relationships to known CC.

CLUES AND CAUTIONS FROM ASTEROID 2008 TC3 AND THE ALMAHATA SITTA METEORITE. C. A. Goodrich1, M.E. Zolensky2, A.M. Fioretti3, M.H. Shaddad4, H. Downes5, T. Hiroi6, I. Kohl7, E.D. Young7, N.T. Kita8, V.E. Hamilton9, M. Riebe10, H. Busemann10, R.J. Macke11, M. Fries2, M. Sanborn12, Q-Z. Yin12, D.K. Ross13, P. Jenniskens14. 1Lunar and Planetary Institute, USRA, Houston TX 77058 USA (goodrich@lpi.usra.edu); 2ARES, NASA-JSC, Houston TX USA; 3CNR, Padova Italy; 4Univ. Khartoum, Khartoum Sudan; 5Birkbeck Univ. London, London UK; 6Brown Univ., Providence, RI USA; 7UCLA, Los Angeles, CA USA; 8Univ. Wisconsin, Madison, WI USA; 9SwRI, Boulder, CO USA; 10ETH, Zürich Switzerland; 11Specola Vaticana, Vatican City State; 12UC Davis, Davis, CA USA; 13Jacobs-JETS, NASA-JSC, Houston TX USA; 14SETI, Mountain View, CA USA.

There is an increasing number of Cr-O-Ti isotope studies that show that solar system materials are divided into two main populations, one carbonaceous chondrite (CC)-like and the other is non-carbonaceous (NCC)-like, with minimal mixing between them attributed to a gap opened in the propoplanetary disk due to Jupiter's formation. The Grand Tack model suggests that there should be a particular time in the disk history when this gap is breached and ensuring a subsequent large-scale mixing between S- and C-type asteroids (inner solar system and outer solar system materials), an idea supported by our recent work on chondrule (Delta)17O-(epsilon)54Cr isotope systematics.

The Almahata Sitta (AhS) polymict ureilite fell in 2008 when asteroid 2008 TC3 impacted over Sudan]. It is the first meteorite to originate from an asteroid that had been tracked and studied in space (with spectral classification) before impact, and provides a unique opportunity to correlate properties of meteorites with those of their parent asteroid. More than 700 monolithologic stones from the AhS fall were collected. Of those previously studied, approx. 70% were ureilites and approx. 30% were chondrites. It has been inferred that 2008 TC3 was loosely aggregated and porous and disintegrated in the atmosphere, with only its most coherent clasts falling as stones. However, understanding the structure of this asteroid is limited by incomplete study of the heterogeneous stones, and the loss of most of the mass of the asteroid. The University of Khartoum (UOK) AhS collection contains over >600 AhS stones with find coordinates. We are studying this collection to determine: 1) the proportion of ureilitic to various non-ureilitic stones; 2) the distribution of types of stones in the strewn field; and 3) the compositional and physical structure of 2008 TC3. We report on 61 new stones, including a unique sample that may represent the bulk of the material lost from 2008 TC3.

Measured and modeled Ca and Ti isotopic fractionation effects in a diverse suite of refractory inclusions are used to understand processes of condensation in the solar protoplanetary disk where they and their precursor materials formed. This coordinated approach reveals largely decoupled isotopic signatures and implies that few, if any, of the studied inclusions can be considered primary condensates. All studied inclusions are enriched in light Ca isotopes (∼−0.2 to −2.8‰/amu), but only two show correspondingly light Ti isotopes. Studied inclusions exhibit both heavy and light Ti isotope enrichments (∼0.3 to −0.4‰/amu). These refractory element isotopic signatures, therefore, suggest admixture and reprocessing of earlier formed materials with distinct condensation histories. Along with coordinated measurements of 50Ti isotopic anomalies, which span a range from ∼0 to ∼40 epsilon-unit excesses, the comparison of measured and modeled fractionation of Ca and Ti isotopes provides a powerful approach to understanding primitive nebular processes and environments in the protoplanetary disk. Remarkable evidence for Ca isotopic zoning within a typical Type B1 inclusion exemplifies the potential record of the earliest solar nebula that is likely lost and/or overprinted in the isotopic compositions of more volatile elements (e.g., Mg, Si, and O) by later modification processes.

Calcium-aluminum-rich inclusions (CAIs) are the oldest surviving solids to have formed in the Solar System. Their chemical and isotopic compositions provide a record of the conditions present in the protoplanetary disk where they formed and can aid our understanding of how solids formed in the solar nebula, an important step in the eventual process of planet building. The isotopic compositions of CAIs are primarily controlled by volatility. Evaporation/sublimation are well understood through both theory and experimental work to produce an enrichment in the heavy isotopes of an element, but less is understood about the effects of condensation. Mass-dependent fractionation can potentially provide a record of nebular condensation. Ti is not likely to experience evaporation due to its refractory nature, making it a useful tool for assessing the effects of condensation. We have undertaken a study of the stable isotope fractionation of Ti isotopes as a tracer of processes that predate the last evaporation events affecting CAIs. We compare the 49Ti/47Ti stable isotope ratio with excess 50Ti common in CAIs. We have collected Ti, Mg, Si, and Ca isotope data for a suite of CAIs in order to search for heterogeneity in each of these isotope systems, and for potential correlations among them. We compare our results to expectations for condensation.

We report measurements of resolved 12CH2D2 and 13CH3D at natural abundances in a variety of methane gases produced naturally and in the laboratory. The ability to resolve 12CH2D2 from 13CH3D provides unprecedented insights into the origin and evolution of CH4. The results identify conditions under which either isotopic bond order disequilibrium or equilibrium are expected. Where equilibrium obtains, concordant Δ12CH2D2 and Δ13CH3D temperatures can be used reliably for thermometry. We find that concordant temperatures do not always match previous hypotheses based on indirect estimates of temperature of formation nor temperatures derived from CH4/H2 D/H exchange, underscoring the importance of reliable thermometry based on the CH4 molecules themselves. Where Δ12CH2D2 and Δ13CH3D values are inconsistent with thermodynamic equilibrium, temperatures of formation derived from these species are spurious. In such situations, while formation temperatures are unavailable, disequilibrium isotopologue ratios nonetheless provide novel information about the formation mechanism of the gas and the presence or absence of multiple sources or sinks. In particular, disequilibrium isotopologue ratios may provide the means for differentiating between methane produced by abiotic synthesis vs. biological processes. Deficits in 12CH2D2 compared with equilibrium values in CH4 gas made by surface-catalyzed abiotic reactions are so large as to point towards a quantum tunneling origin. Tunneling also accounts for the more moderate depletions in 13CH3D that accompany the low 12CH2D2 abundances produced by abiotic reactions. The tunneling signature may prove to be an important tracer of abiotic methane formation, especially where it is preserved by dissolution of gas in cool hydrothermal systems (e.g., Mars). Isotopologue signatures of abiotic methane production can be erased by infiltration of microbial communities, and Δ12CH2D2 values are a key tracer of microbial recycling.

Introduction: Recently published triple-oxygen isotope data from lunar basalts has improved constraints on plausible scenarios for the Moon forming event [1,2]. In these studies extremely high precision ∆'O data are presented that in principle would allow for distinguishing materials on the basis of differences of just a few ppm. In order to evaluate the influences of different mass fractionation laws for both Earth and Moon on these results, anorthositic rocks and minerals were measured [1]. While falling within the theoretical envelope for mass fractionation the ∆'O values of these anorthositic samples are too low to be explained by differences in mass fractionation laws; the anorthosites have δO values not far removed from those of their parental material and the fractionation exponents (three-isotope slopes) required to generate the observed ∆'O values are far lower than one would expect for igneous processes (Figure 1).

We investigated the magnitude and reproducibility of instrumental mass-dependent fractionation of oxygen isotopes in secondary ion mass spectrometry (SIMS) analyses of olivine crystals of different major element chemistry (from Mg-rich to Fe-rich) in order to improve the accuracy of in-situ O-isotope measurements in geochemical/cosmochemical olivine samples. We found that oxygen isotope SIMS matrix effects are reproducible, and developed a model curve that can be used for correcting instrumental mass fractionation of olivine samples of intermediate chemical composition. The changes in instrumental mass fractionations were likely caused by differing Cs concentrations in the near surface regions of the samples due to different sample sputtering rates.