A XANES and Raman investigation of sulfur speciation and structural order in Murchison and Allende meteorites

1M. Bose, 2R. A. Root, 3S. Pizzarello
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12811]
1School of Molecular Sciences, Arizona State University, Tempe, Arizona, USA
2Department of Soil, Water & Environmental Science, University of Arizona, Tucson, Arizona, USA
3School of Molecular Sciences, Arizona State University, Tempe, Arizona, USA
Published by arrangement with John Wiley & Sons

Insoluble organic matter (IOM) and hydrothermally treated IOM extracted from two carbonaceous chondrites, Murchison and Allende, was studied using sulfur K-edge XANES (X-ray absorption near edge structure) and μ-Raman spectroscopy, with the aim to understand their IOM’s sulfur speciation and structural order, and how aqueous alteration or thermal metamorphism may have transformed these materials. We found that the sulfur-functional group chemistry of both the Murchison IOM and hydrothermally treated IOM samples have a large chemical variability ranging from oxidation states of S−2 to S+6, and exhibit a transformation in their oxidation state after the hydrothermal treatment (HT) to produce thiophenes and thiol compounds. Sulfoxide and sulfite peaks are also present in Murchison. Sulfates considered intrinsic to Murchison are most likely preaccretionary in nature, and not a result of reactions with water at high temperatures on the asteroid parent body. We argue that the reduced sulfides may have formed in the CM parent body, while the thiophenes and thiol compounds are a result of the HT. Micro-Raman spectra show the presence of aliphatic and aromatic moieties in Murchison’s material as observed previously, which exhibits no change after HT. Because the Murchison IOM was modified, as seen by XANES analysis, absence of a change observed using micro-Raman indicated that although the alkyl carbons of IOM were cleaved, the aromatic network was not largely modified after HT. By contrast, Allende IOM contains primarily disulfide and elemental sulfur, no organic sulfur, and shows no transformation after HT. This nontransformation of Allende IOM after HT would indicate that parent body alteration of sulfide to sulfate is not feasible up to temperatures of 300°C. The reduced sulfur products indicate extreme secondary chemical processing from the precursor compounds in its parent body at temperatures as high as 624°C, as estimated from μ-Raman D band parameters. The Raman parameters in Allende IOM that was interpreted in terms of amorphous carbon with regions of large clusters of benzene rings, was transformed after the HT to those with fewer benzene rings.

Uranium, thorium and REE partitioning into sulfide liquids: implications for reduced S-rich bodies

1Anke Wohlers, 1Bernard J. Wood
Geochimica et Cosmochimica acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.01.050]
1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK
Copyright Elsevier

We have performed experiments at 1.5 GPa over the temperature range 1400-2100°C to determine the partitioning of lithophile elements (U, Th, Eu, Sm, Nd, Zr, La, Ce, Yb) between sulfide liquid, low-S metals and silicate melt. The data demonstrate pronounced increases in partitioning of all the lithophile elements into sulfide at very low FeO contents (10 in some cases. Similarly DSm may be > 2 under the same conditions of low silicate FeO. This strong partitioning behaviour is found only be important in S-rich metals, however because the observed effect of low FeO on partitioning is uniquely confined to metallic melts close to stoichiometric FeS in composition.
The results and the effects of FeS content of the metal and FeO content (or activity) of the silicate may be understood in terms of exchange reactions such as:

UO2+2FeS=2FeO+US2UO2+2FeS=2FeO+US2
silicate sulfide silicate sulfide

High concentrations of FeS (in metal) and low FeO contents of the silicate melts drive the reaction to the right, favouring high US2 in the sulfide and hence high DU. The effect is, we find, enhanced by the high solubility of S in the silicate (up to 11 wt%) at low FeO contents. This S content greatly reduces the activity coefficient of FeO in the silicate melt, enhancing the displacement of the reaction to the right.
For sulfide-silicate partitioning at 1.5GPa and 1400°C we obtain DNd/DSm of about 1.4 and DTh ∼ 0.1DU. With increasing temperature the differences between these geochemically similar element pairs decreases such that, at 2100°C DNd/DSm is 1.0 and DTh/DU is about 0.3. We used these results, together with DU and DSm to model addition of a putative Mercury-like component (with FeS core) to early Earth. We find that the 1400o results could lead to a significant (∼11ppm) 142Nd anomaly in silicate Earth and add >8 ppb U to the core, but lead to an unreasonably high Th/U of silicate Earth (4.54). Based on the 2100°C results the 142Nd anomaly would be 0 but addition of the sulfur-rich body could add up to 10 ppb of U to the core, generating, when the accompanying 21 ppb Th is also considered, ∼3 TW of the energy required for the geodynamo. In this case, the Th/U ratio of silicate Earth would approximate 4.3, within the range of some estimates.

Origin of highly siderophile and chalcogen element fractionations in the components of unequilibrated H and LL chondrites

1Yogita Kadlag, 1Harry Becker
Chemie der Erde (in Press) Link to Article [http://dx.doi.org/10.1016/j.chemer.2017.01.004]
1Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, D-12249 Berlin, Germany
Copyright Elsevier

Osmium isotopic compositions, abundances of highly siderophile elements (HSE: platinum group elements, Re and Au), the chalcogen elements S, Se and Te and major and minor elements were analysed in physically separated size fractions and components of the ordinary chondrites WSG 95300 (H3.3, meteorite find) and Parnallee (LL3.6, meteorite fall). Fine grained magnetic fractions are 268-65 times enriched in HSE compared to the non-magnetic fractions. A significant deviation of some fractions of WSG 95300 from the 4.568 Ga 187Re-187Os isochron was caused by redistribution of Re due to weathering of metal. HSE abundance patterns show that at least four different types of HSE carriers are present in WSG 95300 and Parnallee. The HSE carriers display (i) CI chondritic HSE ratios, (ii) variable Re/Os ratios, (iii) lower than CI chondritic Pd/Ir and Au/Ir and (iv) higher Pt/Ir and Pt/Ru than in CI chondrites. These differences between components clearly indicate the loss of refractory HSE carrier phases before accretion of the components. Tellurium abundances correlate with Pd and are decoupled from S, suggesting that most Te partitioned into metal during the last high-temperature event. Tellurium is depleted in all fractions compared to CI chondrite normalized Se abundances. The depletion of Te is likely associated with the high temperature history of the metal precursors of H and LL chondrites and occurred independent of the metal loss event that depleted LL chondrites in siderophile elements. Most non-magnetic and slightly magnetic fractions have S/Se close to CI chondrites. In contrast, the decoupling of Te and Se from S in magnetic fractions suggests the influence of volatility and metal-silicate partitioning on the abundances of the chalcogen elements. The influence of terrestrial weathering on chalcogen element systematics of these meteorites appears to be negligible.

The nature, origin and modification of insoluble organic matter in chondrites, the major source of Earth’s C and N

1C.M.O’D. Alexander, 2G.D. Cody, 3B.T. De Gregorio, 1L.R. Nittler, 3R.M. Stroud
Chemie der Erde (in Press) Link to Article [http://dx.doi.org/10.1016/j.chemer.2017.01.007]
1Dept. Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC 20015, USA
2Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, Washington, DC 20015, USA
3Materials Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA
Copyright Elsevier

All chondrites accreted ∼3.5 wt.% C in their matrices, the bulk of which was in a macromolecular solvent and acid insoluble organic material (IOM). Similar material to IOM is found in interplanetary dust particles (IDPs) and comets. The IOM accounts for almost all of the C and N in chondrites, and a significant fraction of the H. Chondrites and, to a lesser extent, comets were probably the major sources of volatiles for the Earth and the other terrestrial planets. Hence, IOM was both the major source of Earth’s volatiles and a potential source of complex prebiotic molecules.

Large enrichments in D and 15N, relative to the bulk solar isotopic compositions, suggest that IOM or its precursors formed in very cold, radiation-rich environments. Whether these environments were in the interstellar medium (ISM) or the outer Solar System is unresolved. Nevertheless, the elemental and isotopic compositions and functional group chemistry of IOM provide important clues to the origin(s) of organic matter in protoplanetary disks. IOM is modified relatively easily by thermal and aqueous processes, so that it can also be used to constrain the conditions in the solar nebula prior to chondrite accretion and the conditions in the chondrite parent bodies after accretion.

Here we review what is known about the abundances, compositions and physical nature of IOM in the most primitive chondrites. We also discuss how the IOM has been modified by thermal metamorphism and aqueous alteration in the chondrite parent bodies, and how these changes may be used both as petrologic indicators of the intensity of parent body processing and as tools for classification. Finally, we critically assess the various proposed mechanisms for the formation of IOM in the ISM or Solar System.

X-ray computed tomography of planetary materials: A primer and review of recent studies

1Romy D. Hanna, 1Richard A. Ketcham
Chemie der Erde (in Press) Link to Article [http://dx.doi.org/10.1016/j.chemer.2017.01.006]
1Jackson School of Geosciences, University of Texas, 2275 Speedway Stop C9000, Austin, TX 78712, USA
Copyright Elsevier

X-ray computed tomography (XCT) is a powerful 3D imaging technique that has been used to investigate meteorites, mission-returned samples, and other planetary materials of all scales from dust particles to large rocks. With this technique, a 3D volume representing the X-ray attenuation (which is sensitive to composition and density) of the materials within an object is produced, allowing various components and textures to be observed and quantified. As with any analytical technique, a thorough understanding of the underlying physical principles, system components, and data acquisition parameters provides a strong foundation for the optimal acquisition and interpretation of the data. Here we present a technical overview of the physics of XCT, describe the major components of a typical laboratory-based XCT instrument, and provide a guide for how to optimize data collection for planetary materials using such systems. We also discuss data processing, visualization and analysis, including a discussion of common data artifacts and how to minimize them. We review a variety of recent studies in which XCT has been used to study extraterrestrial materials and/or to address fundamental problems in planetary science. We conclude with a short discussion of anticipated future directions of XCT technology and application.

Correlated XANES, TEM, and NanoSIMS of Presolar Graphite Grains

1,2,3Evan E. Groopman, 4Larry R. Nittler
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.02.011]
1Laboratory for Space Science, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
2National Research Council Postdoctoral Fellow at the U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA
3Materials Science and Technology Division, U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA
4Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5251 Broad Branch Rd NW, Washington, DC 20015, USA
Copyright Elsevier

We report correlated XANES, TEM, and NanoSIMS measurements of twelve presolar graphite grains extracted from primitive meteorites and for which isotopic data indicate predominantly Type-II supernovae origins. We find continued evidence for isotopic heterogeneities in presolar graphite grains, including the first observation of a radial gradient in the inferred initial 26Al/27Al within a presolar graphite grain. The XANES spectra of these samples show a variety of minor absorbances near the C K-edge, attributable to vinyl-keto, aliphatic, carboxyl, and carbonate molecules, as well as possible damage during sample preparation. Each sample exhibits homogeneous C K-edge XANES spectra within the graphite, however, showing no correlation with isotopic heterogeneities. Gradients in the isotope ratios of C, N, O, and Al could be due to both processes during condensation, e.g., mixing in stellar ejecta and granular transport, and post-condensation effects, such as isotope dilution and exchange with isotopically normal material in the early Solar System or laboratory, the latter of which is a significant issue for high-density presolar graphite grains. It remains unknown whether the mechanisms behind isotope exchange would also affect the local chemistry and therefore the XANES spectra. Ti L-edge XANES from most Ti-rich subgrains match standard spectra for TiC and potentially TiCN. A rare rutile (TiO2) subgrain has been identified, though it lacks the lowest energy L3 peak typically seen in standard spectra. Ca has also been identified by EDXS in TiC subgrains, likely due to the decay of live 44Ti at the time of formation. Future NanoSIMS measurements will determine the variability of initial 44Ti in TiC subgrains, an important constraint on mixing in the ejecta of the grains’ parent supernovae.