¹Stephen M. Elardo, ¹Anat Shahar
Nature Geoscience (in Press) Link to Article [doi:10.1038/ngeo2896]
¹Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA

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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.

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.