^1,3E.S.Steenstra,²A.X.Seegers,² R.Putter,³ J.Berndt,³S.Klemme,⁴S.Matveev,¹ E.Bullock,²W.van Westrenen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113391]
¹The Geophysical Laboratory, Carnegie Institution of Science, Washington D.C., the United States of America
²Faculty of Science, Vrije Universiteit Amsterdam, the Netherlands
³Institute of Mineralogy, University of Münster, Germany
⁴Department of Geosciences, Utrecht University, the Netherlands
Copyright Elsevier

The differences in FeO mantle contents and core masses between the terrestrial planets suggest the oxygen fugacity (fO₂) during their differentiation likely varied significantly. The metal-silicate partitioning of siderophile (iron-loving) elements is a function of fO₂ and of their valence state(s) in silicate melts. Silicon (Si) is known to partition into metal at low fO₂ and has been proposed as a possible light element in the cores of Mercury and the aubrite parent body (AuPB).

To systematically study the metal-silicate partitioning behavior of siderophile elements into Si-bearing metal, 69 high pressure (P) – temperature (T) metal-silicate partitioning experiments were performed under moderately to highly reducing conditions. Oxygen fugacities ranged between 1 and 7 units below the iron-wüstite buffer (ΔIW). Experimental pressures and temperatures ranged between 1 and 5 GPa and 1883 to 2273 K, respectively. A comparison of the ΔIW values and the fO₂ based on the Si-SiO₂ buffer (ΔSi-SiO₂) indicates that the activity coefficient of FeO in silicate melts decreases significantly from reducing to highly reducing conditions under C-saturated conditions. It was found that at conditions more reducing than ΔIW = −3 to −4, the metal-silicate partitioning behavior of the majority of the siderophile elements deviates significantly from values corresponding to their expected valence state(s). These results indicate that the activity in metal of the elements considered, including that of Si itself, is decreased as a function of Si metal content, and a thermodynamic approach was used to quantify these effects. Interaction coefficients of trace elements in Si-bearing, Fe-rich alloys (ε_M^Si) derived from the new experiments are in good agreement with previously proposed values at similar pressures below 5 GPa. However, ε_M^Si values obtained for C-free systems decrease within the 1 to 11 GPa range, suggesting extrapolation of lower-pressure parameters may yield erroneous results at much higher pressures. Altogether, the new results provide an extensive experimental foundation for future studies of planetary differentiation under (highly) reduced conditions.

¹Edwin S.Kite,¹Mohit Melwani Daswani
Earth and Planetary Science Letters 524, 1157118 Link to Article [https://doi.org/10.1016/j.epsl.2019.115718]
¹University of Chicago, Chicago, IL, USA
Copyright Elsevier

Ancient hydrology is recorded by sedimentary rocks on Mars. The most voluminous sedimentary rocks that formed during Mars’ Hesperian period are sulfate-rich rocks, explored by the Opportunity rover from 2004–2012 and soon to be investigated by the Curiosity rover at Gale crater. A leading hypothesis for the origin of these sulfates is that the cations were derived from evaporation of deep-sourced groundwater, as part of a global circulation of groundwater. Global groundwater circulation would imply sustained warm Earthlike conditions on Early Mars. Global circulation of groundwater including infiltration of water initially in equilibrium with Mars’ CO₂ atmosphere implies subsurface formation of carbonate. We find that the CO₂ sequestration implied by the global groundwater hypothesis for the origin of sulfate-rich rocks on Mars is 30–5000 bars if the Opportunity data are representative of Hesperian sulfate-rich rocks, which is so large that (even accounting for volcanic outgassing) it would bury the atmosphere. This disfavors the hypothesis that the cations for Mars’ Hesperian sulfates were derived from upwelling of deep-sourced groundwater. If, instead, Hesperian sulfate-rich rocks are approximated as pure Mg-sulfate (no Fe), then the CO₂ sequestration is 0.3–400 bars. The low end of this range is consistent with the hypothesis that the cations for Mars’ Hesperian sulfates were derived from upwelling of deep-sourced groundwater. In both cases, carbon sequestration by global groundwater circulation actively works to terminate surface habitability, rather than being a passive marker of warm Earthlike conditions. Curiosity will soon be in a position to discriminate between these two hypotheses. Our work links Mars sulfate cation composition, carbon isotopes, and climate change.

¹G.Jeffrey Taylor,¹Linda M.V.Martel,¹Paul G.Lucey,¹Jeffrey J.Gillis-Davis,²David F.Blake,³PhilippeSarrazin
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.046]
¹Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii, USA
²NASA Ames Research Center, Moffett Field, California, USA
³SETI Institute, Mountain View, California, USA
Copyright Elsevier

We used X-Ray Diffraction (XRD) and Rietveld refinement to determine the modal mineralogy of 118 lunar regolith samples (<150 µm size fraction) from all landed Apollo missions. Data were calibrated with mineral mixtures and compared to results based on an X-ray digital imaging procedure for six soils obtained by the Lunar Soil Characterization Consortium. Agreement between XRD and digital imaging for all minerals detectable in the six soils is excellent (R²=0.953). XRD-based ternary plots (plagioclase-total pyroxene-olivine) vary from plagioclase-dominated (highlands as represented by Apollo 16 samples) to substantial mafic abundances at the mare sites. Olivine varies in relative abundance, with the Apollo 17 mare sites having the largest abundances. Olivine reaches 20 wt% at Apollo 17, but is a minor component at Apollo 14. The results agree with trends in mineral abundances obtained from reflectance spectroscopy for the Apollo sites. In a global context, however, the spectral data display a trend of increasing olivine at roughly constant pyroxene/plagioclase, reaching values of 40% olivine in the plagioclase-pyroxene-olivine ternary plot (e.g., Eratosthenian flows in Procellarum), indicating the presence of significant volumes of olivine-rich rock types on unsampled regions of the lunar surface.

¹Zachary A.Torrano,²Gregory A.Brennecka,³Curtis D.Williams,¹Stephen J.Romaniello,¹Vinai K.Rai,¹Rebekah R.Hines,¹Meenakshi Wadhwaa
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.051]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85287, USA
²Institut für Planetologie, University of Münster, Münster, Germany
³Department of Earth and Planetary Sciences, University of California Davis, Davis, CA 95616, USA
Copyright Elsevier

Calcium-aluminum-rich inclusions (CAIs) are the first solids to form in the early Solar System, and they exhibit nucleosynthetic anomalies in many isotope systems. The overwhelming majority of isotopic data for CAIs has been limited to inclusions from the CV chondrite Allende and a select few other CV, CO, CM, and ordinary chondrites. It is therefore important to ascertain whether previously reported values for CAIs are representative of the broader CAI-forming region and to make a more rigorous assessment of the extent and implications of isotopic heterogeneity in the early Solar System. Here, we report the mass-independent Ti isotopic compositions of a suite of 23 CAIs of diverse petrologic and geochemical types, including 11 from Allende and 12 from seven other CV3 and CK3 chondrites; the data for CAIs from CK chondrites represent the first reported measurements of Ti isotope compositions of refractory inclusions from this meteorite class. The resolved variation in the mass-independent Ti isotopic compositions of these CAIs indicates that the CAI-forming region of the early Solar System preserved isotopic variability at their time of formation. Nevertheless, the range of Ti isotope compositions reported here for CAIs from CV and CK chondrites falls within the range observed in previously analyzed CAIs from CV, CO, CM, and ordinary chondrites. This implies that CAIs from CV, CK, CO, CM, and ordinary chondrites originated from a common nebular source reservoir characterized by mass-independent isotopic variability in Ti (and other select elements). We further interpret these data to indicate that the Ti isotope anomalies in CAIs represent the isotopic signatures of supernova components in presolar grains that were incorporated into the Solar System in an initially poorly mixed reservoir that was progressively homogenized over time. We conclude that the differing degrees of isotopic variability observed for different elements in normal CAIs are the result of distinct carrier phases and that these CAIs were likely formed towards the final stages of homogenization of the large-scale isotopic heterogeneity that initially existed in the solar nebula.

¹KeZhu朱柯,¹Paolo A.Sossi,¹Julien Siebert,^1,2FrédéricMoynier
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.043]
¹Institut de Physique du Globe de Paris, Université de Paris, CNRS, 1 rue Jussieu, Paris 75005, France
²Institut Universitaire de France, Paris, France
Copyright Elsevier

Although Solar System bodies exhibit large variations in their volatile element abundances, the mechanisms and conditions that lead to these variations remain ambiguous. The howardite-eucrite-diogenite (HED) meteorites that likely sample the asteroid 4 Vesta, provide evidence for extensive volatile depletion on their parent body. Isotopic variations in moderately volatile elements, such as Zn, have been used to track the origin of such volatile loss. Although not nominally volatile, Cr is useful because it has several oxidized gas species that render it volatile under the oxidizing conditions that characterize planetary accretion. As such, volatile loss of Cr has the potential to produce an isotopically light evaporation residue under an equilibrium regime. This contrasts with other moderately volatile elements that show heavy isotope enrichments in the residue following both kinetic or equilibrium fractionation. Here, we report the Cr stable isotope composition of 11 eucrites and four diogenites. The eucrites possess systematically lighter Cr isotope compositions than diogenites, which is onset by the accumulation of isotopically heavy Cr³⁺-rich orthopyroxene and spinel in diogenites during their magmatic evolution. We estimate for the primary eucrite melt with Mg# ≈ 50, a δ⁵³Cr (⁵³Cr/⁵²Cr deviation relative to NIST SRM 979 in per mile) of -0.22 ± 0.03 ‰ (2SD), lighter than any chondritic meteorite group by ∼0.1 ‰. This deficit may result from either partial melting with residual Cr³⁺-bearing phases (e.g. chromite) that retain heavy isotopes, or from vapor loss that occurred at equilibrium with a magma ocean on Vesta. Isotopic fractionation during partial melting would necessitate implausibly high Cr contents in the Vestan mantle, and oxygen fugacities high enough to stabilize chromite in the mantle source. Isotopic fractionation during evaporation would require an oxidized vapor and a reduced residue, as predicted by thermodynamic constraints on the composition of the vapor phase above a silicate magma ocean. Therefore, this Cr isotopic deficit between Vesta and chondrites may be caused by Cr loss at relatively high oxygen fugacity in a gas phase at equilibrium with the liquid from which it evolved. Temperatures of volatile loss are estimated to be lower than 2300 K, consistent with loss from a large-scale magma ocean model for formation of Vesta, which may be a common evolutionary stage in accreting planetesimals.

^1,2Noun, M. et al. (>10)
Life 9, 44 Link to Article [DOI: 10.3390/life9020044]
¹Institut de Physique Nucléaire d’Orsay, UMR 8608, CNRS/IN2P3, Université Paris-Sud, Université Paris-Saclay, Orsay, F-91406, France
²Lebanese Atomic Energy Commission, NCSR, Beirut, 11-8281, Lebanon

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^1,2Neveu, M.,³Vernazza, P.
Astronophysical Journal 875, 30 Link to Article [DOI: 10.3847/1538-4357/ab0d87]
¹University of Maryland, 4296 Stadium Dr., College Park, MD 20742, United States
²NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20770, United States
³Aix-Marseille Universite, CNRS, Laboratoire d’Astrophysique de Marseille, 38 Rue Frederic Joliot Curie, Marseille, F-13013, France

The parent bodies of ordinary chondrites, carbonaceous CM chondrites, and interplanetary dust particles (IDPs) represent most of the mass of the solar system’s small (D ≤ 250 km) bodies. The times of formation of the ordinary and carbonaceous CM chondrite parent bodies have previously been pinpointed, respectively, to ≈2 and 3–4 million years after calcium–aluminum-rich inclusions (CAIs). However, the timing of the formation of IDP parent bodies such as P- and D-type main-belt asteroids and Jupiter Trojans has not been tightly constrained. Here, we show that they formed later than 5–6 million years after CAIs. We use models of their thermal and structural evolution to show that their anhydrous surface composition would otherwise have been lost due to melting and ice-rock differentiation driven by heating from the short-lived radionuclide ²⁶Al. This suggests that IDP-like volatile-rich small bodies may have formed after the gas of the protoplanetary disk dissipated and thus later than the massive cores of the giant planets. It also confirms an intuitive increase in formation times with increased heliocentric distance, and suggests that there may have been a gap in time between the formation of carbonaceous chondrite (chondrule-rich) and IDP (chondrule-poor) parent bodies.

^1,2Guitreau, M.,³Flahaut, J.
Nature Communications 10, 2457 Link to Article [DOI: 10.1038/s41467-019-10382-y]
¹School of Earth and Environmental Sciences, University of Manchester, Oxford road, Manchester, M13 9PL, United Kingdom
²Université Clermont Auvergne, Laboratoire Magmas et Volcans, 6 avenue Blaise Pascal, Aubière, 63178, France
³CRPG, CNRS/Université de Lorraine, Vandœuvre-lès-Nancy, 54500, France

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^1,2Smith, K.E.,²House, C.H.,³Arevalo, R.D., Jr.,^4,5Dworkin, J.P.,^1,4,5Callahan, M.P.
Nature Communications 10, 2777 Link to Article [DOI: 10.1038/s41467-019-10866-x]
¹Department of Chemistry and Biochemistry, Boise State University, Boise, ID 83725, United States
²Department of Geosciences and Penn State Astrobiology Research Center, Pennsylvania State University, University Park, PA 16801, United States
³Department of Geology, University of Maryland, College Park, MD 20742, United States
⁴Goddard Center for Astrobiology, NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States
⁵Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States

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^1,2Maxwell M. Thiemens,^1,3Peter Sprung,¹Raúl O. C. Fonseca,^4,5Felipe P. Leitzke,¹Carsten Münker
Nature Geoscience (in Press) Link to Article [https://doi.org/10.1038/s41561-019-0398-3]
^{1Institut für Geologie und Mineralogie, Universität zu Köln, Köln, Germany
2Laboratoire G-Time, Département Géosciences, Environnement et Société, Université Libre de Bruxelles, Brussels, Belgium
3Hot Laboratory Division (AHL), Paul Scherrer Institut, Villigen, Switzerland
4Steinmann Institut, Universität Bonn, Bonn, Germany
5Isotope Geology Laboratory, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil}

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