^1,2,3E.S.Steenstra,³J.Berndt,³A.Rohrbach,¹E.SBullock,²W.van Westrenen,³S.Klemme,¹M.J.Walter
Geochimica et Cosmochmica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.08.021]
¹Earth and Planets Laboratory, Carnegie Institution for Science, Washington D.C, U.S.A
²Faculty of Science, Vrije Universiteit Amsterdam, the Netherlands
³Institute of Mineralogy, University of Münster, Germany
Copyright Elsevier

The abundances of highly siderophile elements (HSE) in planetary mantles and achondrites potentially provide important constraints on several aspects of planet formation, including the nature and composition of late accreted materials. Here, we experimentally and systematically assess the distribution of the HSE between silicate melts, sulfide and/or metal liquids at the highly to moderately reduced conditions thought to have characterized Earth accretion. The results show that the chalcophile behavior of all elements, except for Re, is strongly decreased at low FeO and/or high S concentrations in the silicate melt. There are considerable differences between how FeO and/or S contents of the silicate melt affect the D values of the various HSE, with the largest effects observed for Pd, Pt, Ir and Au. If liquid metal is Si-rich and S-poor, the siderophile behavior of the HSE mimics that in the presence of sulfide liquids, but with an offset due to differences in HSE activities in metal and sulfide liquids.

Using our new experimental data, we quantify the relative effects of O in sulfide and S in silicate melt on the sulfide liquid-silicate melt partitioning behavior of the HSE using a thermodynamic approach. The resulting expressions were used to model the distribution of the HSE in highly reduced and differentiated EH- and EL chondritic parent bodies and during differentiation of the aubrite parent body. Our results show that even with their strongly decreased chalcophile and siderophile behavior at highly reduced conditions, HSE abundances in the mantles of these parent bodies remain extremely low. However, if such bodies accreted to Earth, any residual metal present in the parent body mantle and subsequently retained in Earth’s mantle would dramatically affect HSE abundances and produce chondritic ratios, making it impossible to track the potential accretion of a large reduced impactor to the BSE using HSE abundance systematics. In terms of the aubrite parent body, our results confirm previous hypotheses related to the importance of (un)differentiated core forming metals in establishing the HSE contents of unbrecciated aubrites. Finally, our results confirm that sulfides are likely a minor source of HSE abundances in aubrites, particularly for Re, consistent with sample observations.

¹Jérôme Esvan,²Gilles Berger,³Sébastien Fabre,⁴Eric Bêche,¹Yannick Thébault,²Alain Pages,¹Cédric Charvillat
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.08.017]
¹CIRIMAT, CNRS-UPS-INPT, ENSIACET, 4 allée Emile Monso, 31030 Toulouse, France
²IRAP, CNRS, Observatoire Midi-Pyrénées, 14 av. Edouard Belin, 31400 Toulouse, France
³IRAP, Université Paul Sabatier, Observatoire Midi-Pyrénées, 14 av. Edouard Belin, 31400 Toulouse, France
⁴PROMES, PCM-ASI-CNRS, 7 rue du Four Solaire, 66120 Font-Romeu, France
Copyright Elsevier

Extreme conditions encountered in some geological contexts (deep serpentinization, interaction of Venus atmosphere with its basaltic surface, volcanic degassing) activate mechanisms and rates of silicate alteration that are poorly understood. In the present study, we investigate the mechanisms of mineral reactions in a natural geological system at high temperature, under conditions where the low solvation of cations by fluids likely promotes surface reactions such as surface diffusion and/or local recrystallization. We focus on vitreous glasses and olivine, reputed to be the most alterable phases in volcanic rocks, by reacting samples for one week in a Ni-based alloy experimental vessel. For the framework of our experimental study, we chose to apply the deep atmosphere conditions on Venus: 470°C and 90 bar of reconstituted Venus-like gas. We also tested the effect of water (Early Venus or wet volcanic degassing) by adding water vapor at up to 320 bar total pressure. The mineral reactions affecting the samples were identified by a set of spectroscopic surface analyses of the altered samples: Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, X-Ray Diffraction in grazing incidence mode, X-ray Photo electron Spectroscopy and Raman spectroscopy.

Samples of obsidian and tholeiitic glasses are found to be sensitive to a threshold water pressure, depending on glass composition, below which the reaction is limited to some elemental mobility in the glass (alkali enrichment, calcium loss) leading to a possibly more stable surface layer of tens to hundreds of microns. Above this threshold water pressure (ca. 50 bar H2O for the obsidian but >250 bar H2O for the tholeiitic glass), water promotes the depolymerization of the glass and the crystallization of stable minerals. This crystalline rim is less protective that the chemically modified layer.

Olivine samples react differently depending on whether the olivine is isolated or included in a basaltic rock. In the latter case only, iron coatings are formed, which are identified as hematite, suggesting that this phase is not fed by olivine itself but rather by surface diffusion from neighboring Fe-rich phases. This supports the conclusions from experimental studies and orbital observations on the short-term visibility of unaltered olivine in Venus lava flows: such a coating is enhanced when Fe-bearing minerals are in the proximity of olivine. Under high water vapor pressure, Fe-bearing talc (and not serpentine) forms by a likely topotactic reaction that also incorporates silica from the gas. This talc layer may form a protective layer, implying that serpentinization of ultramafic rocks at high temperature may not be as prevalent as one might think in a gas-dominated system like the Early Venus surface.