Geochemical constraints on core-mantle differentiation in Mercury and the aubrite parent body

1,2,3E.S.Steenstra,2W.van Westrenen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113621]
1The Geophysical Laboratory, Carnegie Institution of Science, Washington, DC, USA
2Faculty of Science, Vrije Universiteit Amsterdam, the Netherlands
3Institute of Mineralogy, University of Münster, Germany
Copyright Elsevier

Differentiation of Mercury and the aubrite parent body (AuPB) at extremely reducing conditions is implied from the low FeO and high S contents of their mantles. Here, the mantle abundances of these elements as derived from remote sensing (in the case of Mercury) and aubrite meteorite analysis are used in conjunction with new experimentally determined metal-silicate partition coefficients at reducing conditions presented in Steenstra et al. (2020a) to quantify the geochemical consequences of core formation in highly reduced planetary bodies. Plausible core compositions of Mercury and the AuPB are assessed, and the distribution of Si and other elements during core formation are quantified.

Combining the new results with previous remote sensing observations of the surface of Mercury it is found that its core is likely to be Si-rich (>4.5–21 wt%). The estimated core Si contents depend mostly on the assumption of C saturation in Mercury’s mantle and the type of bulk composition considered. Calculations for C-free conditions also yield significant quantities of Si in the AuPB core (>6–20 wt%, depending on bulk composition and redox state). The amount of Si in the AuPB core is greatly decreased if C-saturation is assumed.

The new experimental data and related thermodynamic parameterization for predicting the C contents at graphite saturation of FeSi alloys (CCGS) shows that Mercury’s mantle was likely C-saturated following formation of a Si-rich core, allowing for the separation of a graphitic flotation crust. Due to the lower Si contents calculated for the AuPB core under C-saturated conditions, a graphitic flotation crust is unlikely to form in smaller-sized reduced asteroids such as the AuPB.

The Si-rich nature of the AuPB core for C-undersaturated scenarios is expected to have resulted in preferential partitioning of the majority of the volatile siderophile elements (VSE) into the AuPB core. However, depletions for the most volatile VSE cannot be reconciled with core formation depletion only. Their depletions in aubrites require additional depletion from (I) segregating sulfide liquids during AuPB differentiation and/or (II) compatible behavior during mineral-melt fractionation and/or (III) loss of these elements in a vapor phase during and/or after AuPB differentiation.

An experimental assessment of the chalcophile behavior of F, Cl, Br and I: implications for the fate of halogens during planetary accretion and the formation of magmatic ore deposits

1,2,3E.S.Steenstra,2F.van Haaster,2R.van Mulligen, 3S.Flemetakis, 3J.Berndt, 3S.Klemme,2W.van Westrenen
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.006]
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
Copyright Elsevier

The elemental and isotopic abundances of halogens (F, Cl, Br, I) are used to constrain planetary volatile loss and volatile delivery processes, but their behavior during magmatic differentiation in general, and sulfide liquid segregation in particular, is currently not well constrained. To test whether sulfide liquid segregation could affect halogen behavior during magmatic processes, we performed high-pressure experiments to systematically quantify the sulfide liquid – silicate melt partition coefficients (Dsulliq-silmelt values, defined as the ratio between the wt.% concentration of the halogen in the sulfide liquid and silicate melt, respectively) of F, Cl, Br and I at a pressure of 1 GPa and temperatures of 1683–1883 K.

Results show that dry-polishing target surfaces is crucial for obtaining representative halogen concentrations of sulfide liquids. The results also show that no appreciable amounts of F partition into sulfide liquids, whereas Cl, Br and I behave increasingly chalcophile with increasing atomic radius (i.e.,DFsulliq-silmelt < DClsulliq-silmelt < DBrsulliq-silmelt < DIsulliq-silmelt ), presumably as a result of an increasingly covalent nature of Fe-halogen bonds with increasing radius. This results in I behaving chalcophile (DIsulliq-silmelt >1) in several experiments. In contrast to previous observations, DCl/Brsulliq-silmelt was found to be <1. The DCl,Br,Isulliq-silmelt predominantly vary with sulfide liquid melt composition, showing an increase with increasing O in the sulfide liquid, which itself is correlated with more oxidizing conditions (i.e., higher fO2 ) or silicate melt FeO contents. The DCl,Br,Isulliq-silmelt values remain constant and/or potentially decrease again at the highest O concentrations of the sulfide liquids in this study (∼2.5 wt.% O).

Results indicate that the magnitude of halogen depletions in the terrestrial, martian and lunar mantle are not strongly affected by segregation of sulfide liquids during their accretion, given the expected low modal abundance of sulfide liquids and/or relatively low DCl,Br,Isulliq-silmelt values. Core formation remains the most important process in establishing iodine depletion in the terrestrial mantle, whereas volatility-related loss seems most likely for F, Cl, Br and I, in case of the martian mantle. However, segregation of sulfide liquids during accretion could have resulted in a relative increase of the offset between the mantle depletions of the lighter and heavier halogens. The experimental results confirm the previously proposed feasibility of sulfide liquids as reservoirs for halogens in magmatic sulfide ore environments. As proposed by Mungall and Brenan (2003), fractional crystallization of these sulfide liquids in the absence of a silicate melt can lead to the formation of halide melts or fluids, consistent with the association between halide minerals and magmatic sulfide ores in some localities.

Comprehensive Investigation of Some Ordinary Chondrites Based on X-Ray Methods and Mössbauer Spectroscopy

1Guda, L.V.,1Kravtsova, A.N.,1Guda, A.A.,2Kubrin, S.P.,3Mazuritskiy, M.I.,1Soldatov, A.V.
Journal of Surface Investigation 13, 995-1004 Link to Article [DOI: 10.1134/S1027451019060089]
1Smart Materials Research Institute, Southern Federal University, Rostov-on-Don, 344090, Russian Federation
2Research Institute of Physics, Southern Federal University, Rostov-on-Don, 344090, Russian Federation
3Physics Faculty, Southern Federal University, Rostov-on-Don, 344090, Russian Federation

 

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An unrevealed treasure: a new Italian meteorite from the Royal Mineralogical Museum of Naples

1Cecchi, V.M.,2Rossi, M.,2,3Ghiara, M.R.,4Pratesi, G.,4Franza, A.
Geology Today 35, 212-216 Link to Article [DOI: 10.1111/gto.12293]
1Natural History Museum-SMA, University of Florence, Florence, 50121, Italy
2Department of Earth, Environmental and Resources Sciences, University of Naples Federico II, Naples, 80126, Italy
3Centre of Natural and Physical Sciences, University of Naples Federico II, Naples, 80134, Italy
4Department of Earth Sciences, University of Florence, Florence, 50121, Italy

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Distribution of Germanium and Other Elements in Samples of the Chelyabinsk Meteorite, Determined via Scanning Synchrotron Radiation X-ray Fluorescence Microanalysis

1Darin, F.A.,1Rakshun, Y.V.,1Sorokoletov, D.S.,2Darin, A.V.,2,3Rashchenko, S.V.,2,4Sharygin, V.V.,5Senin, R.A.,5Gogin, A.A.
Bulletin of the Russian Academy of Sciences: Physics 83, 1433-1436 Link to Article [DOI: 10.3103/S1062873819110078]
1Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090, Russian Federation
2Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090, Russian Federation
3Novosibirsk National Research State University, Novosibirsk, 630090, Russian Federation
4Yeltsin Ural Federal University, Yekaterinburg, 620075, Russian Federation
5National Research Center Kurchatov Institute, Moscow, 123098, Russian Federation

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Characterization of a newly fallen Nigerian meteorite

1Gismelssed, A.,2Okunlola, O.,1Al-Rawas, A.,1Yousif, A.,2Oyedokun, M.,3Adetunji, J.,1Widatallah, H.,1Elzai, M.
Hyperfine Interactions 241, 22 Link to Article [DOI: 10.1007/s10751-019-1683-7]
1Physics Department, College of Science, Sultan Qaboos university, Muscat, Oman
2Department of Geology University of Ibadan, Ibadan, Nigeria
3School of Environmental Sciences, University of Derby, Kedleston Road, Derby, DE22 1GB, United Kingdom

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