No FeS layer in Mercury? Evidence from Ti/Al measured by MESSENGER

1,2,3C.Cartier,4O.Namur,5 L.R.Nittler,5S.Z.Weider,6E.Crapster-Pregont,6A.Vorburger,6E.A.Franck,1B.Charlier
Earth and Planetary Science Letters 534, 116108 Link to Article [https://doi.org/10.1016/j.epsl.2020.116108]
1Département de Géologie, Université de Liège, 4000, Sart Tilman, Belgium
2Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, 63038, France
3CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy, 54501, France
4Department of Earth and Environmental Sciences, KU Leuven, Leuven, 3001, Belgium
5Carnegie Institution of Washington, Department of Terrestrial Magnetism, Washington, DC 20015, USA
6Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
Copyright Elsevier

In this study we investigate the likeliness of the existence of an iron sulfide layer (FeS matte) at the core-mantle boundary (CMB) of Mercury by comparing new chemical surface data obtained by the X-ray Spectrometer onboard the MESSENGER spacecraft with geochemical models supported by high-pressure experiments under reducing conditions. We present a new data set consisting of 233 Ti/Si measurements, which combined with Al/Si data show that Mercury’s surface has a slightly subchondritic Ti/Al ratio of 0.035 ± 0.008. Multiphase equilibria experiments show that at the conditions of Mercury’s core formation, Ti is chalcophile but not siderophile, making Ti a useful tracer of sulfide melt formation. We parameterize and use our partitioning data in a model to calculate the relative depletion of Ti in the bulk silicate fraction of Mercury as a function of a putative FeS layer thickness. By comparing the model results and surface elemental data we show that Mercury most likely does not have a FeS layer, and in case it would have one, it would only be a few kilometers thick (<13km). We also show that Mercury’s metallic Fe(Si) core cannot contain more than ∼1.5 wt.% sulfur and that the formation of this core under reducing conditions is responsible for the slightly subchondritic Ti/Al ratio of Mercury’s surface.

 

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