Oxidation state of iron and Fe-Mg partitioning between olivine and basaltic martian melts

1Andrew K. Matzen,2Alan Woodland,3John R. Beckett,1Bernard J. Wood
American Mineralogist 107, 1442-1452 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2022/Abstracts/AM107P1442.pdf]
1Department of Earth Sciences, University of Oxford, Oxford, OX1 3AN, U.K.
2Goethe-Universität Institut für Geowissenschaften, Frankfurt, D-60438, Germany
3California Institute of Technology, MC 170-25, Pasadena, California 91125, U.S.A.
Copyright: The Mineralogical Society of America

We performed a series of experiments at 1 atm pressure and temperatures of 1300–1500 °C to
determine the effect of oxygen fugacity on the oxidation state of Fe in a synthetic martian basalt. Ferricferrous ratios were determined on the quenched glasses using Mössbauer spectroscopy. Following the conventional doublet assignments in the spectrum, we obtain a Fe3+/ΣFe value of 0.19 at 1450 °C and
an oxygen fugacity corresponding to the QFM buffer. If we apply the Berry et al. (2018) assignments
the calculated Fe3+/ΣFe drops to 0.09, and the slope of log(XFe melt O1.5/XFe melt O) vs. log( fO2) changes from 0.18 to 0.26.
Combining oxidation state data together with results of one additional olivine-bearing experiment
to determine the appropriate value(s) for the olivine (Ol)-liquid (liq) exchange coefficient, KD,Fe2+-Mg
= (FeO/MgO)Ol/(FeO/MgO)liq (by weight), suggests a KD,Fe2+-Mg of 0.388 ± 0.006 (uncertainty is one
median absolute deviation) using the traditional interpretation of Mössbauer spectroscopy and a value
of 0.345 ± 0.005 following the Mössbauer spectra approach of Berry et al. (2018).
We used our value of KD,Fe2+-Mg to test whether any of the olivine-bearing shergottites represent liquids.
For each meteorite, we assumed a liquid composition equal to that of the bulk and then compared that
liquid to the most Mg-rich olivine reported. Applying a KD,Fe2+-Mg of ~0.36 leads to the possibility that
bulk Yamato 980459, NWA 5789, NWA 2990, Tissint, and EETA 79001 (lithology A) represent liquids.


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