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

Matzen, Andrew K. and Woodland, Alan and Beckett, John R. and Wood, Bernard J. (2022) Oxidation state of iron and Fe-Mg partitioning between olivine and basaltic martian melts. American Mineralogist, 107 (7). pp. 1442-1452. ISSN 0003-004X. doi:10.2138/am-2021-7682.

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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(X_(FeO_(1.5))^(melt)/X_(FeO)^(melt)) 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, K)(D,Fe²⁺-Mg) = (FeO/MgO)^(Ol)/(FeO/MgO)^(liq) (by weight), suggests a K_(D,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 K_(D,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 K_(D,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.

Item Type:Article
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URLURL TypeDescription ItemDeposit item AM-22-77682
Wood, Bernard J.0000-0003-1038-7301
Additional Information:© 2022 by the Mineralogical Society of America. received: 18 Jun 2020. accepted: 07 Jul 2021. first online: 01 Jul 2022. We thank Victoria Smith for her efforts to run and maintain the electron microprobe laboratory, Steven Wyatt for hood space and safe HF handling, and Jamie Long, and all the other workshop technicians, who help to keep the experimental laboratory running. We also thank Maryjo Brounce, Elizabeth Cottrell, and Glenn Gaetani for thoughtful and through reviews. This research began with support from ERC Advanced Grant 267764 and was completed with support from STFC (U.K.) grant ST/R000999/1.
Funding AgencyGrant Number
European Research Council (ERC)267764
Science and Technology Facilities Council (STFC)ST/R000999/1
Subject Keywords:Olivine-phyric shergottites, Fe²⁺-Mg partitioning, olivine, basalt, Mars
Issue or Number:7
Record Number:CaltechAUTHORS:20220712-629335000
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:115506
Deposited By: George Porter
Deposited On:13 Jul 2022 21:34
Last Modified:13 Jul 2022 21:34

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