^1,2,4Tomohiro Usui,^2,4Kevin Righter,^3,4Charles K. Shearer,⁴John H. Jones
American Mineralogist 107, 357–368 Link to Article [DOI: https://doi.org/10.2138/am-2021-7743]
¹Institute of Space and Astronautical Sciences, Japan Aerospace Exploration Agency, Kanagawa, 252-5210, Japan
²NASA Johnson Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, Texas 77058, U.S.A.
³Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
⁴Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, U.S.A.
Copyright: The Mineralogical Society of America

Ni and Co variations in primary martian magmas exhibit anomalous incompatible behavior, which
has remained an unexplained conundrum. Because martian magmas are S-rich, and some trace metals
are reported to have enhanced solubility in S-bearing magmas, we have carried out a series of experi–
ments to evaluate the effect of high-S melts on the olivine/melt partitioning of Ni, Co, Mn, V, and Cr.
Near-liquidus experiments on a synthetic primary martian mantle melt (Yamato-980459 [Y98]) were
completed in a piston-cylinder apparatus at 0.75GPa. Previous studies in S-free systems illustrate that
the partition coefficients for these elements are dependent chiefly on DMg(Ol/melt) (the partition coefficient
defined as wt% Mg in olivine/wt% Mg in melt, a proxy for temperature), and were used to calibrate a
predictive expression that includes the effects of temperature [i.e., DMg(Ol/melt)], melt composition, and
oxygen fugacity. These predictive expressions are then used to isolate any effect in DM olivine/melt
due to dissolved sulfur. The results show that S might have a small effect for Co, but not enough to
change Co partitioning from compatible to incompatible in our experiments. The addition of a sulfur
term to the DCo predictive expressions shows that nearly 8000 ppm of sulfur would be required in the
melt (at liquidus temperature of Y98) for DCo to become <1. These S contents are two times higher
than those of a sulfide-saturated melt at the P–T conditions of a martian mantle source region. Therefore,
the anomalous incompatible behavior observed in these primary magma suites must be due to another
mechanism. High temperature, oxygen fugacity, and diffusion are not viable mechanisms, but magma
mixing, assimilation, or kinetic crystallization effects remain possibilities.