Siderophile and chalcophile element abundances in shergottites: Implications for Martian core formation

Abstract

Elemental abundances for volatile siderophile and chalcophile elements for Mars inform us about processes of accretion and core formation. Such data are few for Martian meteorites, and are often lacking in the growing number of desert finds. In this study, we employed laser ablation inductively coupled plasma–mass spectrometry (LA‐ICP‐MS) to analyze polished slabs of 15 Martian meteorites for the abundances of about 70 elements. This technique has high sensitivity, excellent precision, and is generally accurate as determined by comparisons of elements for which literature abundances are known. However, in some meteorites, the analyzed surface is not representative of the bulk composition due to the over‐ or underrepresentation of a key host mineral, e.g., phosphate for rare earth elements (REE). For other meteorites, the range of variation in bulk rastered analyses of REE is within the range of variation reported among bulk REE analyses in the literature. An unexpected benefit has been the determination of the abundances of Ir and Os with a precision and accuracy comparable to the isotope dilution technique. Overall, the speed and small sample consumption afforded by this technique makes it an important tool widely applicable to small or rare meteorites for which a polished sample was prepared. The new volatile siderophile and chalcophile element abundances have been employed to determine Ge and Sb abundances, and revise Zn, As, and Bi abundances for the Martian mantle. The new estimates of Martian mantle composition support core formation at intermediate pressures (14 ± 3 GPa) in a magma ocean on Mars.

Siderophile Elements in Tracing Planetary Formation and Evolution

“…High MgO ultramafic rocks have the highest HSE abundances, whereas the most highly-evolved basalts with lowest MgO also have the lowest abundances of typically compatible HSE, such as Os and Ir. More recent papers by Riches et al (2011), Brandon et al (2012, Dale et al (2012) and Yang et al (2015) confirm the earlier results with many additional HSE data (Fig. 4.7).…”

“…Collectively, these observations led Brandon et al (2012) to conclude that the HSE abundances in the martian mantle are within a factor of 2 of terrestrial mantle abundances. Using a different set of data, Yang et al (2015) came to a similar conclusion.…”

Noble gases in 18 Martian meteorites and angrite Northwest Africa 7812—Exposure ages, trapped gases, and a re‐evaluation of the evidence for solar cosmic ray‐produced neon in shergottites and other achondrites

“…; Yang et al. ). Previously such specimens were termed “lherzolitic,” but this term is inappropriate on several grounds (see Nyquist et al.…”

Highly siderophile element (HSE) abundances in the mantle of Mars are due to core formation at high pressure and temperature

“…Recent work shows that 17 siderophile elements (Fe, Ni, Co, Mo, W, Mn, V, Cr, P, Ga, Cu, Sn, Ge, In, Zn, As, and Sb; Righter and Chabot ; Yang et al. ) in the Martian mantle can be explained as pressure attains approximately 14 GPa, so this is a relevant pressure to examine here. During such an accretion process and at these PT conditions, the calculated HSE mantle abundances (and corresponding D metal/silicate values) are: Au = 0.63 ppb (1000), Pd = 4.22 ppb (600), Pt = 4.98 ppb (900), Re = 0.05 ppb (3200), Ru = 1.8 ppb (1800), Os = 1.8 ppb (1000), and Ir = 2.2 ppb (1000) (Fig.…”