¹K. Righter, ²L. R. Danielson, ²K. M. Pando, ³J. Williams, ³M. Humayun, ⁴R. L. Hervig, ⁴T. G. Sharp4
¹Mailcode KT, NASA Johnson Space Center, Houston, Texas, USA
²Jacobs Technology, JETS, NASA Johnson Space Center, Houston, Texas, USA
³National High Magnetic Field Laboratory and Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida, USA
⁴ASU School of Earth and Space Exploration, Tempe, Arizona, USA

Highly siderophile elements (HSEs) can be used to understand accretion and core formation in differentiated bodies, due to their strong affinity for FeNi metal and sulfides. Coupling experimental studies of metal–silicate partitioning with analyses of HSE contents of Martian meteorites can thus offer important constraints on the early history of Mars. Here, we report new metal–silicate partitioning data for the PGEs and Au and Re across a wide range of pressure and temperature space, with three series designed to complement existing experimental data sets for HSE. The first series examines temperature effects for D(HSE) in two metallic liquid compositions—C-bearing and C-free. The second series examines temperature effects for D(Re) in FeO-bearing silicate melts and FeNi-rich alloys. The third series presents the first systematic study of high pressure and temperature effects for D(Au). We then combine our data with previously published partitioning data to derive predictive expressions for metal–silicate partitioning of the HSE, which are subsequently used to calculate HSE concentrations of the Martian mantle during continuous accretion of Mars. Our results show that at midmantle depths in an early magma ocean (equivalent to approximately 14 GPa, 2100 °C), the HSE contents of the silicate fraction are similar to those observed in the Martian meteorite suite. This is in concert with previous studies on moderately siderophile elements. We then consider model calculations that examine the role of melting, fractional crystallization, and sulfide saturation/undersaturation in establishing the range of HSE contents in Martian meteorites derived from melting of the postcore formation mantle. The core formation modeling indicates that the HSE contents can be established by metal–silicate equilibrium early in the history

Reference
Righter K, Danielson LR, Pando KM, Williams J, Humayun M, Hervig RL, Sharp TG (2015) Highly siderophile element (HSE) abundances in the mantle of Mars are due to core formation at high pressure and temperature. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12393]

Published by arrangement with John Wiley&Sons

^1,2Shuying Yang, ^1,2Munir Humayun, ³Kevin Righter, ^1,4Gwendolyn Jefferson, ^1,5Dana Fields, ⁶Anthony J. Irving
¹National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida, USA
²Department of Earth, Ocean & Atmospheric Science, Florida State University, Tallahassee, Florida, USA
³National Aeronautics and Space Administration, Johnson Space Center, Houston, Texas, USA
⁴Carter High School, Rialto, California, USA
⁵Rickards High School, Tallahassee, Florida, USA
⁶Department of Earth & Space Sciences, University of Washington, Seattle, Washington, USA

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.

Reference
Yang S, Humayun M, Righter K, Jefferson G, Fields D, Irving AJ (2015) Siderophile and chalcophile element abundances in shergottites: Implications for Martian core Formation. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12384]

Published by Arrangement with John Wiley&Sons

¹Miriam Sharp, ²Kevin Righter,¹Richard J. Walker
¹Department of Geology, University of Maryland, College Park, Maryland, USA
²NASA Johnson Space Center, Houston, Texas, USA

This study uses experimentally determined plagioclase-melt D values to estimate the trace element concentrations of Sr, Hf, Ga, W, Mo, Ru, Pd, Au, Ni, and Co in a crystallizing lunar magma ocean at the point of plagioclase flotation. Similarly, experimentally determined metal-silicate partition experiments combined with a composition model for the Moon are used to constrain the concentrations of W, Mo, Ru, Pd, Au, Ni, and Co in the lunar magma ocean at the time of core formation. The metal-silicate derived lunar mantle estimates are generally consistent with previous estimates for the concentration of these elements in the lunar mantle. Plagioclase-melt derived concentrations for Sr, Ga, Ru, Pd, Au, Ni, and Co are also consistent with prior estimates. Estimates for Hf, W, and Mo, however, are higher. These elements may be concentrated in the residual liquid during fractional crystallization due to their incompatibility. Alternatively, the apparent enrichment could reflect the inappropriate use of bulk anorthosite data, rather than data for plagioclase separates.

Reference
Sharp M, Righter K, Walker RJ (2014) Estimation of trace element concentrations in the lunar magma ocean using mineral- and metal-silicate melt partition coefficients. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12396]

Published by arrangement with John Wiley&Sons