Experimentally determined subsolidus metal-olivine element partitioning with applications to pallasites
Patrick H. Donohue, Eddy Hill, Gary R. Huss
Geochimica et Cosmochimica Acta,
Volume 222, 1 February 2018, Pages 305-318
“Pallasite meteorites, which consist primarily of olivine and metal, may be remnants of disrupted core-mantle boundaries of differentiated asteroids or planetesimals. The early thermal histories of pallasites are potentially recorded by minor- and trace-element zonation in olivine. However, constraining this history requires knowledge of element behavior under the conditions of pallasite formation, which is lacking for many of the main elements of interest (e.g., Co, Cr, Mn). In this study, we experimentally determined metal/olivine partition coefficients for Fe, Ni, Co, Cr, and Mn in a pallasite analogue at subsolidus temperatures. Metal/olivine partition coefficients (KM) increase in the order KMn << KCr << 1 << KFe << KCo << KNi, with five orders of magnitude separating KMn from KNi. Transition metals also become more siderophile with increasing experimental temperature (900–1550 °C). The experiments incidentally produced diffusion profiles in olivine for these elements; our results suggest they diffuse through olivine at similar rates. Core compositions of pallasite olivines are consistent with high-temperature equilibration with FeNi-metal. Olivine zonation toward crystal rims varies significantly for the investigated transition metals. We suggest rim zonation results from partial re-equilibration during late stage crystallization of minor phases (e.g., chromite, phosphates). This re-equilibration occurred over short timescales relative to overall pallasite cooling, likely tied to initial cooling rates on the order of 100–300 °C/Myr."