Valerie J. Hillgren

High pressure and high temperature metal-silicate partitioning of siderophile elements: The importance of silicate liquid composition

We have determined liquid metal-liquid silicate partition coefficients for Ni, Co, Mo, W, P, Ga, and Ge at 10 GPa and 2000°C in a multi-anvil apparatus. Our sample containers were either MgO or Al2O3. The two different capsule materials imposed different redox states on our samples as well as producing very different silicate liquid compositions. The samples run in Al2O3 containers were at an fO2 1.1–1.4 log units below the iron-wurstite (IW) buffer, and samples run in the MgO containers were 2.3 log units below the IW buffer. Although the partition coefficients we determine for Ni, Co, Mo, and W agree well with our previous work, we have obtained some rather surprising results. In the experiments reported here the partition coefficients for W, P, and Ge decrease with decreasing oxygen fugacity which is opposite to what is expected. We also find that partition coefficients for Mo and Ga do not increase as much as expected with decreasing oxygen fugacity, and although Ni and Co behave in a more normal manner, their partition coefficients also do not increase as much as expected with decreasing oxygen fugacity as a +2 valence for the two elements would predict. Simple analysis of the structures of the two different silicate melts shows that the MgO-rich melt is considerably more depolymerized than the Al2O3-rich melt. The partitioning behavior of all the elements indicates that all the cations are stabilized in the more depolymerized MgO-rich melt. This is in accord with previous work on compositional effects on forsterite-liquid partitioning and the partitioning of metal cations between immiscible silicate liquids. Our results emphasize the need for thorough investigation of all variables that could potentially effect partitioning including the composition of the silicate melt. In addition, they suggest that mantle composition may play an important role during core formation.

A Synthesis of Instrumental Analytical Techniques for Examination of the Thermal History of Pallasite Meteorites

Pallasites are a unique group of meteorites consisting of a mixture of approximately equal proportions of olivine grains in a matrix of Fe-Ni metal. These meteorites provide physical samples of the interior of a differentiated planetary body. As such, they provide direct geochemical clues to planetary differentiation processes. In this study, one large collaborative effort is undertaken to analyze a suite of 14 main-group pallasite specimens by several different instrumental analytical techniques in an attempt to provide a comprehensive picture of formation processes, temperatures and timescales. The suite of specimens chosen come from both the main group pallasites and the Eagle Station trio, and encompass a range of fayalite composition and of olivine shape and distribution. Samples have been optically imaged and then mapped EDS using a JEOL JSM-6500F field-emission SEM equipped with an Oxford X-Max 80 mm 2 silicon drift detector to determine major element composition and distribution. The Oxford AZtec software allows large-format montaged mapping so that the entire specimen can be mapped to the same scale. Smaller region EDS mapping highlights minor mineral phases and the interfaces between the mineral grains and the metal phases. These maps quantitatively verify the mineral phases present and the Fe/Mg ratio of the olivine phase and the Fe/Ni ratio of the metal phase. Electron microprobe WDS analyses of mineral phases are being done on a JEOL 8900 to verify major and minor element composition in regions of interest. These analyses also highlight areas that are targeted for focused ion beam (FIB) analysis. The FIB is used to examine the interface between olivine and metal and create a 3-D reconstruction. Nano-scale phases are then highlighted for future TEM preparation. Between the SEM and microprobe results, areas were highlighted for mass spectrometry analysis. Several samples were analyzed with laser ablation ICPMS for trace element content. Pallasites can provide information about highly-siderophile elements (HSE) present in both mantle and core of differentiated bodies. Many differentiated planetary bodies show enrichment in HSE concentration in their mantle relative to their core. Our study indicates that both HSE concentrations and the O-isotope fractionation share a common trend with iron III-AB meteorites. The HSE concentrations are also comparable to Mars and the Angrite parent body. In addition, diffusion profiles are being obtained in the microprobe for both the metal and olivine phases, which indicate the cooling rates of these meteorites. The iron metal region contains regions of taenite interlaced with kamacite, in which Ni zonation and diffusion has traditionally been used to determine the metallographic cooling rate within the taenite [1]. More recently, zonation of the olivine rim has been used to determine cooling rates [2], however these indicate a much faster cooling rate than that estimated by the metallic rates. This study aims to examine diffusion in both phases to better