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Abstract

The intracrystalline distribution and intercrystalline partitioning of the trace elements P, Sc, Co and Zn, of eight peridotitic mantle-derived xenoliths, from the French Massif Central, Saudi Arabia and Kimberly in South Africa were investigated using spatially resolved secondary ion mass spectrometry (SIMS). The major and minor elements of the mineral phases, namely olivine, orthopyroxene, clinopyroxene and, depending on their origin, spinel or garnet, were determined in advance by spatially resolved electron probe micro analysis (EPMA). In spite of focusing on well-equilibrated samples, high-resolution profiles show that the mineral phases rarely represent thermodynamic equilibration of their bulk mineral chemistry. Mostly, the minerals show intracrystalline heterogeneities, at least at the outermost rims of the crystals, which indicate that the mineral phases left a state of "equilibration". At best these are mineral zonings with distinctive homogeneous cores. To examine the investigated partition coefficients of the trace elements for possible pressure or temperature dependencies, the last stable pressure and temperature conditions for all samples were determined by geothermobarometric calculations. It could be shown that P greatly differs from all the trace elements analyzed. In most cases, the extremely slow diffusion of P in silicates causes insufficient equilibration. Despite P showing distinguishable temperature dependencies for the partition coefficients between olivine, orthopyroxene and clinopyroxene, it is not qualified for geothermobarometry of mantle-derived peridotitic xenoliths. In addition, the intracrystalline distribution of P in olivine seems to be sensitive to microtectonic deformation, which can provide information about the deformation processes in the Upper Mantle. The partition coefficients of Sc, Co and Zn show temperature dependencies to varying degrees. The partitioning of Co shows the strongest dependency on temperature for every possible mineral pairing. Furthermore, only pressure dependencies for the partitioning coefficients with garnet, but no influence of mineral chemistry of the partitioning of Co could be observed within the represented ranges of pressure and temperature conditions from 966 to 1209°C and 10.0 to 51.6 kbar. The divalent trace element Co has the advantage of fast diffusion in silicates and the accompanied ability for fast reeqilibration. Therefore, it is qualified for the geothermobarometric reconstruction of the last pressures and temperatures a peridotite suffered, which did not last long enough to achieve reequilibration of the remaining mineral chemistry. Zn presents itself similarly to Co, but the applied SIMS setup is not accurate enough to get comparable results as for Co. It can be assumed that, if analyzed with a specific SIMS setup, the partitioning of Zn could provide better correlations with temperature as well. In contrast, Sc shows temperature and, albeit to a lower degree, pressure dependencies. The slower diffusion of the trivalent Sc compared to Co would be suited to reconstruct older events in the peridotites history due to its higher resistance against short lived events.