Incompatible trace element partitioning and residence in anhydrous spinel peridotites and websterites from the Ronda orogenic peridotite

We report solution-ICPMS analyses of Rb, Ba, Th, U, Nb, Ta, REE, Sr, Zr and Hf for acid-leached minerals of anhydrous spinel peridotites and websterit es from the Ronda peridotite (S. Spain). The same elements were also analyz ed by LA-ICPMS in the silicates of three peridotites. The results obtained by solution-ICPMS and LA-ICPMS are similar for the less (HREE) and the most incompatible (Rb-Ba) elements, and provide comparable inter-element distri bution coefficients (D-xt/cpx) for these elements. However, moderately inco mpatible elements (typically LREE) show significant discrepancies between s olution and in situ analyses. D-opx/cpx and D-ol/cpx for these elements are generally lower for solution than for in situ analyses. D-xt/cpx for MREE, HREE, Zr and Hf are consistent with experimental values. In contrast, D-xt /cpx for highly incompatible elements and LREE are higher than expected fro m available experimental data and/or crystal-chemical considerations. The o bserved D-xt/cpx for the most incompatible trace elements may be explained by very small amounts of melt/fluid, or solid, inclusions trapped in these minerals. Inclusions would affect both solution- and LA-ICPMS data, but the ir proportion would be less important for LA-ICPMS analyses. We show with a mixing model that an extremely small amount of equilibrium partial melt (t ypically 0.01-0.1%) trapped in minerals is sufficient to increase the D-opx /cpx for HIE and LREE by a factor of 5-20 and the D-ol/cpx by two or three orders of magnitude. Similar effects may be produced by sub-percent amounts of HIE-rich fluids of solid microphases. Such very small volumes of inclus ions may pass unnoticed during mineral handpicking and LA-ICPMS analysis. H ence, D-xt/cpx for HIE and LREE should be considered cautiously when minera l analyses are used to constrain melt processes and mantle composition. Mas s balance calculations were performed for a nominally anhydrous spinel harz burgite sample. Similar to previous studies, the mass balance indicates imp ortant discrepancies for HIE between peridotite composition reconstructed f rom mineral analyses (bulk and in situ) and whole rock composition. The maj or silicate minerals are the main repositories for REE, Zr and Hf (> 75% of the whole rock budget), and also host greater than or equal to 65% of Th a nd U. In contrast, more than 80% of the budget of Rb, Ba and Nb, and about 60% of Ta and Sr, is hosted by micro-components in grain boundaries (GBC) o r trapped in minerals (inclusions). Alone, the GBC accounts for 50% of the budget of Nb and Ta. The inclusions are an important repository for Rb (39% ), Nb (40%) and Sr (49%). The GBC and inclusion repositories display very s imilar trace element signatures, suggesting that they were once a single re pository (< 1 wt%) now re-distributed in different textural components. Thi s repository could be a combination of hydrous phases and/or Ti oxides, and /or melt/fluid inclusions of mantle origin. (C) 2000 Elsevier Science B.V. All rights reserved.