Distribution of Trace Elements in Minerals from Anhydrous Spinel Peridotites and Websterites From the Ronda Peridotite: Implications for the Nature of LILE, REE and HFSE Reservoirs in the Subcontinental Lithospheric Mantle

Abstract

Intermineral distribution coefficients (DXt/r We report dissolution ICP-MS analyses ofRb, Ba, Th, U, Nb, Ta, REE, Sr, Zr, Hf and Sc in leached separates of clinopyroxene, orthopyroxene, olivine and spinel in anhydrous spinel peridotites (12 samples) and websterites (6 samples) from the Ronda peridotite (S. Spain). None of the selected samples display evidence of cryptic metasomatism. This data-set provides a good example of the distribution of a large group of trace elements in anhydrous minerals from the subcontinental lithospheric mantle, as exemplified by orogenic massifs. The average partition coefficients (relative to clinopyroxene: D xt/cpx) of the studied trace elements are shown in Fig. 1. For most elements, there are no statistical differences in the D ~ between spinel peridotites and websterites. For the REE, Zr and Hf, the small standard deviation is suggestive of the lack of an important compositional effect in the partitioning of the studied compositions ('fertiles' to 'depleted'). As previously suggested, clinopyroxene is the main host of REE and other incompatible elements in anhydrous spinel mantle rocks. The D ~ for Zr and Hf is a factor 10 higher than for MREE, indicating the potential of orthopyroxene to fractionate these High Field Strength Elements (HFSEZr, Hf, Nb, Ta) relative to REE 1. When plotted in increasing order of compatibility in a melt/peridotite system (Fig. 1), the D xt/cpx of the Highly Incompatible trace Elements (HIE = Rb, Ba, Th, U, Nb, Ta) systematically increases from the MREE (Fig. 1). This feature is more marked in the olivine/cpx and spinel/cpx partitioning (Fig.l). Although the distribution of MREE, HREE, Zr and Hf in clinopyroxenes, orthopyroxenes and, partly, in olivines might be accounted for by crystallographic control in these minerals. However, the distribution of the HIE is inconsistent with a crystallographic control, especially in the case of spinels and olivines. The fact that the relative increase of the D xtjcpx of the HIE is correlated with their degree of incompatibility in a melt/peridotite system, and that olivines and spinels from different samples display similar normalized trace element patterns and abundances of HIE, indicates that an important part of their budget may be hosted in melt/fluid inclusions within these minerals. In Ronda minerals, two-phase fluid/(melt?) inclusions (~ lgm) typically occur as inclusions trails similar to secondary fluid/melt inclusions documented in peridotite xenoliths worldwide 2.