Clinopyroxene/Orthopyroxene Trace Element Partition Coefficients in Spinel Peridotite Xenoliths

Abstract

The distribution of incompatible trace elements between clinoand ortho-pyroxenes from spinel peridotite xenoliths has been investigated. The xenoliths came from three localities within the Pannonian Basin, Eastern Central Europe, namely Szigliget, Szentbekalla and Bondoro-hegy (EmbeyIsztin et al., 1989). All upper mantle xenoliths selected for this study are anhydrous and include the following textural types; protogranular, porphyroclastic, equigranular and poikilitic. Both metasomatised and unmetasomatised xenoliths were analysed. However, there were no significant interor intragranular trace element variations in the clinoand orthopyroxenes, and the homogeneity of the major elements in the mineral phases also suggest equilibrium. The four major mineral phases (clinoand orthopyroxene, olivine and spinel) were analysed by laser ablation microprobe inductively coupled plasma mass spectrometry (LAM-ICP-MS) at Memorial University of Newfoundland; for analysis details see Jenner et al. (1993). REE, Sr, Nb, Zr, Hf and Ti are above detection limits for the clinoand orthopyroxenes; however, these elements are generally too low in the olivines and spinels for quantitative analysis. The U, Th and Ta content of orthopyroxene was also below detection limits. The averages and standard deviations of the calculated orthopyroxene/clinopyroxene partition coefficients are given in Fig 1. This figure which is based on the averages of 37 xenoliths strongly confirms the observation of Rampone et al. (1991), McDonough et al. (1992) and Sun and Kerrich (1995) that the orthopyroxene/clinopyroxene partition coefficients of the HFSE elements (which substitute in the M1 sites in the pyroxenes) are significantly higher than those of the neighbouring REE (substituting in the M2 sites). The figure also illustrates orthopyroxene/clinopyroxene partition coefficients from some spinel peridotite xenoliths from British Columbia (Sun and Kerrich, 1995). These later data were calculated using solution ICPMS data on separated clinoand ortho-pyroxenes. The U-shaped pattern for the REE partition coefficients (i.e. the increase of the partition coefficients towards the LREE) is probably the result of impurities in the orthopyroxene separate. Highly incompatible trace elements reside not only in the pyroxene lattice but in fluid inclusions and along the grain boundaries. The REE partition coefficients based on in situ analysis show regular variation with ionic radii, e.g. Fig. 2, where there is illustrated an almost linear trend of the logarithm of the partition coefficients in the D (opx/cpx) versus ionic radii. There is increased scatter of the D values as lower trace element abundances in orthopyroxene are approached. However, the higher scattering in the case of some LREE and Sr (all have relatively large ionic radii) is not simply a result of analytical uncertainty; the orthopyroxene/clinopyroxene partition coefficents of these elements tend to increase with increasing equilibrium temperatures (Fig. 3.). This variation is intrepreted to be the result of a temperature dependence of Ca partitioning between