Partitioning of trace elements between clinopyroxene and garnet: data from mantle eclogites

Concentrations of REE, Ba, Sr, Y and Zr have been measured by ion microprobe (SIMS) for garnet and clinopyroxene in a suite of eclogite xenoliths from the Roberts Victor kimberlite pipe in South Africa. The xenoliths have a restricted set of temperatures of formation estimated at 1100 ± 100°C at 5 GPa, but span a wide range of compositions; garnet Ca/(R2+) ratios range from 0.08 to 0.50, whilst the associated clinopyroxenes also show increasing jadeite solid solution as their Ca/(R2+) ratios increase. The cpx/grt partition coefficients (Di) for REE, Sr and Y decrease progressively as Ca/(R2+ in the minerals increase; in the LREE this change amounts to approximately three orders of magnitude. Evidence for such large changes in partition coefficients as a function of Ca content is also seen in other data for mantle eclogites from South Africa and Siberia. Cpx and grit with low Ca/(R2+ ratios have (Di) for HREE which are similar to those implied by mineral/liquid data for basic-ultrabasic melts. Poor correlations of Ba, La and Lu partition coefficients with cpx-grt Ca contents in the present dataset, are the result of late-stage partial alteration and relatively large errors of measurement associated with low concentrations. Zr partition coefficients show only a weak correlation with mineral Ca/(R2+). correlations are demonstrated between cpx/grt Di (calculated on a ppm or molar basis) and molar cpx/grt DCa for most REE, Sr and Y. These may be expressed in the form: In Di∗cpxgrt =A ln DCa∗cpxgrt + BB (Eq. (1)), where A and B are empirical constants for each element and vary with ionic radius in the REE. The partitioning of a given trace element progressively favours garnet rather than pyroxene as increasing amounts of Ca are more strongly partitioned into garnet than clinopyroxene; and the magnitude of this effect increases with ionic radius in the REE. The empirical linear calibrations (Eq. (1)) given for REE, Y and Sr allow calculation of the (Di) of a cpx-grt pair from their Ca contents (for 1100 ± 100°C, ∼ 5 GPa). Using the well-documented R3+ Al[4] Ca−1Si−1 (YAG-type) substitution relationship for REE and Y in garnets as the basis of an exchange reaction, it is suggested that the A term in empirical Eq. (1) may be related to variations in activity coefficients as the major element (Ca) composition changes, whilst term B is largely dependent on the standard state free energy.