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 degrees C at 5 GPa, but span a wide range of compositions; garnet Ca/(R(2+)) ratios range from 0.08 to 0.50, whilst the associated clinopyroxenes also show increasing jadeite solid solution as their Ca/(R(2+)) ratios increase. The cpx/grt partition coefficients (D-i) for REE, Sr and Y decrease progressively as Ca/(R(2+)) 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 grt with low Ca/(R(2+)) ratios have D-i 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/(R(2+)). Strong correlations are demonstrated between cpx/grt D-i (calculated on a ppm or molar basis) and molar cpx/grt D-Ca for most REE, Sr and Y. These may be expressed in the form: ln D-i*(cpk/grt) = A ln D-Ca*(cpx/grt) + B (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 D-i of a cpx-grt pair from their Ca contents (for 1100 +/- 100 degrees C, similar to 5 GPa). Using the well-documented R(3+) (AlCa-1Si-1)-Ca-[4] (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.