Seeking the primary compositions of mantle xenoliths: Isotopic and elemental consequences of sequential leaching treatments on an eclogite suite

Abstract

Three leaching methods have been tested on garnet and clinopyroxene separates from Roberts Victor xenolithic eclogites, in an attempt to remove secondary materials and uncover the primary compositions of the samples. A “typical” leaching treatment commonly used to clean mineral separates proved insufficient. However, the secondary material can be completely removed by treating the separates in 80°C 6N HCl for approximately 14h, followed by 6N HNO3 and then washing in MQ water ultrasonically for 20min three times. Further leaching produced no change in the isotopic composition (Sr, Nd, Hf) of the residual minerals. The leaching process produces largest relative changes in Sr-isotope compositions, and less in Nd- and Hf-isotope compositions. After this leaching process, measurement of the primary isotopic compositions of Sr, Nd and Hf in the eclogitic garnets can be achieved. The HREE patterns of the leachates are similar to those of the host minerals, but the leachates are enriched in LREE and LILE and show pronounced positive Eu anomalies. The leachable material is a mixture of the primary host minerals and secondary minerals (±fluid inclusions) deposited by a metasomatic melt/fluid. The leachates have very radiogenic Sr-isotope compositions (87Sr/86Sr=0.706–0.712 at the time of kimberlite eruption time (128Ma)) and this range suggests that the melt/fluid was heterogeneous in terms of Sr-isotope composition, and that it may be related to diamond-forming fluids. The results for these samples suggest that some Eu anomalies, commonly used to shallow crustal processes, such as plagioclase fractionation, in eclogite protoliths were actually introduced by a secondary melt/fluid percolating in the lithospheric mantle.