Oxygen Isotope Ratios in Eclogites from Kimberlites

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Science  04 Jun 1971:
Vol. 172, Issue 3987, pp. 1025-1027
DOI: 10.1126/science.172.3987.1025


The oxygen isotope compositions (δ18O) of eclogitic xenoliths from the Roberts Victor kimberlite range from 2 to 8 per mil relative to SMOW (standard mean ocean water). This surprising variation appears to be due to fractional crystallization: the eclogites rich in oxygen-18 represent early crystal accumulates; the eclogites poor in oxygen-18 represent residual liquids. Crystal-melt partitioning probably exceeded 3 per mil and is interpreted to be pressure-dependent. Anomalous enrichment of oxygen-18 in cumulate eclogites relative to ultramafic xenoliths suggests that crystal-melt partitioning increased after melt-formation but prior to crystallization.