Recent fluid processes in the Kaapvaal Craton, South Africa: coupled oxygen isotope and trace element disequilibrium in polymict peridotites

Oxygen-isotope mapping of thin sections of polymict peridotite xenoliths sh ows that significant oxygen isotope disequilibrium is preserved on a sub-mi llimetre scale in primary and secondary minerals. Primary porphyroblastic p hases (e.g., olivine, orthopyroxene, garnet, diopside) tend to have higher delta(18)O ratios than secondary minerals (e.g., mica, ilmenite, neoblastic olivine, orthopyroxene rims). Polymict minerals have a lower oxygen isotop e composition than 'average mantle' (delta(18)O=5.2+/-0.3 parts per thousan d) and show clear evidence of inter- and intra-mineral oxygen isotope diseq uilibrium. Disequilibrium is also evident in the elemental geochemistry of the mantle minerals and a general correlation exists between oxygen isotope s and major (Si, Mg, Ca, Fe) and trace elements (Ce, Cr, Zr, Nb, REE). The interpretation that isotopic heterogeneity may relate to melt processes is supported by delta(18)O zonation in garnets, significant isotopic variation close to secondary veins, delta(18)O (primary phases) > delta(18)O (second ary phases) and oxygen isotope disequilibria in many minerals. In addition, a positive correlation between delta(18)O and grain size indicates a role for deformation processes as a result of diffusion reactions perhaps inextr icably linked to melt processes. We suggest that polymict peridotites forme d as a result of movement along mantle shear zones which led to the juxtapo sition of minerals of varied provenance. Contemporaneous melt transfer reac ted with these mantle breccias and rapid entrainment by 'kimberlite' meant that any associated mineral disequilibrium was very effectively preserved. (C) 2000 Elsevier Science B.V. All rights reserved.