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

摘要

Oxygen-isotope mapping of thin sections of polymict peridotite xenoliths shows that significant oxygen isotope disequilibrium is preserved on a sub-millimetre scale in primary and secondary minerals. Primary porphyroblastic phases (e.g., olivine, orthopyroxene, garnet, diopside) tend to have higher δ~(18)O ratios than secondary minerals (e.g., mica, ilmenite, neoblastic olivine, orthopyroxene rims). Polymict minerals have a lower oxygen isotope composition than 'average mantle' (δ~(18)O = 5.2 ± 0.3‰) and show clear evidence of inter- and intra-mineral oxygen isotope disequilibrium. Disequilibrium is also evident in the elemental geochemistry of the mantle minerals and a general correlation exists between oxygen isotopes 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 δ~(18)O zonation in garnets, significant isotopic variation close to secondary veins, δ~(18)O (primary phases) ＞ δ~(18)O (secondary phases) and oxygen isotope disequilibria in many minerals. In addition, a positive correlation between δ~(18)O and grain size indicates a role for deformation processes as a result of diffusion reactions perhaps inextricably linked to melt processes. We suggest that polymict peridotites formed as a result of movement along mantle shear zones which led to the juxtaposition of minerals of varied provenance. Contemporaneous melt transfer reacted with these mantle breccias and rapid entrainment by 'kimberlite' meant that any associated mineral disequilibrium was very effectively preserved.