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

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

Feb 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000e%26psl.176...57z&link_type=abstract

Earth and Planetary Science Letters, Volume 176, Issue 1, p. 57-72.

Computer Science

9

Scientific paper

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 δ18O ratios than secondary minerals (e.g., mica, ilmenite, neoblastic olivine, orthopyroxene rims). Polymict minerals have a lower oxygen isotope composition than `average mantle' (δ18O=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 δ18O zonation in garnets, significant isotopic variation close to secondary veins, δ18O (primary phases)>δ18O (secondary phases) and oxygen isotope disequilibria in many minerals. In addition, a positive correlation between δ18O 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.

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