Oxygen and hydrogen isotopic composition of phlogopites and amphiboles in diamond-bearing kimberlite hosted MARID xenoliths: Constraints on fluid-rock interaction and recycled crustal material in the deep continental lithospheric mantle

Abstract

Abstract MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) xenoliths are transported from the deep-cratonic lithosphere to the Earth's surface by Cretaceous kimberlites emplaced in the Kimberley area of the Kaapvaal Craton. MARID xenoliths have high modal abundances (70–80 vol%) of mica and amphibole, indicating their origin from a hydrous source. The δ18O values (4.7 ≤ δ18O ≤ 6.9‰) of phlogopite micas from 14 MARID samples indicate that these minerals are both 18O–enriched and 18O–depleted with respect to the average upper mantle δ18O value of 5.8 ± 0.6‰. The range of δ2H values of phlogopites (−83 ≤ δ2H ≤ −53‰, n = 14) of MARID xenoliths are slightly larger than the average mantle δ2H value (−70 ± 10‰). The oxygen (Δ18Ophlogopites-amphibole = −0.4 and 0.4‰) and hydrogen (Δ2Hphlogopite-amphibole = 14 and 36‰) isotopic disequilibrium recorded from two MARID xenoliths suggests the duration of the last isotopic exchange, possibly just before the kimberlite emplacement, between these minerals and metasomatic fluids was too short to reach isotopic equilibrium. Our model calculation indicates that the phlogopites of MARID xenoliths underwent isotopic exchange with fluids of δ18O = 5.5 to 10‰, δ2H = −62 to −90‰. The range of δ18O value of the calculated metasomatic fluids resembles the oxygen isotopic composition of the primary mantle carbonate (~ 6–9‰) suggesting interaction between carbonatic melt and MARID xenoliths in the continental lithospheric mantle. Furthermore, δ18O values of phlogopites together with previously published nitrogen isotope data (−11 ≤ δ15N ≤ 9‰; Banerjee et al., 2015) indicates incorporation of inhomogeneously distributed recycled crustal material from subducted crust within their source magma. Therefore, O-H-N isotope data for MARID xenoliths indicates their crystallization from geochemically heterogeneous magma in the upper continental mantle and subsequent metasomatism with mantle fluids.