Hf isotope compositions and HREE variations in off-craton garnet and spinel peridotite xenoliths from central Asia

摘要

Garnet-facies continental mantle is poorly understood because the vast majority of mantle xenoliths in continental basalts are spinel peridotite. Peridotite xenoliths from Vitim (southern Siberia) and Mongolia provide some of the best samples of garnet and garnet-spinel facies off-craton lithospheric mantle. Garnets in those fertile to moderately depleted lherzolites show a surprisingly broad range of HREE abundances, which poorly correlate with modal and major oxide compositions. Some garnets are zoned and have Lu-rich cores. We argue that these features indicate HREE redistribution after the partial melting, possibly related to spinel-garnet phase transition on isobaric cooling. Most peridotites from Vitim have depleted to ultra-depleted Hf isotope compositions (calculated from mineral analyses: epsilon(Hf(0)) = +17 to +45). HREE-rich garnets have the most radiogenic epsilon(Hf) values and plot above the mantle Hf-Nd isotope array while xenoliths with normal HREE abundances usually fall within or near the depleted end of the MORB field. Model Hf isotope ages for the normal peridotites indicate an origin by ancient partial melt extraction from primitive mantle, most likely in the Proterozoic. By contrast, an HREE-rich peridotite yields a Phanerozoic model age, possibly reflecting overprinting of the ancient partial melting record with that related to a recent enrichment in Lu. Clinopyroxene-garnet Lu-Hf isochron ages (31-84 Ma) are higher than the likely eruption age of the host volcanic rocks (similar to 16 Ma). Garnet-controlled HREE migration during spinel-garnet and garnet-spinel phase transitions may be one explanation for extremely radiogenic Hf-176/Hf-177 reported for some mantle peridotites; it may also contribute to Hf isotope variations in sub-lithospheric source regions of mantle-derived magmas. Copyright (c) 2005 Elsevier Ltd.