Lithium isotopic signatures of peridotite xenoliths and isotopic fractionation at high temperature between olivine and pyroxenes

摘要

We determined the Li isotopic composition of coexisting olivine, clinopyroxene, and orthopyroxene from spinel- and garnet-bearing peridotite xenoliths from San Carlos (Arizona), Vitim (Siberia), Dreiser Weiher (Eifel, Germany), and Kapfenstein (Austria). These samples represent the upper mantle that experienced varying degrees of partial melting followed by different metamorphic and metasomatic histories. Ultrasonic cleaning of handpicked mineral separates with Milli-Q water (18 MOmega cm) removed significant amounts (10-30%) of the lithium from unwashed separates and resulted in systematically heavier lithium isotopic compositions in the washed minerals. delta(7)Li values for washed minerals range from +1.4% to +4.5% in olivines, from -1.0% to +3.9% in orthopyroxenes, and from -2.4% to +3.0% in clinopyroxenes, delta(7)Li=[(Li-7/Li-6)(sample)/ (Li-7/Li-6)(L-SVEC) (stadard)-1] x 1000. Olivines in mildly depleted lherzolites (Eifel, Vitim, San Carlos) are isotopically the heaviest (delta(7)Li +3.4 - +4.5%). A metasomatised, amphibole-bearing lherzolite from the Eifel gave the lightest value (+1.4%). Olivines have similar delta(7)Li values to that of MORB, while the pyroxenes are isotopically lighter. In all but one sample, the delta(7)Li changes systematically between the minerals; olivine has the heaviest delta(7)Li value followed by orthopyroxene, and clinopyroxene has the lightest values. The degree of intramineral fractionation (Deltadelta(7)Li=[delta(7)Li(ol) - (delta(7epx))]) correlates negatively with equilibration temperature. Deltadelta(7) Li is about 3.5 for low-temperature (similar to950 degreesC) xenoliths and 1.5 for high-temperature (similar to1150 degreesC) xenoliths. We therefore suggest Li isotope fractionation may occur at high, magmatic temperatures. (C) 2004 Elsevier B.V. All rights reserved.