Lithium abundances and isotopic compositions in mantle xenoliths from subduction and intra-plate settings: Mantle sources vs. eruption histories

摘要

We report Li concentrations and isotopic compositions for olivine and pyroxene separates and bulk rocks of variably metasomatised harzburgite xenoliths hosted by andesitic tuffs of the active Avacha volcano in the Kamchatka arc, and for a single lava-hosted harzburgite from the intra-plate Vitim volcanic field, Siberia. The Li-isotope data on the Avacha samples (delta Li-7=0 to + 5 parts per thousand) mainly fall within the range reported previously for 'normal', MORB-type mantle (+2 to +6 parts per thousand) and show no significant inter-mineral disequilibria. These results indicate that the shallow mantle in the active Kamchatka arc, and probably in other subduction zones, has roughly (i.e. within +/- 2 parts per thousand) MORB-type Li isotopic signatures. By contrast, pyroxenes from the Vitim xenolith yield extremely low delta Li-7 (- 17 parts per thousand and - 18 parts per thousand) and are out of equilibrium with coexisting olivine, which has high delta Li-7 (+6 parts per thousand). Taken together, these results indicate that anomalous Li elemental and isotopic compositions in mantle xenoliths cannot be considered signatures of shallow sub-are mantle or evidence for recent or ancient slab-related metasomatism. We review published data on Li abundances and isotopic compositions in intra-plate mantle xenoliths to show that the intermineral disequilibria do not appear to be related to characteristics of the xenoliths themselves (tectonic settings, rock types, chemical compositions) but rather depend on what kind of volcanic rocks host the xenoliths. The disequilibria are extremely rare in xenoliths from pyroclastic rocks, which cooled almost instantaneously on eruption, but are common in xenoliths found in massive basalts (which took longer to cool down after the eruption). The anomalous Li elemental and isotopic compositions in lava-hosted xenoliths may be related to re-distribution of Li between minerals during the long cooling driven by temperature-controlled variations of inter-mineral partition coefficients and/or infiltration of residual fluids produced by crystallization of host lava. The diffusive re-distribution of Li can be accompanied by isotopic fractionation whose magnitude, in addition to cooling time, may depend on modal ratios of olivine and pyroxenes, grain size and other factors. We further speculate that significant variations of delta Li-7 in lava-hosted suites may be related to different cooling times for thicker and thinner flows.