Li isotope fractionation in peridotites and mafic melts

摘要

We have measured the Li isotope ratios of a range of co-existing phases from peridotites and mafic magmas to investigate high-temperature fractionations of Li-7/Li-6. The Li isotopic compositions of seven mantle peridotites, reconstructed from analyses of mineral separates, show little variation (delta Li-7 3.2-4.9 parts per thousand) despite a wide range in fertility and radiogenic isotopic compositions. The most fertile samples yield a best estimate of delta Li-7 similar to 3.5 parts per thousand for the upper mantle. Bulk analyses of olivine separates from the xenoliths are typically similar to 1.5%. isotopically lighter than co-existing orthopyroxenes, suggestive of a small, high-temperature equilibrium isotope fractionation. On the other hand, bulk analyses of olivine phenocrysts and their host melts are isotopically indistinguishable. Given these observations, equilibrium mantle melting should generate melts with 67 Li little different from their sources (< 0.57 parts per thousand lighter). In contrast to olivine and orthopyroxene, that dominate peridotite Li budgets, bulk clinopyroxene analyses are highly variable (delta Li-7 = 6.6 parts per thousand to -8.1 parts per thousand). Phlogopite separated from a modally metasomatised xenolith yielded an extreme delta Li-7 of -18.9 parts per thousand. Such large Li isotope variability is indicative of isotopic disequilibrium. This inference is strongly reinforced by in situ, secondary ion mass-spectrometry analyses which show Li isotope zonation in peridotite minerals. The simplest zoning patterns show isotopically light rims. This style of zoning is also observed in the phenocrysts of holocrystalline Hawaiian lavas. More dramatically, a single orthopyroxene crystal from a San Carlos xenolith shows a W-shaped Li isotope profile with a 40 parts per thousand range in delta Li-7, close to the isotope variability seen in all terrestrial whole rock analyses. We attribute Li isotope zonation in mineral phases to diffusive fractionation of Li isotopes, within mineral phases and along melt pathways that pervade xenoliths. Given the high diffusivity of Li, the Li isotope profiles we observe can persist, at most, only a few years at magmatic temperatures. Our results thus highlight the potential of Li isotopes as a high-resolution geospeedometer of the final phases of magmatic activity and cooling. (c) 2006 Elsevier Inc. All rights reserved.