Observations of Li isotopic variations in the Trinity Ophiolite : evidence for isotopic fractionation by diffusion during mantle melting

Abstract

The Trinity peridotite (northern CA) contains numerous lithologic sequences consisting of dunite to harzburgite to spinel lherzolite to plagioclase lherzolite. Previous workers have documented geochemical gradients in these sequences consistent with melt-rock reaction processes occurring around dunites, interpreted to reflect conduits for melt ascent. We have undertaken a study of Li isotope compositions of clinopyroxene and some olivine within these sequences using ion probe techniques in order to test the hypothesis that the geochemical gradients are related to diffusive fluxing of alkali elements into or away from the melt conduit. Results show large variations in 7Li/6Li occurring in a consistent pattern across three transects from dunite to plagioclase lherzolite within the Trinity peridotite. Specifically, measurements of average δ7Li for single thin sections along the traverse reveal a low in δ7Li in the harzburgite adjacent to the dunite returning to higher values farther from the dunite with a typical offset of ~10 per mil in the low δ7Li trough. This pattern is consistent with a process whereby Li isotopes are fractionated during diffusion through a melt either from the dunite conduit to the surrounding peridotite, or from the surrounding peridotite into the dunite conduit. The patterns in 7Li/6Li occur over a length scale similar to the decrease in REE concentration in these same samples. Explaining both the trace element and Li isotopic gradients requires a combined process of alkali diffusion and melt extraction. We develop a numerical model and examine several scenarios of the combined diffusion-extraction process. Using experimentally constrained values for the change in Li diffusion coefficient with isotope mass, large changes in δ7Li as a function of distance can be created in year to decade time scales. The addition of the melt extraction term allows larger Li concentration gradients to be developed and thus produces larger isotopic fractionations than diffusion only models. The extraction aspect of the model can also account for the observed decrease in rare earth element concentrations across the transects.