Lithium and its isotopes as tracers of subduction zone fluids and metasomatic processes: Evidence from the Catalina Schist, California, USA

Abstract

The Catalina Schist, well-studied for its tectonometamorphic history and geochemical evolution, provides a field area in which to evaluate the extent of Li mobility in subduction-zone fluids and the degree to which Li isotopes can trace metamorphic fluid–rock interactions. A suite of 63 samples of the Catalina Schist that represent a range of metamorphic grades and diverse lithologies was analyzed for Li concentration and isotope composition. Metasedimentary rocks show no evidence for significant loss of Li, with increasing metamorphic grade, despite the wide range of prograde P–T histories represented by the five tectonometamorphic units (with the highest-grade rocks having experienced peak temperatures near 750°C). Metamafic rocks similarly show no change in Li concentration, as a function of grade, that can be directly related to varying degrees of devolatilization. The Li concentrations and δ7Li of metasedimentary rocks appear to best correlate with a chemical alteration index that likely reflects differential extents of weathering in the sediment sources. The δ7Li of most metamafic rocks (including gabbroic metaconglomerate cobbles), veins and mélange matrix reflects overprinting of the Li isotope compositions derived (through transfer of Li in fluids) from sedimentary lithologies in the same tectonometamorphic units. Thus it appears that Li was mobilized in fluids released from metasedimentary rocks but that the loss was insufficient to affect the bulk δ7Li and Li content of the metasedimentary rocks. Veins and mélange matrix have the highest Li concentrations of all samples analyzed, ranging up to 70ppm. Some veins and amphibolite-grade mafic rocks have low δ7Li suggestive of Li diffusion through an intergranular fluid. Taken together, these observations suggest mobilization of Li during high pressure/temperature (P/T) fluid–rock interactions that also resulted in enrichments in B, N, and some large ion lithophile elements (K, Rb, Cs, Ba) in the veins, mélange matrix, and cobbles. This study demonstrates that, although Li is mobile in high P/T metamorphic fluids, it does not partition into fluids to the extent demonstrated for N, B, and Cs, the latter elements showing large depletions in the higher-grade Catalina Schist metasedimentary rocks. Thus, even in relatively warm subduction zones, deeply subducted metasedimentary rocks are expected to largely retain their Li inventory to depths of at least 40km, the maximum depth at which units of the Catalina Schist experienced peak metamorphism.