Using Li and Its Isotopes as a Tracer of Subduction-Related Metamorphic Fluid-Rock Interactions

Whole-rock variations in Li concentrations and δ7Li have been used as tracers of fluid-rock interactions in subduction-related metamorphic rocks. Such observations can help characterize fluid sources and the relative timing and duration of such interactions. Estimates of the duration of fluid infiltration based on diffusion profiles in rocks across features such as veins and reaction zones reveal surprisingly brief events, on the order of 10s to 100s of years. Evidently subduction zones release fluid pulses rather than continuous streams. Inter- and intra-mineral variations in [Li] and δ7Li promise additional insight into the nature and duration of fluid flow and fluid-rock interactions, but require better understanding of Li systematics. Careful consideration of petrologic context, mineral reactions, isotopic fractionation, site occupancy and substitution mechanisms all must be evaluated. Examples of in situ variations in Li concentrations in garnet and phengite in metamorphic rocks of the Franciscan Complex, CA and other localities illustrate the potential for scientific advance that inter- and intra-mineral variation in [Li] and δ7Li offer, but also delineate areas of interpretational ambiguity. Teasing out the effects of site occupancy and substitution mechanisms in numerous minerals is a top priority and will require coupled measurements with other trace elements. Consideration of mineral assemblages and reaction textures provides insight into how mineral reactions direct the partitioning and redistribution of Li. Analysis of minerals for which pressure-temperature and absolute timing of formation (e.g. garnet) can also be constrained may provide additional insight into the conditions of fluid flow events. Overall, in situ measurements of [Li] and δ7Li in metamorphic minerals coupled with whole-rock scale measurements will provide an increasingly accurate approach to determine the periodicity of fluid fluxes, the duration of fluid infiltration events, and the source of fluids.