Lithium and its isotopes as a tracer of fluid flow mechanisms in the Catalina Schist melange zone

Abstract

Mélange zones are areas of highly mixed and deformed rock believed to form from shearing between subducting slab and peridotite mantle wedge. They have high -pressure/low-temperature mineral assemblages and contain a fine-grained matrix with centimeter to meter scale blocks surrounded by rinds, thought to represent a reaction zone between the block and matrix. These rinds are not well understood, but could be formed due to mechanical mixing, diffusion, or infiltration. Lithium is used to determine the role played by fluid-mediated processes in the Catalina Schist mélange zone because it is fluid mobile and has high diffusivity. Samples from amphibolite, lawsonite-blueschist, and lawsonite-albite facies were retrieved from the Catalina Schist subduction complex on Santa Catalina Island. Lithium isotopic compositions and concentrations were determined using mass spectrometry techniques. One-dimensional diffusion models were applied to the data to determine the extent of the different mechanisms responsible for fluid transport throughout the subduction complex.