The Effect of fO2 on Partition Coefficients of U and Th between Garnet and Silicate Melt

Abstract:

Garnet is one of the most important minerals controlling partitioning of U and Th in the upper mantle. U is redox sensitive, while Th is tetra-valent at redox conditions of the silicate Earth. U-series disequilibria have provided a unique tool to constrain the time-scales and processes of magmatism at convergent margins. Variation of ^garnet/meltD_U/Th with fO₂ is critical to understand U-series disequilibria in arc lavas. However, there is still no systematic experimental study about the effect of fO₂ on partitioning of U and Th between garnet and melt.

Here we present experiments on partitioning of U, Th, Zr, Hf, Nb, Ta, and REE between garnet and silicate melts at various fO₂. The starting material was hydrous haplo-basalt. The piston cylinder experiments were performed with Pt double capsules with C-CO, MnO-Mn₃O₄ (MM), and hematite-magnetite (HM) buffers at 3 GPa and 1185-1230 ^oC. The experiments produced garnets with diameters > 50μm and quenched melt. Major elements were measured by EMPA at ETH Zurich. Trace elements were determined using LA-ICP-MS at Northwestern University (Xi’an, China) and SIMS (Cameca1280 at the Institute of Geology and Geophysics, Beijing, China), producing consistent partition coefficient data for U and Th.

With fO₂ increasing from CCO to MM and HM, ^garnet/meltD_U decreases from 0.041 to 0.005, while ^garnet/meltD_Th ranges from 0.003 to 0.007 without correlation with fO₂. Notably, ^garnet/meltD_Th/U increases from 0.136 at CCO to 0.41 at HM. Our results indicate that U is still more compatible than Th in garnet even at the highest fO₂ considered for the subarc mantle wedge (~NNO). Therefore, we predict that if garnet is the dominant phase controlling U-Th partitioning during melting of the mantle wedge, melts would still have ²³⁰Th excess over ²³⁸U. This explains why most young continental arc lavas have ²³⁰Th excess. If clinopyroxene is the dominant residual phase during mantle melting, U could be more incompatible than Th at high fO₂ because increasing fO₂ can increase ^{clinopyroxene/melt}D_Th/U by more than two magnitudes (Lundstrom et al. 1994). In this case, in-growth melting of the mantle can produce ²³⁸U excess over ²³⁰Th observed in the oceanic arc lavas.