High-pressure Hydrous Phase Relations of Radiolarian Clay and Implications for the Involvement of Subducted Sediment in Arc Magmatism

Abstract

Melting of subducted oceanic sediment is considered to play a key role in the generation of the arc magmatic signature. We have carried out an experimental study on hydrous melting of trace element-doped radiolarian clay at 3 GPa and temperatures from 700 to 1250°C; 7–15 wt % H2O was added to the sediment to simulate the effects of flushing by fluids derived from underlying dehydrating lithologies, such as serpentinites. Melting begins at ∼750°C owing to the breakdown of phengite + clinopyroxene + coesite and a hydrous melt coexists with mostly garnet + kyanite ± quartz up to around 1250°C. Rutile and Fe–Ti oxides are present to ∼1000°C. Very high degrees of melting occur at relatively low, supra-solidus temperatures (e.g. with 15 wt % added H2O, the clay is ∼54% molten at 800°C), in marked contrast to fluid-absent melting of similar rock compositions, which yields negligible melt fractions (<∼10%) for similar temperatures. A particular focus of this study is residual monazite, which preferentially incorporates light rare earth elements (LREE) and Th, thereby exerting a powerful control on the Th/La ratio of sediment-derived fluids and melts. In contrast to previous studies, we find that DTh/La varies widely and can be significantly above or below unity. Our dataset suggests that this pattern arises because the various members of the monazite solid solution series are influenced independently by different parameters. We also demonstrate that monazite–melt partition coefficients based on doped experiments cannot be used uncritically to predict fractionation processes in nature because of monazite–huttonite solid solution. However, extrapolation of our results to natural concentration levels suggests fractionation of Th from La in the presence of monazite in most cases. We propose that a solid residue with little or no residual monazite is needed to explain a wide range of geochemical features of arc magmas, including Th/La ratios. A monazite-free residue can be achieved at relatively low sub-arc temperatures provided that enough water is made available (e.g. through antigorite breakdown) to promote sufficient melting to dissolve the entire LREE + Th budget of the sediment.