Monazite solubility in hydrous silicic melts at high pressure conditions relevant to subduction zone metamorphism

Critical to any application of accessory phase stability to subduction zone thermal structure and processes is knowledge of the thermodynamic stability of these minerals in different types of subducted rock, their solubility in the presence of fluids, and the extent to which they fractionate trace element ratios of interest. This study focuses on monazite, which is the principal carrier of light rare earth elements (LREE) and thorium (Th) in CaO-poor subducted sediments. Relatively little is known about the mechanism of monazite dissolution in high-pressure hydrous melts (or supercritical fluids), yet monazite and allanite (the principal carrier of LREE and Th in oceanic basalts and some CaO-rich sediments) solubility has been used recently to quantify subducted slab-top temperatures (Plank, T., Cooper, L.B., Manning, C.E., 2009. Emerging geothermometers for estimating slab surface temperatures. Nature Geosci. 2, 611–615).We have studied monazite solubility at subduction zone conditions (3 GPa, T ≥ 800 °C) in hydrous sediment-melting experiments. Experimental results highlight the important role that phosphorous exerts on monazite solubility in hydrous silicic melts at high pressure. Thermodynamically this corresponds to a case where monazite dissolves predominantly as its dissociated constituent ions (LREE3 + and PO43 −). This is in contrast to monazite solubility in granitic melts at low pressures (0.2 GPa) where it appears to dissolve predominantly as associate LREEPO4 species, such that its solubility is essentially independent of dissolved phosphorous. Our results have implications for monazite-based thermometry, as the error introduced by not taking phosphorous into account in high-pressure fluids can amount to > 100 °C.

► Monazite dissolves predominantly as its dissociated constituent ions at high pressures.
► Monazite solubility is highly dependent on the phosphorous content of fluids.
► This has implications for subduction zone geothermometry (LREE < P) using monazite.