Phase Relations of a Depleted Peridotite Fluxed by a CO2-H2O fluid-Implications for the Stability of Partial Melts Versus Volatile-Bearing Mineral Phases in the Cratonic Mantle

We present phase-equilibria experiments of a K-bearing, depleted peridotite (Mg# 92) fluxed with a mixed CO2-H2O fluid (0.5 wt.% CO2 and 0.94 wt.% H2O in the bulk) to gain insight into the stability of volatile-bearing partial melts versus volatile-bearing mineral phases in a depleted peridotite system. Experiments were performed at 850-1150 degrees C and 2-4 GPa using a piston-cylinder and a multianvil apparatus. Olivine, orthopyroxene, clinopyroxene, and spinel/garnet are present at all experimental conditions. Textural confirmation of partial melt is made at temperatures as low as 1000 degrees C at 2 GPa, 950 degrees C at 3 GPa, and 1000 degrees C at 4 GPa marking the onset of melting at 900-1000 degrees C at 2 GPa, 850-950 degrees C at 3 GPa, and 950-1000 degrees C at 3 GPa. Phlogopite and magnesite breakdown at 900-1000 degrees C at 2 GPa, 950-1000 degrees C at 3 GPa, and 1000-1050 degrees C at 4 GPa. Comparison with previously published experiments in depleted peridotite system with identical CO2-H2O content introduced via a silicic melt show that introduction of CO2-H2O as fluid lowers the temperature of phlogopite breakdown by 150-200 degrees C at 2-4 GPa and stabilizes partial melts at lower temperatures. Our study thus, shows that the volatile-bearing phase present in the cratonic mantle is controlled by bulk composition and is affected by the process of volatile addition during craton formation in a subduction zone. In addition, volatile introduction via melt versus aqueous fluid, leads to different proportion of anhydrous phases such as olivine and orthopyroxene. Considering the agent of metasomatism is thus critical to evaluate how the bulk composition of depleted peridotite is modified, leading to potential stability of volatile-bearing phases as the cause of anomalously low shear wave velocity in mantle domains such as mid lithospheric discontinuities beneath continents.