High Pressure Phase Relations of a Depleted Peridotite Fluxed by CO2‐H2O‐Bearing Siliceous Melts and the Origin of Mid‐Lithospheric Discontinuity

Abstract

AbstractWe present phase equilibria experiments on a depleted peridotite (Mg# 92) fluxed with variable proportions of a slab‐derived rhyolitic melt (with 9.4 wt.% H2O, 5 wt.% CO2), envisaging an interaction that could occur during formation of continents by imbrication of slabs/accretion of subarc mantles. Experiments were performed with 5 wt.% (Bulk 2) and 10 wt.% (Bulk 1) melt at 950–1175°C and 2–4 GPa using a piston‐cylinder and a multi‐anvil apparatus, to test the hypothesis that volatile‐bearing mineral‐phases produced during craton formation can cause reduction in aggregate shear‐wave velocities (VS) at mid‐lithospheric depths beneath continents. In addition to the presence of olivine, orthopyroxene, clinopyroxene, and garnet/spinel, phlogopite (Bulk 1: 3–7.6 wt.%; Bulk 2: 2.6–5 wt.%) at 2–4 GPa, and amphibole (Bulk 1: 3–9 wt.%; Bulk 2: 2–6 wt.%) at 2–3 GPa (≤1050°C) are also present. Magnesite (Bulk 1: ∼1 wt.% and Bulk 2: ∼0.6 wt.%) is present at 2–4 GPa (<1000°C at 3 and < 1050°C at 4 GPa) and its thermal breakdown coincides with the visual appearance of trace‐melt. However, an extremely small fraction of melt is inferred at all experiments based on the knowledge of fluid‐saturated peridotite solidus and the difference between bulk H2O and total H2O stored in the hydrous phases. Calculated mineral end‐member volume‐proportions were used to calculate VS of the resulting assemblage at experimental conditions and along representative continental geotherms (surface heat flow of 40–50 mWm−2). We note that reactive crystallization of phlogopite ± amphibole by infiltration of 3–10% slab‐derived hydrous‐silicic melt can cause up to 6% reduction in VS and that the estimated reduction in VS increases with increasing melt:rock ratio. The presence of phlogopite limits amphibole‐stability, making phlogopite a more likely candidate for MLDs at >100 km depth.