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

Abstract:

We 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 (V_S) 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 V_S 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 V_S and that the estimated reduction in V_S 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.

Saha, S., Dasgupta, R., & Tsuno, K. (2018). High pressure phase relations of a depleted peridotite fluxed by CO₂-H₂O-bearing siliceous melts and the origin of Mid-Lithospheric Discontinuity. Geochemistry, Geophysics, Geosystems, 19. https://doi.org/10.1002/2017GC007233