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 (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.

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

“…This approach to establish chemical equilibrium was adopted in previous studies on melting of volatile‐bearing peridotite systems (Condamine et al, ; Condamine & Médard, ; Mallik et al, ; Parman et al, ). (3) Finally, the Fe‐Mg K D between orthopyroxene and clinopyroxene varies from 0.75 to 1.34, which lies within the range of previous hydrous mantle melting experimental studies, i.e., 0.40 to 1.77, where approach to equilibrium has been established (Conceicao & Green, ; Condamine et al, ; Condamine & Médard, ; Green et al, ; Mallik et al, , ; Mandler & Grove, ; Niida & Green, ; Pirard & Hermann, ; Saha et al, ; Thibault et al, ; Tumiati et al, ; Wallace & Green, ).…”

“…The complete breakdown of phlogopite at low temperatures, observed in this study, is of interest considering that phlogopite can be stable to temperatures as high as 1400 °C between 2 and 4 GPa (Allen et al, ; Barton & Hamilton, ; Conceicao and Conceicao & Green, ; Condamine & Médard, ; Elkins‐Tanton & Grove, ; Enggist et al, ; Enggist & Luth, ; Esperança & Holloway, ; Konzett & Ulmer, ; Kushiro et al, ; Mallik et al, ; Mitchell, ; Modreski & Boettcher, ; Sato et al, ; Yoder & Kushiro, ). We note that despite identical bulk volatile content (0.94 wt.% H 2 O and 0.5 wt.% CO 2 ), the upper temperature limit of phlogopite in this study is lower by at least 150–200 °C compared to that in the study by Saha et al (). The inferred solidus location of this study is also lower compared to the study of Saha et al ().…”

“…In order to gain insight into the differences in aggregate V S of the solid, residual assemblage caused by differences in the mode of volatile addition, we compare these estimates with values of V S for the bulk composition explored by Saha et al (). For these calculations, P‐T conditions along a representative 44‐mW/m 2 cratonic geotherm were chosen such that pressures along the geotherm matched experimental pressures in the two studies.…”

“…CO 2 ‐bearing, experimental studies are represented by blue circles (Brey et al, ; Dasgupta & Hirschmann, ; Falloon & Green, ; Ghosh et al, ; Hirose, ). H 2 O‐bearing studies are represented in orange (Balta et al, ; Conceicao & Green, ; Falloon & Danyushevsky, ; Fumagalli et al, ; Fumagalli & Poli, ; Green, ; Grove et al, ; Kawamoto & Holloway, ; Konzett & Ulmer, ; Mandler & Grove, ; Niida & Green, ; Parman & Grove, ; Pirard & Hermann, ; Tenner et al, ; Till et al, ), and mixed H 2 O and CO 2 ‐bearing studies are represented in yellow (Dvir & Kessel, ; Olafsson & Eggler, ; Saha et al, ; Thibault et al, ; Tumiati et al, ; Wallace & Green, , ). Plotted for comparison in gray circles are cratonic peridotite xenolith compositions (Lee et al, ; Luguet et al, ; Maier et al, ; Pearson & Wittig, ) that have been filtered for contamination.…”

“… Note . Major element compositions of starting mixes used in this study compared to the bulk composition used by Saha et al (), that is, S18, with identical CO 2 and H 2 O contents introduced via a hydrous silicic melt. FeO* indicates that all Fe is reported as FeO.…”