The effect of CO2 on the partitioning of H2O between clinopyroxene and melts and melting of volatile-bearing eclogites

Abstract

H₂O and CO₂ are critical components for the development and sustainment of life on Earth and affect a wide variety of geological processes. It has been suggested that the presence of CO₂ in the mantle could alter how H₂O is stored and partitioned between minerals and melts, implying dramatic effects on both melting processes and H₂O storage capacities of the Earth’s mantle. Eclogites, which are present as heterogeneities in the mantle, have been shown to participate in the production of oceanic island basalts such as the HIMU (High-µ, µ = ²³⁸U/²⁰⁴Pb). They can host significant amounts of H₂O in clinopyroxenes and their H₂O storage capacity and melting temperature can be significantly influenced by the presence of CO₂. Here, the effect of CO₂ on the partition coefficient of H₂O between clinopyroxene and melt ($D_{H_{2}O}^{\mathrm{cpx}/melt}=C_{H_{2}O}^{\mathrm{cpx}}/C_{H_{2}O}^{\mathrm{melt}}$) was explored at high-pressure and high-temperature by conducting piston cylinder experiments at 1200 °C and 2 GPa. The experiments employed a basaltic starting mixture with different concentrations of H₂O and CO₂ and produced an assemblage consisting of more than 60 wt% basaltic-andesitic melt in equilibrium with high-Al (Al₂O₃ = 7.83–11.18 wt%) clinopyroxene crystals. The H₂O concentration in clinopyroxene, measured by Fourier Transform Infrared spectroscopy, ranges from 869 ± 71 to 1950 ± 134 ppm wt, and shows a negative correlation with the concentration of CO₂ in the system and a positive correlation with tetrahedrally-coordinated Al³⁺ in the clinopyroxene. The quenched melts have H₂O and CO₂ concentrations ranging from 5.18 to 6.97 wt% and from 0.19 to 2.59 wt%, respectively. The calculated $D_{H_{2}O}^{\mathrm{cpx}/melt}$ varies from 0.031 ± 0.005 at XCO₂ = 0.03 to 0.017 ± 0.003 at XCO₂ = 0.34 (where XCO₂ $=C{O}_2/\left(H_{2}O+C{O}_2\right)$ in wt). The variation of $D_{H_{2}O}^{\mathrm{cpx}/melt}$ as a function of XCO₂ is described by a power law in the form: $D_{H_{2}O}^{\mathrm{cpx}/melt}=0.0123\ast XC{O}_2^{-0.237}$, with an extrapolated $D_{H_{2}O}^{\mathrm{cpx}/melt}$ of 0.033 at XCO₂ = 0. The calculated $D_{H_{2}O}^{\mathrm{cpx}/melt}$ is combined with $D_{H_{2}O}^{\mathrm{grt}/cpx}$ to constrain the effect of CO₂ on the H₂O partition coefficient between an eclogite and a melt ($D_{H_{2}O}^{\mathrm{eclo}/melt}$), which is then used to predict the melting temperature of eclogite in a hydrous-carbonated system. The results show that melting of hydrous-carbonated eclogite (600 ppm wt H₂O-300 ppm wt CO₂) occurs at temperatures drastically lower than those of both purely hydrous and purely carbonated melting. In this scenario, the temperature of hydrous carbonated melting of eclogite remains lower than the mantle adiabat and crosses the geotherm of the hottest subduction zones at P > 5.8 GPa, indicating that the oceanic crust in a subducting slab could undergo partial melting at this pressure. In addition, $D_{H_{2}O}^{\mathrm{eclo}/melt}$ is used in a simple mass balance model to constrain the H₂O content of a mantle source lithology of HIMU melts, suggested to be made mainly of recycled oceanic crust. The model, using $D_{H_{2}O}^{\mathrm{eclo}/melt}$ at XCO₂ values of 0, 0.1 and 0.3, indicates that partial melting of eclogite with a bulk H₂O content ranging from ∼300 to ∼720 ppm wt can potentially explain the H₂O abundance measured in magmas originating from the HIMU mantle reservoir.