Curtolo A., Condamine P., Schiavi Federica, Bolfan-Casanova N., Novella D. (2025). The effect of CO2 on the partitioning of H2O between clinopyroxene and melts and melting of volatile-bearing eclogites. Geochimica et Cosmochimica Acta, 398, 54-66. ISSN 0016-7037.
Titre du document
The effect of CO2 on the partitioning of H2O between clinopyroxene and melts and melting of volatile-bearing eclogites
Curtolo A., Condamine P., Schiavi Federica, Bolfan-Casanova N., Novella D.
Source
Geochimica et Cosmochimica Acta, 2025,
398, 54-66 ISSN 0016-7037
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-mu, mu = U-238/Pb-204). 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-H2O(cpx/melt )= C-H2O(cpx)/C-H2O(melt)) was explored at high-pressure and high-temperature by conducting piston cylinder experiments at 1200 degrees 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-H2O(cpx/melt) varies from 0.031 +/- 0.005 at XCO2 = 0.03 to 0.017 +/- 0.003 at XCO2 = 0.34 (where XCO2 = CO2/[H2O+CO2 in wt). The variation of D-H2O(cpx/melt) as a function of XCO2 is described by a power law in the form: D-H2O(cpx/melt )= 0.0123 & lowast;XCO2-0.237, with an extrapolated D-H2O(cpx/melt) of 0.033 at XCO2 = 0. The calculated D-H2O(cpx/melt) is combined with DH2Ogrt/cpx to constrain the effect of CO2 on the H2O partition coefficient between an eclogite and a melt (D-H2O(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-H2O(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-H2O(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 similar to 300 to similar to 720 ppm wt can potentially explain the H2O abundance measured in magmas originating from the HIMU mantle reservoir.
Plan de classement
Géologie et formations superficielles [064]
;
Géophysique interne [066]
