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

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Title: The effect of CO2 on the partitioning of H2O between clinopyroxene and melts and melting of volatile-bearing eclogites.
Authors: Curtolo, Andrea1 (AUTHOR) andrea.curtolo.2@phd.unipd.it, Condamine, Pierre2 (AUTHOR), Schiavi, Federica2 (AUTHOR), Bolfan-Casanova, Nathalie2 (AUTHOR), Novella, Davide1 (AUTHOR)
Source: Geochimica et Cosmochimica Acta. Jun2025, Vol. 398, p54-66. 13p.
Subjects: Fourier transform infrared spectroscopy, Slabs (Structural geology), Earth's mantle, Aluminum oxide, Carbon dioxide
Abstract: H 2 O and CO 2 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 CO 2 in the mantle could alter how H 2 O is stored and partitioned between minerals and melts, implying dramatic effects on both melting processes and H 2 O 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 H 2 O in clinopyroxenes and their H 2 O storage capacity and melting temperature can be significantly influenced by the presence of CO 2. Here, the effect of CO 2 on the partition coefficient of H 2 O between clinopyroxene and melt (D H 2 O cpx / m e l t = C H 2 O cpx / C H 2 O 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 H 2 O and CO 2 and produced an assemblage consisting of more than 60 wt% basaltic-andesitic melt in equilibrium with high-Al (Al 2 O 3 = 7.83–11.18 wt%) clinopyroxene crystals. The H 2 O 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 CO 2 in the system and a positive correlation with tetrahedrally-coordinated Al3+ in the clinopyroxene. The quenched melts have H 2 O and CO 2 concentrations ranging from 5.18 to 6.97 wt% and from 0.19 to 2.59 wt%, respectively. The calculated D H 2 O cpx / m e l t varies from 0.031 ± 0.005 at XCO 2 = 0.03 to 0.017 ± 0.003 at XCO 2 = 0.34 (where XCO 2 = C O 2 / H 2 O + C O 2 in wt). The variation of D H 2 O cpx / m e l t as a function of XCO 2 is described by a power law in the form: D H 2 O cpx / m e l t = 0.0123 ∗ X C O 2 - 0.237 , with an extrapolated D H 2 O cpx / m e l t of 0.033 at XCO 2 = 0. The calculated D H 2 O cpx / m e l t is combined with D H 2 O grt / c p x to constrain the effect of CO 2 on the H 2 O partition coefficient between an eclogite and a melt (D H 2 O eclo / m e l t ), 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 H 2 O-300 ppm wt CO 2) 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 eclo / m e l t is used in a simple mass balance model to constrain the H 2 O 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 eclo / m e l t at XCO 2 values of 0, 0.1 and 0.3, indicates that partial melting of eclogite with a bulk H 2 O content ranging from ∼300 to ∼720 ppm wt can potentially explain the H 2 O abundance measured in magmas originating from the HIMU mantle reservoir. [ABSTRACT FROM AUTHOR]
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Abstract:H 2 O and CO 2 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 CO 2 in the mantle could alter how H 2 O is stored and partitioned between minerals and melts, implying dramatic effects on both melting processes and H 2 O 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 H 2 O in clinopyroxenes and their H 2 O storage capacity and melting temperature can be significantly influenced by the presence of CO 2. Here, the effect of CO 2 on the partition coefficient of H 2 O between clinopyroxene and melt (D H 2 O cpx / m e l t = C H 2 O cpx / C H 2 O 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 H 2 O and CO 2 and produced an assemblage consisting of more than 60 wt% basaltic-andesitic melt in equilibrium with high-Al (Al 2 O 3 = 7.83–11.18 wt%) clinopyroxene crystals. The H 2 O 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 CO 2 in the system and a positive correlation with tetrahedrally-coordinated Al3+ in the clinopyroxene. The quenched melts have H 2 O and CO 2 concentrations ranging from 5.18 to 6.97 wt% and from 0.19 to 2.59 wt%, respectively. The calculated D H 2 O cpx / m e l t varies from 0.031 ± 0.005 at XCO 2 = 0.03 to 0.017 ± 0.003 at XCO 2 = 0.34 (where XCO 2 = C O 2 / H 2 O + C O 2 in wt). The variation of D H 2 O cpx / m e l t as a function of XCO 2 is described by a power law in the form: D H 2 O cpx / m e l t = 0.0123 ∗ X C O 2 - 0.237 , with an extrapolated D H 2 O cpx / m e l t of 0.033 at XCO 2 = 0. The calculated D H 2 O cpx / m e l t is combined with D H 2 O grt / c p x to constrain the effect of CO 2 on the H 2 O partition coefficient between an eclogite and a melt (D H 2 O eclo / m e l t ), 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 H 2 O-300 ppm wt CO 2) 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 eclo / m e l t is used in a simple mass balance model to constrain the H 2 O 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 eclo / m e l t at XCO 2 values of 0, 0.1 and 0.3, indicates that partial melting of eclogite with a bulk H 2 O content ranging from ∼300 to ∼720 ppm wt can potentially explain the H 2 O abundance measured in magmas originating from the HIMU mantle reservoir. [ABSTRACT FROM AUTHOR]
ISSN:00167037
DOI:10.1016/j.gca.2025.03.013