In situ determination of magnesite solubility and carbon speciation in water and NaCl solutions under subduction zone conditions
The dissolution behavior of carbonates in subduction zone fluids has not been well constrained. In this study we investigated the solubility of magnesite in pure water and NaCl solutions (with up to 19 wt% NaCl) in situ with a hydrothermal diamond anvil cell, and determined carbon speciation in the...
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Elsevier
2022-09-01
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| Series: | Solid Earth Sciences |
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| Online Access: | http://www.sciencedirect.com/science/article/pii/S2451912X22000204 |
| _version_ | 1811274960121888768 |
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| author | Wan-Cai Li Qinxia Wang |
| author_facet | Wan-Cai Li Qinxia Wang |
| author_sort | Wan-Cai Li |
| collection | DOAJ |
| description | The dissolution behavior of carbonates in subduction zone fluids has not been well constrained. In this study we investigated the solubility of magnesite in pure water and NaCl solutions (with up to 19 wt% NaCl) in situ with a hydrothermal diamond anvil cell, and determined carbon speciation in the fluid by Raman spectroscopy. The solubility of magnesite in pure water falls in the range of 0.01–0.05 mol/kg at 0.7–2.4 GPa and 635–940 °C, enhanced strongly by temperature. Least squares fitting of experimental data leads to the following empirical expression for magnesite solubility in pure water: CMgCO3=(964510±41362)exp(−6953±690T)exp[0.0200±0.0098T(P−1)] where CMgCO3 is given in μg/g, T is the temperature in K, and P is the pressure in bar, respectively. The solubility of MgCO3 is about an order of magnitude lower than that of CaCO3 at 0.7–2.4 GPa, 640–940 °C. The solubility enhancement factor by NaCl (m/m° with m and m° being magnesite solubility in NaCl solution and water, respectively) presents as a parabolic trend with the mole fraction of NaCl in the range of 0–0.07, with a maximum amplification of 5.2 at XNaCl = 0.035, which is different from the continuously increase of solubility with salinity increasing at high salinity conditions in previous studies and suggests the dissolution reaction of magnesite in dilute NaCl solution is different. Despite slight contamination of CH4 formed by the reaction of the diamond anvils, we were able to identify CO32− and HCO3- to be the aqueous carbon species, HCO3- was predominant over CO32− in the range of 200–800 °C and 1.9–3.8 GPa and its proportion was affected by temperature, but not affected by pressure at 400–600 °C. Our experimental data suggest that in the absence of melting, only a small amount of magnesite can be mobilized by the slab-released fluid at subarc depths. |
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| issn | 2451-912X |
| language | English |
| last_indexed | 2024-04-12T23:28:19Z |
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| series | Solid Earth Sciences |
| spelling | doaj.art-93f943ac681140b28e610d2ca41305db2022-12-22T03:12:20ZengElsevierSolid Earth Sciences2451-912X2022-09-0173200214In situ determination of magnesite solubility and carbon speciation in water and NaCl solutions under subduction zone conditionsWan-Cai Li0Qinxia Wang1CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China; CAS Center for Excellence in Comparative Planetology, Hefei, 230026, China; Corresponding author.CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, ChinaThe dissolution behavior of carbonates in subduction zone fluids has not been well constrained. In this study we investigated the solubility of magnesite in pure water and NaCl solutions (with up to 19 wt% NaCl) in situ with a hydrothermal diamond anvil cell, and determined carbon speciation in the fluid by Raman spectroscopy. The solubility of magnesite in pure water falls in the range of 0.01–0.05 mol/kg at 0.7–2.4 GPa and 635–940 °C, enhanced strongly by temperature. Least squares fitting of experimental data leads to the following empirical expression for magnesite solubility in pure water: CMgCO3=(964510±41362)exp(−6953±690T)exp[0.0200±0.0098T(P−1)] where CMgCO3 is given in μg/g, T is the temperature in K, and P is the pressure in bar, respectively. The solubility of MgCO3 is about an order of magnitude lower than that of CaCO3 at 0.7–2.4 GPa, 640–940 °C. The solubility enhancement factor by NaCl (m/m° with m and m° being magnesite solubility in NaCl solution and water, respectively) presents as a parabolic trend with the mole fraction of NaCl in the range of 0–0.07, with a maximum amplification of 5.2 at XNaCl = 0.035, which is different from the continuously increase of solubility with salinity increasing at high salinity conditions in previous studies and suggests the dissolution reaction of magnesite in dilute NaCl solution is different. Despite slight contamination of CH4 formed by the reaction of the diamond anvils, we were able to identify CO32− and HCO3- to be the aqueous carbon species, HCO3- was predominant over CO32− in the range of 200–800 °C and 1.9–3.8 GPa and its proportion was affected by temperature, but not affected by pressure at 400–600 °C. Our experimental data suggest that in the absence of melting, only a small amount of magnesite can be mobilized by the slab-released fluid at subarc depths.http://www.sciencedirect.com/science/article/pii/S2451912X22000204MagnesiteSolubilityCarbon speciationFluidSubduction zone |
| spellingShingle | Wan-Cai Li Qinxia Wang In situ determination of magnesite solubility and carbon speciation in water and NaCl solutions under subduction zone conditions Solid Earth Sciences Magnesite Solubility Carbon speciation Fluid Subduction zone |
| title | In situ determination of magnesite solubility and carbon speciation in water and NaCl solutions under subduction zone conditions |
| title_full | In situ determination of magnesite solubility and carbon speciation in water and NaCl solutions under subduction zone conditions |
| title_fullStr | In situ determination of magnesite solubility and carbon speciation in water and NaCl solutions under subduction zone conditions |
| title_full_unstemmed | In situ determination of magnesite solubility and carbon speciation in water and NaCl solutions under subduction zone conditions |
| title_short | In situ determination of magnesite solubility and carbon speciation in water and NaCl solutions under subduction zone conditions |
| title_sort | in situ determination of magnesite solubility and carbon speciation in water and nacl solutions under subduction zone conditions |
| topic | Magnesite Solubility Carbon speciation Fluid Subduction zone |
| url | http://www.sciencedirect.com/science/article/pii/S2451912X22000204 |
| work_keys_str_mv | AT wancaili insitudeterminationofmagnesitesolubilityandcarbonspeciationinwaterandnaclsolutionsundersubductionzoneconditions AT qinxiawang insitudeterminationofmagnesitesolubilityandcarbonspeciationinwaterandnaclsolutionsundersubductionzoneconditions |