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CO 2 Induced a Small Water Solubility in Orthopyroxene and Its Implications for Water Storage in the Upper Mantle
Author(s) -
Guo Xinzhuan,
Bai Jiayuan,
Wang Chao,
Wu Xiang,
Zhou Xingfan
Publication year - 2020
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2019jb018745
Subject(s) - solubility , fugacity , peridotite , mineral redox buffer , aqueous solution , mantle (geology) , chemistry , analytical chemistry (journal) , dissolution , olivine , mineralogy , geology , thermodynamics , environmental chemistry , geochemistry , physics
To investigate the effect of CO 2 on water solubility in orthopyroxene coexisting with H 2 O‐CO 2 as a buffering fluid, high‐pressure experiments were conducted at pressures of 1.5 and 3 GPa and a temperature of 1100 °C. The experiments were performed in a Walker‐type multianvil assembly using natural orthopyroxene with various CO 2 to CO 2 ‐H 2 O molar ratios as a starting material. The water contents were measured by polarized Fourier transform infrared spectrometry. At 1.5 GPa and 1100 °C, the H 2 O solubility decreased with increasing CO 2 content in the fluid. The water solubility c (H 2 O) could be quantitatively determined based on water fugacity f (H 2 O) as c (H 2 O) = 25.21 × f (H 2 O) 1.24 . The addition of 57% CO 2 dramatically reduced the water solubility in orthopyroxene from 184 to 90 ppm. In contrast, at 3 GPa and 1100 °C, the water solubility did not change with the CO 2 content in the starting material because CO 2 is unstable in bulk peridotite due to the reaction between CO 2 and olivine at pressures exceeding 2.2 GPa. This study confirms that the additional component in the aqueous fluid can change the water activity and fugacity, thereby directly lowering the water storage capacity in mantle minerals. As a result, previous estimates of the maximum water storage capacity in the shallow mantle may be overestimated by a factor of 3.