Ga isotopic fractionation in sulfides from the Yuhuang and Duanqiao hydrothermal fields on the Southwest Indian Ridge

This study examines the Ga isotopic compositions of sulfides in the Yuhuang and Duanqiao hydrothermal fields on the Southwest Indian Ridge, mid-ocean ridge basalts (MORB), and calcareous sediments around the hydrothermal fields. The delta Ga-71/69(NIST-994) values of the MORB samples vary little (+1.20 parts per thousand to +1.23 parts per thousand, with an average of +1.22 parts per thousand) and are consistent with the delta Ga-71/69(NIST-994) values of two standard basalt samples (BCR-2 and BHVO-2), indicating that Ga isotopes may either not fractionate or fractionate only slightly under high-temperature geological processes; therefore, the delta Ga-71/69(NIST-994) value of oceanic crust may be +1.22 parts per thousand. The sediments (+1.28 parts per thousand to +1.47 parts per thousand, with an average of +1.38 parts per thousand) are rich in heavier Ga isotopes than the basalts, and the Ga present in the sediments may have originated from soluble Ga present in the seawater that was adsorbed by (Mn, Fe) oxides/hydroxides. The Ga contribution of basaltic debris to the sediments was almost negligible. Thus, we speculate that the delta Ga-71/69(NIST-994) value of seawater in the study area fell within a range from +1.92 parts per thousand to +2.36 parts per thousand. The delta Ga-71/69(NIST-994) values of the sulfides in the Yuhuang hydrothermal field range from +0.99 parts per thousand to +1.57 parts per thousand, with an average of +1.25 parts per thousand, and the delta Ga-71/69(NIST-994) values of the sulfides in the Duanqiao hydrothermal field range from +0.93 parts per thousand to +1.55 parts per thousand, with an average of +1.19 parts per thousand. The delta Ga-71/69(NIST-994) ranges of the sulfides in the Yuhuang and Duanqiao hydrothermal fields are similar, with the Ga isotopic fractionation reaching 0.58 parts per thousand and 0.62 parts per thousand, respectively. The average delta Ga-71/69(NIST-994) values in the sulfides are close to those in the MORBs. This suggests that Ga within the sulfides in the Yuhuang and Duanqiao hydrothermal fields mainly originated from MORBs, with seawater and sediments making only small contributions. The Ga isotopic fractionation in the sulfides may be related to processes associated with the formation of sulfides, such as rapid precipitation or the admixture of different stages of sulfide. This study is of great significance for understanding the global distribution of Ga isotopes and the Ga cycle in submarine hydrothermal systems. (C) 2021 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V.