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 δ71/69GaNIST-994values of the MORB samples vary little(+1.20‰ to +1.23‰,with an average of +1.22‰) and are consistent with the δ71/69GaNIST-994values 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 δ71/69GaNIST-994value of oceanic crust may be +1.22‰.The sediments(+1.28‰to +1.47‰,with an average of +1.38‰) 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 δ71/69GaNIST-994value of seawater in the study area fell within a range from +1.92‰ to +2.36‰.The δ71/69GaNIST-994values of the sulfides in the Yuhuang hydrothermal field range from +0.99‰ to +1.57‰,with an average of +1.25‰,and the δ71/69GaNIST-994values of the sulfides in the Duanqiao hydrothermal field range from+0.93‰ to +1.55‰,with an average of +1.19‰.The δ71/69GaNIST-994ranges of the sulfides in the Yuhuang and Duanqiao hydrothermal fields are similar,with the Ga isotopic fractionation reaching 0.58‰ and 0.62‰,respectively.The average δ71/69GaNIST-994values 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.