Behavior of Li isotopes during the alteration of oceanic crust: A review

Before subduction into the mantle, the oceanic crust formed at mid-ocean ridges would undergo fluid-rock interaction on the seafloor and within the crust. Study in this regard can enhance our understanding of the seafloor hydrothermal system and crust-mantle recycling. Lithium (Li) is strongly mobilized by hydrous fluids and has significant isotopic fractionation during many geological processes (e.g. weathering, seawater, and hydrothermal alteration), thus the variation in Li content and isotopic ratio provide information of the oceanic crust alteration. At present, geochemical data of altered oceanic crust are still lack, and the interpretations of available Li data are often controversial, which caused no consensus on the mechanism of the alteration process. We summarized the Li data of altered basalts and serpentinized peridotites from oceanic drilling cores, discussed the main factors controlling Li behaviors during the alteration process (e.g., the temperature of fluid-rock reaction, chemical composition of fluid, water-rock ratio, secondary mineral precipitation), and suggested that future studies shall be strengthened in the following directions: (1) keep adding new Li isotope data into the geochemical reservoirs and improving the accuracy of analysis; (2) conduct studies on Li isotopes at different spatial scales; (3) evaluate the effects of both equilibrium and kinetic fractionation when considering the high-temperature alteration processes; (4) combine Li and other isotope systems that display similar behaviors during the oceanic crust alteration.