Lithium isotopic systematics of submarine vent fluids from arc and back‐arc hydrothermal systems in the western Pacific

Abstract

The Li concentration and isotopic composition (d 7 Li) in submarine vent fluids are important for oceanic Li budget and potentially useful for investigating hydrothermal systems deep under the seafloor because hydrothermal vent fluids are highly enriched in Li relative to seawater. Although Li isotopic geochemistry has been studied at mid-ocean-ridge (MOR) hydrothermal sites, in arc and back-arc settings Li isotopic composition has not been systematically investigated. Here we determined the d 7 Li and 87 Sr/ 86 Sr values of 11 end-member fluids from 5 arc and back-arc hydrothermal systems in the western Pacific and examined Li behavior during high-temperature water-rock interactions in different geological settings. In sediment-starved hydrothermal systems (Manus Basin, Izu-Bonin Arc, Mariana Trough, and North Fiji Basin), the Li concentrations (0.23-1.30 mmol/kg) and d 7 Li values (14.3& to 17.2&) of the end-member fluids are explained mainly by dissolution-precipitation model during high-temperature seawater-rock interactions at steady state. Low Li concentrations are attributable to temperature-related apportioning of Li in rock into the fluid phase and phase separation process. Small variation in Li among MOR sites is probably caused by low-temperature alteration process by diffusive hydrothermal fluids under the seafloor. In contrast, the highest Li concentrations (3.40-5.98 mmol/kg) and lowest d 7 Li values (11.6& to 12.4&) of end-member fluids from the Okinawa Trough demonstrate that the Li is predominantly derived from marine sediments. The variation of Li in sediment-hosted sites can be explained by the differences in degree of hydrothermal fluid-sediment interactions associated with the thickness of the marine sediment overlying these hydrothermal sites. Key Points:First Li isotopic report on submarine vent fluids from arc and back-arc hydrothermal systems Li in vent fluids can be explained mainly by dissolution-precipitation model during seawater-rock and seawater-sediment interactions Variation in Li due to diffusive hydrothermal fluid and the degree of hydrothermal fluid-sediment interaction

Downstream and seasonal changes of lithium isotope ratios in the Ganges‐Brahmaputra river system

“…Hydrothermal activity can contribute water with extremely high Li concentrations and low δ 7 Li values [ Araoka et al ., ; Chan et al ., ; Henchiri et al ., ; Millot et al ., ; Tomascak et al ., ]. In fact, Kısakűrek et al .…”

“…Isotopic data are reported as per mil (‰) deviations relative to the NIST L‐SVEC standard. The uncertainty of the δ 7 Li value was better than ±0.3‰ (2 standard deviation), as estimated from its long‐term reproducibility from July 2011 to November 2011 using our in‐house Li standard [ Araoka et al ., ]. Our δ 7 Li value for the proposed seawater standard IRMM BCR‐403 of +30.9‰ [ Araoka et al ., ] agrees well with the previously reported value of +31.0 ± 0.5‰ (2 standard deviation, n = 30) [ Millot et al ., ] within an analytical uncertainty of ±0.3‰.…”

Application of B, Mg, Li, and Sr Isotopes in Acid‐Sulfate Vent Fluids and Volcanic Rocks as Tracers for Fluid‐Rock Interaction in Back‐Arc Hydrothermal Systems

“…Li is a fluid-mobile element, and while it is taken up from seawater into basalt at low to moderate temperatures, it is leached from oceanic crust during high-temperature water-rock interaction (Chan et al, 1992;Chan et al, 2002;James et al, 2003;Seyfried et al, 1984;Verney-Carron et al, 2015). Li concentrations and δ 7 Li values in hydrothermal vent fluids are mainly influenced by (1) Li concentrations and isotope ratios of the basement (fresh versus altered basement) (Chan et al, 1992;Chan et al, 2002), (2) w/r ratios during water-rock interaction (Araoka et al, 2016), and/or (3) the reaction temperature (James et al, 2003;Millot et al, 2010;Seyfried et al, 1984). Equilibrium Li isotope fractionation factors between solid and fluid range from α rock-fluid = 0.992 at 200°C to 0.995 at 300°C and 0.997 at 400°C (Millot et al, 2010).…”

“…Geochemistry, Geophysics, Geosystems high temperatures leads usually to an isotope fractionation of Li between fluid and solid phases (e.g., Araoka et al, 2016;Seyfried et al, 1984). Negligible Li isotope fractionation during dissolution of primary minerals is also observed during low-temperature processes, for example, weathering of silicate rocks (Pistiner & Henderson, 2003).…”

“…Hydrothermal vent fluids in submarine arc and back‐arc environments are influenced by the complex interplay between mantle wedge, subducting slab, seafloor morphology, and volcanic activity. In consequence, fluids venting in these tectonic settings are characterized by a broad range of chemical and physical variability (Araoka et al, ; Gamo et al, ; Gena et al, ; Reeves et al, ; Resing et al, ; Seewald et al, ; Wilckens et al, ; Yamaoka et al, ). A unique type of vent fluid, which is known from several arc and back‐arc sites in the Western Pacific, are acid‐sulfate fluids.…”