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

Abstract

The Manus Basin hosts a broad range of vent fluid compositions typical for arc and back-arc settings, ranging from black smoker to acid-sulfate styles of fluid venting, as well as novel intermediate temperature and composition "hybrid" smokers. We investigated B, Li, Mg, and Sr concentrations and isotopic compositions of these different fluid types as well as of fresh and altered rocks from the same study area to understand what controls their compositional variability. In particular, the formation of acid-sulfate and hybrid smoker fluids is still poorly understood, and their high Mg concentrations are explained either by dissolution of Mg-bearing minerals in the basement or by mixing between unmodified seawater and magmatic fluids. Mg isotope ratios of the acid-sulfate fluids from the Manus Basin are seawater-like, which supports the idea that acid-sulfate fluids in this study area predominantly form by mixing between unmodified seawater and a Mg-free magmatic fluid. Changes in the B, Li, and Sr isotope ratios relative to seawater indicate water-rock interaction in all acid-sulfate fluids. Further, the combination of δ 7 Li with B concentrations of the same fluids links changes in δ 7 Li to changes in (1) basement alteration, (2) water-to-rock ratios during water-rock interaction, and/or (3) the reaction temperature. These isotope systems, thus, allow tracing of basement composition and acid-sulfate-driven alteration of the back-arc crust and help increase our understanding of hydrothermal fluid-rock interactions and the behavior of fluid-mobile elements.

Sr isotopic ratios of hydrothermal fluids from the Okinawa Trough and the implications of variation in fluid–sediment interactions

“…The low Sr isotopic ratios and consistency between fluids and volcanic rocks demonstrate that virtually all seawaterderived 87 Sr recharged into the subseafloor was removed, and 87 Sr-depleted Sr was leached from volcanic rocks into the fluid. This observation-based model is consistent with experimental simulation studies on hydrothermal fluid-mineral interactions, in which Sr in the liquid phase (i.e., recharged seawater) is removed through co-precipitation with anhydrite when heated to 200 °C (Albarède et al 1981;Wilckens et al 2019), whereas Sr in some minerals (e.g., from volcanic rock and sediment) is released into fluids at temperatures above 200 °C (Wei 2007;Berndt et al 1988;James et al 2003). The removal of seawater-derived Sr through co-precipitation with anhydrite is also expected in sediment-associated hightemperature hydrothermal fluids because of its sulfatedepleted nature at temperatures higher than 200 °C (Albarède et al 1981;Wilckens et al 2019).…”

“…This observation-based model is consistent with experimental simulation studies on hydrothermal fluid-mineral interactions, in which Sr in the liquid phase (i.e., recharged seawater) is removed through co-precipitation with anhydrite when heated to 200 °C (Albarède et al 1981;Wilckens et al 2019), whereas Sr in some minerals (e.g., from volcanic rock and sediment) is released into fluids at temperatures above 200 °C (Wei 2007;Berndt et al 1988;James et al 2003). The removal of seawater-derived Sr through co-precipitation with anhydrite is also expected in sediment-associated hightemperature hydrothermal fluids because of its sulfatedepleted nature at temperatures higher than 200 °C (Albarède et al 1981;Wilckens et al 2019). This suggests that Sr derived from recharged seawater accounts for a negligible proportion of Sr in the hydrothermal vent fluids in the Okinawa Trough.…”

Tracking the Lithium and Strontium Isotope Signature of Hydrothermal Plume in the Water Column: A Case Study at the EMSO-Azores Deep-Sea Observatory

“…The mixing between hydrothermal fluid and seawater involves physicochemical gradients that lead to mineral precipitation (sulfate-bearing minerals, polymetallic sulfides, iron oxyhydroxides, and manganese oxides) which can be further dissolved and reprecipitated in the buoyant to the nonbuoyant hydrothermal plume (Chavagnac et al, 2018b;Lee et al, 2021). Such mineral-seawater interactions (mineral precipitation and dissolution, element adsorption) can induce Li isotope fractionation, i.e., the preferential adsorption of 6 Li over 7 Li on mineral (Decitre et al, 2004;Chan and Hein, 2007;Vigier et al, 2008;Araoka et al, 2016;Wilckens et al, 2019). The δ 7 Li and DLi datasets of all casts do not follow the conservative mixing curve between hydrothermal fluid and seawater endmembers (Figures 4A,B).…”

Strontium, Hydrogen and Oxygen Behavior in Vent Fluids and Plumes from the Kueishantao Hydrothermal Field Offshore Northeast Taiwan: Constrained by Fluid Processes

“…The intensity of the water-rock interaction has been recorded by these isotopes in the hydrothermal fluids, and they are particularly useful in the research of fluid origin and hydrothermal processes in subseafloor hydrothermal systems [1,6,7]. The mobile Sr, H and O isotopes are good markers of host rock composition and subseafloor alteration processes in hydrothermal systems [2,4,8,9]. Sr, H and O in hydrothermal fluids come from different sources, relying on the hydrothermal circulation system, seawater and fluid mixing, host rocks and magma [2,4,10,11].…”