Abstract
AbstractThe 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 δ7Li with B concentrations of the same fluids links changes in δ7Li 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.
Highlights
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
Mg isotope ratios show that the high Mg concentrations in the acid‐sulfate fluids from the Manus Basin cannot be explained by dissolution of Mg‐bearing minerals
Mg appears to be seawater derived, since the Mg isotope ratios of all acid‐sulfate fluids resemble those of seawater, which confirms the hypotheses that acid‐sulfate fluids form in principal through direct injection of magmatic volatiles into unmodified seawater
Summary
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. Acid‐sulfate fluids are characterized by low pH, high Mg and sulfate concentrations, and high gas contents (Gamo et al, 1997; Lupton et al, 2006., 2011; Resing et al, 2007; de Ronde et al, 2011; Butterfield et al, 2011; Seewald et al, 2015). Black smoker fluids form mainly through water‐rock interaction of seawater delivered to the subsurface via hydrothermal convection (Figure 1a).
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