Temporal evolution of magmatic-hydrothermal systems in the Manus Basin, Papua New Guinea: Insights from vent fluid chemistry and bathymetric observations

Abstract:

The temporal evolution of hydrothermal fluids from back-arc systems is poorly constrained, despite growing evidence for dynamic magmatic–hydrothermal activity, and imminent commercial mining. Here we discuss surveys of diverse vent fluids from multiple hydrothermal fields in the Manus back-arc basin, Papua New Guinea, sampled in 2006 and 2011. Effects of host rock composition, and dynamic magmatic volatile inputs on fluid chemistry are evaluated to understand changes in these systems. Highly acidic and SO₄-rich moderate temperature fluids (~48–215°C), as well as SO₄-poor black smoker fluids (up to 358°C), were collected at the PACMANUS, SuSu Knolls and DESMOS areas in 2006 and 2011. Acidic, milky white SuSu and DESMOS fluids, rich in elemental S and SO₄, exit the seafloor with Na, K, Mg, and Ca diluted conservatively up to 30% relative to seawater, implying subsurface mixing of seawater with SO₂-rich aqueous fluids exsolved from magma, analogous to subaerial fumarole discharge. SO₂ disproportionation during cooling and mixing of magmatic fluids contributes acidity, SO₄, H₂S and S⁽⁰⁾_(s), as well as widespread S outcrops on the seafloor. Nearby black smoker fluids indicate entrainment and reaction of magmatic fluid into convecting fluids at depth, and additional hybrid-type fluids appear to consist of evolved seawater and unreacted magmatic fluid SO₂ derivatives. Fluids at DESMOS in 2006 indicate increased magmatic SO₂ relative to 1995, despite constant low venting temperatures (~120°C). In contrast, dramatic changes in bathymetry and seafloor morphology point to substantial continuous eruption of volcaniclastic material between 2006 and 2011 at SuSu Knolls, burying fumarolic vents from 2006. Compositions of new 2011 acidic, sulfate-rich fluids there suggest reaction with less altered, fresher rock. At the PACMANUS area, farther from the arc, direct magmatic degassing to the seafloor is not occurring presently, but entrainment and reaction of similar acid-sulfate fluids deep within the reaction zone has persisted from 2006-2011 based on black smoker compositions. Overall, these results help to elucidate temporal trends in processes responsible for the formation of hot-spring fluids in magmatically active back-arc environments, and resulting chemical exchange between the crust and water column.