Chemistry of hydrothermal plumes above submarine volcanoes of the Mariana Arc

Fifty submarine volcanoes of the Mariana Arc, covering 1200 km from 13.5°N to 23.2°N, were surveyed for hydrothermal activity. Of these 50 volcanoes, eight showed limited evidence of hydrothermal activity, while another 10 volcanoes displayed intense, chemically rich, hydrothermal plumes that allow detailed chemical characterization and insight into the hydrothermal activity forming the fluids that generate the plumes. The most active volcanoes exhibit a wide range of CO2 to pH relationships, from the venting of acid‐rich fluids, to CO2‐rich fluids, to fluids rich in alkalinity and CO2. These pH‐CO2‐alkalinity relationships are partially responsible for the wide range of Fe:Mn (3 to 32) observed at the different volcanoes. This chemical heterogeneity is further manifest in a wide range of CO2:3He (3 × 109 to 55 × 109), indicating that for all but one, >80% of the CO2 venting from these volcanoes has a slab source. The helium, by contrast, has an upper mantle isotopic signature. The range of chemical conditions suggests that these volcanoes occupy various states of eruptive evolution ranging from ongoing magmatic activity to highly evolved hydrothermal systems.