Geochemical characterization of highly diverse hydrothermal fluids from volcanic vent systems of the Kermadec intraoceanic arc

Abstract During the R/V Sonne cruise SO253 in 2016/2017, hydrothermal vent sites along the Kermadec intraoceanic arc were sampled for hydrothermal fluids at four active volcanoes: Macauley, Haungaroa, Brothers and Rumble III, respectively. Water depths ranged between 290 m and 1700 m. A new vent field was discovered at Haungaroa. The samples were taken from diffuse-flow sites as well as from white and black smokers – rich in metals and gases – with discharge temperatures as high as 311 °C. Their fluid composition is very variable but basically divides into two types: one that indicates distinct magmatic input and another that shows evidence for intense water-rock interaction under hot, acidic conditions. Fluid samples from Macauley, the shallowest sampling site (~300 m), had Fe concentrations as high as 1.7 mM, Al concentrations up to 122 μM and H2S up to 10 mM at a pH of only 1.2. At Brothers, the deepest sampling site (down to 1600 m), we identified two different fluid types: 1) A magmatically-influenced type at the Upper and Lower Cone with highest temperatures of 115 °C, up to 95.6 mM Mg (the highest Mg concentration measured in fluids from intraoceanic arc systems so far), elevated SO42− (76.9 mM), high H2S (5.0 mM), but Fe concentrations of only 15 μM and 2) A fluid with low Mg (5.4 mM), low H2S (1.1 mM), temperatures reaching 311 °C and high Fe contents (12.4 mM) at the Upper Caldera and NW Caldera Wall, typical of a black smoker fluid. Chloride concentrations in all fluids were similar, or highly enriched when compared to seawater (e.g. up to 787 mM, brine fluids), with also one low-chlorinity vapor-phase fluid sample recovered, indicating that phase separation is occurring at Brothers. Unusual highly elevated Mg concentrations in fluids from the Brothers Lower Cone (95.6 mM, compared to 53.2 mM in ambient seawater) combined with highly elevated concentrations of SO42− (76.9 mM, compared to 29.0 mM in ambient seawater) indicate dissolution of Mg- and SO42−-bearing minerals in the subsurface, such as caminite. Our data show how highly diverse and variable island arc systems can be with respect to their fluid chemistry, both spatially and temporally. It adds to the still limited data set of arc systems compared to mid-ocean ridges and supplies an important contribution towards a better understanding of geochemical processes along arc volcanoes. The higher range in fluid chemistry together with shallower water depth implies that the fluids from intraoceanic arcs may contribute a significant fraction of dissolved metals not only to the global oceanic biogeochemical cycle but also into the photic zone, the area of highest bioproductivity.