Arsenic in marine hydrothermal fluids

Hydrothermal fluids emerging at the seafloor near mid-ocean ridges (MOR) or in back-arc basins (BAB) and island arc (IA) settings are known to contain a considerable amount of dissolved metals (e.g. Fe, Zn, Cu, Cd) due to water–rock interaction at elevated temperatures and potentially magmatic degassing. The chemical input from these fluids plays an important role for ocean chemistry and the cycling of elements. Despite its notoriety as an environmental toxin and its abundance in hydrothermal ore deposits in combination with gold, arsenic (As) is one of the few elements, which has been rarely investigated.

Compared to the amount of As in open ocean seawater of around 1.7 μg L− 1, hydrothermal fluids can have significantly higher concentrations. Fluids from MOR hydrothermal systems such as the East Pacific Rise can contain up to 80.5 μg L− 1 As and at the Mid-Atlantic Ridge the highest values were around 24 μg L− 1. Those values, although elevated are surpassed by As concentrations in BAB and IA hosted hydrothermal systems, which can be as high as 1386 µg L− 1 in BAB settings and even higher with values up to 5850 µg L− 1 (~3900-times seawater) in IA shallow-water settings, occurring near shore hydrothermal systems.

The most important factors controlling the amount of As in hydrothermal fluids are the different – mostly physicochemical – conditions at and beneath the seafloor. These include temperature (controlling phase separation of the fluids and leaching processes in the host rock), pressure as a function of depth, pH directly influencing leaching processes, As mobility and speciation, reaction time and maturity of the system in combination with redox reactions and diverse chemical reactions like adsorption and desorption. The concentration of As in the underlying host rock may also play a role, although its mineralogical association may be more important than bulk rock concentration. Additional input of As could be caused by degassing magmatic metal-rich volatiles or sediment-fluid interaction in sediment covered hydrothermal systems. The contribution from magmatic volatiles is hard to quantify and therefore often neglected in calculations and discussions. Of the two As redox species, arsenate (AsV) and arsenite (AsIII), arsenite is easier transported in the vapor phase. However, As speciation has not been traditionally part of chemical analyses. Prior to discharge at the seafloor, the concentration of As in hydrothermal fluids may decrease rapidly, if precipitation of As-bearing and As-scavenging minerals, such as pyrite, orpiment, claudetite and hydrous ferric/manganese oxides, occurs in the shallow subsurface as shown by thermodynamic calculations. This is consistent to the observation that those hydrothermal fluids high in Fe and Mn are often low in As.

Arsenic concentrations in fluids from shallow BAB/IA hydrothermal systems are higher than those in deep situated MOR hydrothermal systems most likely due to their different physicochemical conditions. Basaltic host rocks (MOR) and dacitic/andesitic host rocks (BAB/IA) have As concentrations which are more or less identical and cannot account for the huge differences in fluid As concentration.

Arsenic is one of the few trace elements whose concentration in seawater is higher than in river water, which could be caused by the flux of As from hydrothermal systems. Estimating that between 3.0 × 103 and 1.25 × 108 kg As could be discharged at the MOR annually, which is a substantial amount compared to 53.9 × 106 kg transported annually by rivers. Higher flux can be expected at BAB and IA settings, however data is sparse. A single IA system, confined to a very small area of 60 by 100 m (Tutum Bay, Papua New Guinea), discharges around 5.5 × 102 kg As annually.