Effects of Subduction on Arc Seafloor Hydrothermal Systems

Abstract:

Systematic surveys of SW Pacific intraoceanic arcs have shown hydrothermal activity related to arc volcanoes is commonplace, e.g., the 1220 km long Kermadec arc (KA) is host to 33 volcanic centres where 25 are hydrothermally active, or an incidence of venting of 76%. Similarly, the 1310 km long Tofua arc has ~30% of all its volcanoes hydrothermally active and the 1360 km long Mariana arc 44%; a worldwide estimate has 43% of submarine arc volcanoes venting. The minimum density inferred for active submarine volcanic centers along intraoceanic arcs is 1.1/100 km. Venting related to the ~6900 km of submarine arcs is thus considered to be equivalent of ~9% of that associated with the ~60,000 km of MORs. The hydrothermal systems can be divided between volcanic (5%), magmatic-hydrothermal (70%) and water-rock dominated (~25%) systems. The chemical composition of discharing hydrothermal fluids from arc vent fields is much more heterogenous than those of MORs, with much higher magmatic gas contents (i.e., CO₂ and SO₂/H₂S). Some vent fluids show evidence for subseafloor phase separation while others have higher than seawater values of Mg and SO₄. The latter fluids commonly have pH values <2 with the deposition of native sulfur commonplace, a direct result of the disproportionation of S gases. Initial ²²⁶Ra/Ba values for recent mineralization are comparable for samples collected from volcanoes along the same arc, but differ between arcs. For example, Brothers and Clark volcanoes of the KA have ²²⁶Ra/Ba values of 22.7 ± 2.0 and 19.1 ± 0.4 Bq.g^-1, respectively, compared to 47.4 ± 2.6 and 41.1 ± 0.5 Bq.g^-1 for East Diamante and Nikko volcanoes, respectively, of the Mariana arc. Average spacing between KA volcanic centers increases from 30 km in the volcanic front of the southern KA, to 45 km in the mid-KA, to 58 km for those situated on the Kermadec Ridge in the northern KA. The centers are dominated by mafic cones in the south and by calderas on the Kermadec Ridge. The 15-23 km thick, southward moving Hikurangi Plateau is today subducting beneath the southern KA and may be responsible for the formation of large, volatile-rich magma chambers beneath the region, leading to prolonged volatile/fluid (and hence metal) transfer to hydrothermal systems, as exemplified by ≤20 ka old mineralization at Clark and ≤1.2 ka mineralization at Brothers.