SUBMARINE HYDROTHERMAL SYSTEMS ALONG INTRAOCEANIC ARCS

Volcanic arcs with a submarine component (n=21) include both intraoceanic and island arcs that combined total 21,700 km (cf. ~60,000 km for all MORs). About 93% of the arcs (~20,000 km) occur in the Pacific region. The number of volcanoes known to occur along these arcs is 693, with at least 206 (29%) being submarine.

Arcs that have been surveyed in detail for their hydrothermal plumes between 1999 and 2003 include; the Kermadec (840 km), Tofua (425), Solomons (125), Tabar-Lihir-Tanga-Feni (225) and Mariana arcs (1290), or ~13% of all arcs (37% of the 6,900 km of intraoceanic arcs). If we include the less systematic surveys done over volcanoes of the Izu-Bonin arc (1,200 km), then ~19% of all arcs have been surveyed, or over half (54%) of the intraoceanic arcs. The number of volcanoes that have been surveyed along intraoceanic arcs is 137, or one every 27 km of arc length, the same as for all arcs combined. Actively venting volcanoes along intraoceanic arcs total 43 (31%), or one vent site every 87 km of arc length. Extrapolation of this frequency of venting to the remaining sections of unsurveyed intraoceanic arcs means 36 vent sites have yet to be discovered. Thus submarine hydrothermal venting associated with intraoceanic arcs is significant on a global scale.

Most volcanoes along intraoceanic arcs are simple cones, although caldera volcanoes are well represented. Depths to vent sites range from ~130 m to 1,1650 m thus hydrothermal emissions are being injected into the mid- to shallow-parts of the oceans. The vast majority of vents occur at, or very near, the summits of the cones with subordinate flank vent sites up to 200 m deeper. Vent sites within caldera volcanoes are mainly located adjacent to the caldera walls.

Venting along the Kermadec arc is characterized by hydrothermal plumes that are chemically heterogeneous when compared to MOR sites, i.e., they range from being highly enriched in dissolved ionic species (e.g., Fe) and ³He, CO₂ and sulfur gases, to ³He-rich but with very low concentrations of ionic species. Evidence for a magmatic component in arc vent systems is given by the nature and concentrations of various gases, and Fe. The venting of hydrothermal systems at relatively shallow depths and with high gas contents will assist phase separation and should promote the formation of massive sulfide (Cu-Zn ± Pb ± Au) deposits.