The ~2500 km long Kermadec-Tonga arc northeast ofNew Zealand is
the longest continuous intraoceanic arc in the world. At least 94 volcanic
centres along the arc are submarine, most (87 per cent) within the Kermadec arc
(de Ronde et al, 2003a). Volcanic
rocks range from basalt to rhyodacite with trace element and isotopic data
indicating significant magma source heterogeneity both along and across the
arc.

The
entire ~1300 km of the Kermadec arc has now been surveyed for hydrothermal
emissions (de Ronde et al, 2004).
First-stage surveys utilised surface vessels. In February 1996, the first
sulfide samples from the southern Kermadec arc were dredged from the Brothers
and Rumble II West caldera volcanoes (R/V Tangaroa cruise TAN96/03;
Wright et al, 1998; de Ronde et al, 2003b). In September 1998,
low-temperature, diffuse venting was discovered at Monowai, Rumble III and Clark
volcanoes, and high-temperature, black smoker venting was found within Brothers
caldera (R/V Sonne cruise SO-135; Stoffers et al, 1999). The NZAPLUME cruise
of
March 1999 surveyed 13
volcanic centres (each mainly comprised of single, large volcanoes) of the
southern Kermadec arc and documented hydrothermal emissions above seven of them
(de Ronde et al, 2001; Baker et al, 2003; Massoth et al, 2003). In July 2001, additional
mineralised and hydrothermally altered samples were dredged from Brothers
volcano (R/V Tangaroa cruise TAN01/07). The NZAPLUME II cruise of May
2002 confirmed venting was still occurring at all the known hydrothermal sites,
and that ten active vent sites occurred along the mid-part of the arc, between
Brothers and Raoul
Island (de Ronde et al, 2002). In September-October 2004,
NZAPLUME III surveyed the northern-most part of the Kermadec arc where another
nine volcanic centres are hydrothermally active.

Second-stage
exploration of the Kermadec arc hydrothermal systems utilised deep submergence
vehicles. In October-November 2004, the Japanese submersible Shinkai 6500
made four dives on Brothers. Samples recovered include vent fluids,
chimneys, altered rocks and various animals and microbes. In April-May 2005, a
more extensive expedition using the American submersible Pisces V made 23
dives on nine different volcanic centres, eight of them for the first time.

Extensive plume surveys
show Brothers volcano is host to the most hydrothermally active sites of all
those known along the Kermadec arc. In total, nine dives have been made on
Brothers, six on the NW caldera site and three on the cone site. The NW caldera
vent site is a long-term hydrothermal system that is today dominated by evolved
seawater but has had episodic injections of magmatic fluid. The cone site is a
nascent magmatic-hydrothermal system where crack zones localise upwelling acidic
waters (de Ronde et al, 2005). A SE
caldera site represents the main upflow of a relatively well-established
magmatic-hydrothermal system on the seafloor where sulfide-rich chimneys are
extant. Each of these different vent sites represent diverse parts of an
evolving hydrothermal system, any one of which may be typical of submarine
volcanic arcs.