Back-Arc Hydrothermal Systems in the Lau and Manus Basins
M.K. Tivey*, W. Bach, P.R. Craddock, M. Mottl, J. Seewald, M.A. Tivey, C.G.
Wheat
* Woods Hole Oceanographic Insitution, Woods Hole, MA, USA; Email:
mktivey@whoi.edu
Hydrothermal systems in back-arc basins (BABs) can differ from those along mid-ocean
ridges (MORs) as a result of different rock composition (e.g., andesite to rhyolite versus
basalt), water depths, depths to magma lenses, crustal structure, temperatures of
underlying melt, and/or input of magmatic volatiles not found in MOR magmas (e.g.,
SO2). These differences can affect compositions of hydrothermal fluids and related
deposits. In 2005 and 2006, comprehensive suites of high temperature fluids and paired
vent deposits were collected from nine unsedimented hydrothermal areas along the
Eastern Lau Spreading Center and in the Manus Basin. Data from five of the vent areas
document a lack of significant input of magmatic acid volatiles, and the effect that
different substrate compositions (basalt to andesite-rhyolite) have on fluid and deposit
compositions (e.g., enrichments of Ba and Pb in fluids and deposits in systems with more
felsic substrate; enrichments of Zn in deposits despite low fluid Zn concentrations
because of high fluid pH relative to MOR vent fluids). Data from four of the vent areas
provide evidence for significant input of magmatic acid volatiles, which results in low
fluid pH, enhanced metal mobility, and consequent differences in deposit compositions.
Data from the Lau and Manus Basin hydrothermal areas demonstrate that, while BAB
hydrothermal systems are in some ways more complex than MOR systems, they can be
used to better our understanding of volcanogenic-hosted massive sulfide deposit
formation.