Leybourne, M. I., Schwarz-Schampera, U., de Ronde, C. E. J., Baker, E. T., Faure, K., Walker, S. L., Butterfield, D. A., Resing, J. A., Lupton, J. E., Hannington, Mark D., Gibson, H. L., Massoth, G. J., Embley, R. W., Chadwick, W. W., Clark, M. R., Timm, Christian , Graham, I. J. and Wright, I. C. (2012) Submarine Magmatic-Hydrothermal Systems at the Monowai Volcanic Center, Kermadec Arc. Economic Geology, 107 (8). pp. 1669-1694. DOI 10.2113/econgeo.107.8.1669.

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Abstract

The Monowai volcanic center is located at the midpoint along the ~2,530-km-long Tonga-Kermadec arc system.
The Monowai volcanic center is comprised of a large elongate caldera (Monowai caldera area ~35 km2;
depth to caldera floor 1,590 m), which has formed within an older caldera some 84 km2 in area. To the south
of this nested caldera system is a large composite volcano, Monowai cone, which rises to within ~100 m of the
sea surface and which has been volcanically active for the past several decades. Mafic volcanic rocks dominate
the Monowai volcanic center; basalts are the most common rock type recovered from the cone, whereas
basaltic andesites are common within the caldera. Hydrothermal plume mapping has shown at least three
major hydrothermal systems associated with the caldera and cone: (1) the summit of the cone, (2) low-temperature
venting (<60°C; Mussel Ridge) on the southwestern wall of the caldera, and (3) a deeper caldera
source with higher temperature venting that has yet to be observed. The cone summit plume shows large
anomalies in pH (a shift of −2.00 pH units) and δ3He (≤358%), and noticeable H2S (up to 32 μm), and CH4
(up to 900 nm). The summit plume is also metal rich, with elevated total dissolvable Fe (TDFe up to 4,200
nm), TDMn (up to 412 nm), and TDFe/TDMn (up to 20.4). Particulate samples have elevated Fe, Si, Al, and
Ti consistent with addition to the hydrothermal fluid from acidic water-rock reaction. Plumes extending from
~1,000- to 1,400-m depth provide evidence for a major hydrothermal vent system in the caldera. The caldera
plume has lower values for TDFe and TDMn, although some samples show higher TDMn concentrations than
the cone summit plume; caldera plume samples are also relatively gas poor (i.e., no H2S detected, pH shift of
−0.06 pH units, CH4 concentrations up to 26 nm). The composition of the hydrothermal plumes in the caldera
have higher metal contents than the sampled vent fluids along Mussel Ridge, requiring that the source of the
caldera plumes is at greater depth and likely of higher temperature. Minor plumes detected as light scattering
anomalies but with no 3He anomalies down the northern flank of the Monowai caldera most likely represent
remobilization of volcanic debris from the volcano flanks.
We believe the Monowai volcanic center is host to a robust magmatic-hydrothermal system, with significant
differences in the style and composition of venting at the cone and caldera sites. At the cone, the large shifts
in pH, very high δ3He% values, elevated TDFe and TDFe/TDMn, and the H2S- and CH4-rich nature of the
plume fluids, together with elevated Ti, P, V, S, and Al in hydrothermal particulates, indicates significant magmatic
volatile ± metal contributions in the hydrothermal system coupled with aggressive acidic water-rock
interaction. By contrast, the caldera has low TDFe/TDMn in hydrothermal plumes; however, elevated Al and
Ti contents in caldera particulate samples, combined with the presence of alunite, pyrophyllite, sulfide minerals,
and native sulfur in samples from Mussel Ridge suggest past, and perhaps recent, acid volatile-rich venting
and active Fe sulfide formation in the subsurface.

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