Geochemistry of hydrothermal fluids from the PACMANUS, Northeast Pual and Vienna Woods hydrothermal fields, Manus Basin, Papua New Guinea
Date
2010-10-29Author
Reeves, Eoghan P.
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Seewald, Jeffrey S.
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Saccocia, Peter J.
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Bach, Wolfgang
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Craddock, Paul R.
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Shanks, Wayne C.
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Sylva, Sean P.
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Walsh, Emily
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Pichler, Thomas
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Rosner, Martin
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https://hdl.handle.net/1912/4549As published
https://doi.org/10.1016/j.gca.2010.11.008Abstract
Processes controlling the composition of seafloor hydrothermal fluids in silicic back-arc or neararc
crustal settings remain poorly constrained despite growing evidence for extensive magmatichydrothermal
activity in such environments. We conducted a survey of vent fluid compositions from two
contrasting sites in the Manus back-arc basin, Papua New Guinea, to examine the influence of variations
in host rock composition and magmatic inputs (both a function of arc proximity) on hydrothermal fluid
chemistry. Fluid samples were collected from felsic-hosted hydrothermal vent fields located on Pual
Ridge (PACMANUS and Northeast (NE) Pual) near the active New Britain Arc and a basalt-hosted vent
field (Vienna Woods) located farther from the arc on the Manus Spreading Center. Vienna Woods fluids
were characterized by relatively uniform endmember temperatures (273–285°C) and major element
compositions, low dissolved CO2 concentrations (4.4mmol/kg) and high measured pH (4.2–4.9 at 25°C).
Temperatures and compositions were highly variable at PACMANUS/NE Pual and a large, newly
discovered vent area (Fenway) was observed to be vigorously venting boiling (358°C) fluid. All
PACMANUS fluids are characterized by negative δDH2O values, in contrast to positive values at Vienna
Woods, suggesting substantial magmatic water input to circulating fluids at Pual Ridge. Low measured
pH (25°C) values (~2.6 to 2.7), high endmember CO2 (up to 274 mmol/kg) and negative δ34SH2S values
(down to -2.7‰) in some vent fluids are also consistent with degassing of acid-volatile species from
evolved magma. Dissolved CO2 at PACMANUS is more enriched in 13C (-4.1‰ to -2.3‰) than Vienna
Woods (-5.2‰ to -5.7‰), suggesting a contribution of slab-derived carbon. The mobile elements (e.g. Li,
K, Rb, Cs and B) are also greatly enriched in PACMANUS fluids reflecting increased abundances in the
crust there relative to the Manus Spreading Center. Variations in alkali and dissolved gas abundances
with Cl at PACMANUS and NE Pual suggest that phase separation has affected fluid chemistry despite
the low temperatures of many vents. In further contrast to Vienna Woods, substantial modification of
PACMANUS/NE Pual fluids has taken place as a result of seawater of seawater ingress into the upflow
zone. Consistently high measured Mg concentrations, trends of increasingly non-conservative SO4
behavior, decreasing endmember Ca/Cl and Sr/Cl ratios with increased Mg indicate extensive subsurface
anhydrite deposition is occurring as a result of subsurface seawater entrainment. Decreased pH and
endmember Fe/Mn ratios in higher Mg fluids indicate that the associated mixing/cooling gives rise to
sulfide deposition and secondary acidity production. Several low temperature (≤80°C) fluids at
PACMANUS/NE Pual also show evidence for anhydrite dissolution and water-rock interaction (fixation
of B) subsequent to seawater entrainment. Hence, the evolution of fluid compositions at Pual Ridge
reflects the cumulative effects of water/rock interaction, admixing and reaction of fluids exsolved from
silicic magma, phase separation/segregation and seawater ingress into upflow zones.
Description
Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 75 (2011): 1088-1123, doi:10.1016/j.gca.2010.11.008.
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Preprint: Reeves, Eoghan P., Seewald, Jeffrey S., Saccocia, Peter J., Bach, Wolfgang, Craddock, Paul R., Shanks, Wayne C., Sylva, Sean P., Walsh, Emily, Pichler, Thomas, Rosner, Martin, "Geochemistry of hydrothermal fluids from the PACMANUS, Northeast Pual and Vienna Woods hydrothermal fields, Manus Basin, Papua New Guinea", 2010-10-29, https://doi.org/10.1016/j.gca.2010.11.008, https://hdl.handle.net/1912/4549Related items
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