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Fluid mixing and thermal regimes beneath the PACMANUS hydrothermal field, Papua New Guinea: Helium and oxygen isotope data

Fluid mixing and thermal regimes beneath the PACMANUS hydrothermal field, Papua New Guinea: Helium and oxygen isotope data
Fluid mixing and thermal regimes beneath the PACMANUS hydrothermal field, Papua New Guinea: Helium and oxygen isotope data
Fluid mixing processes and thermal regimes within the Snowcap and Roman Ruins vent sites of the PACMANUS hydrothermal system, Papua New Guinea, were investigated using 3He/4He ratios from fluid inclusions within pyrite and anhydrite and the ?18O signature of anhydrite. Depressed 3He/4He ratios of 0.2–6.91RA appear to be caused by significant atmospheric diffusive exchange, whilst He–Ne diffusive fractionation precludes correction using 20Ne. 40Ar/36Ar ratios of 295–310 are elevated above seawater, indicating the majority of argon is seawater derived but with a magmatic component. ?18O anhydrite ratios are 6.5‰ to 11‰ for Snowcap and 6.4‰ to 11.9‰ for Roman Ruins. Using oxygen isotope fractionation factors for the anhydrite–water system, the temperatures calculated assuming isotopic equilibrium at depth are up to 100 °C cooler than fluid inclusion trapping temperatures. It is likely that anhydrite is precipitated rapidly, preventing ?18O equilibration. By comparing new ?18O values for anhydrite with corresponding published 87Sr/86Sr ratios, seawater is inferred to penetrate deep into the Snowcap system with little conductive heating. A simple fluid mixing model has been constructed whereby the differing venting styles can be explained by a plumbing system at depth which favors delivery of end-member hydrothermal fluid to the high temperature sites.
PACMANUS, hydrothermal, helium isotopes, fluid inclusions
0012-821X
93-102
Webber, A.P.
c9b7b1e3-ea40-4740-805e-914806eeaeb7
Roberts, S.
f095c7ab-a37b-4064-8a41-ae4820832856
Burgess, R.
84917ca4-67d2-4c1a-8c88-d41180c5f08a
Boyce, A.J.
4c7d549e-ca7d-4bed-89b6-a7364513d130
Webber, A.P.
c9b7b1e3-ea40-4740-805e-914806eeaeb7
Roberts, S.
f095c7ab-a37b-4064-8a41-ae4820832856
Burgess, R.
84917ca4-67d2-4c1a-8c88-d41180c5f08a
Boyce, A.J.
4c7d549e-ca7d-4bed-89b6-a7364513d130

Webber, A.P., Roberts, S., Burgess, R. and Boyce, A.J. (2011) Fluid mixing and thermal regimes beneath the PACMANUS hydrothermal field, Papua New Guinea: Helium and oxygen isotope data. Earth and Planetary Science Letters, 304 (1-2), 93-102. (doi:10.1016/j.epsl.2011.01.020).

Record type: Article

Abstract

Fluid mixing processes and thermal regimes within the Snowcap and Roman Ruins vent sites of the PACMANUS hydrothermal system, Papua New Guinea, were investigated using 3He/4He ratios from fluid inclusions within pyrite and anhydrite and the ?18O signature of anhydrite. Depressed 3He/4He ratios of 0.2–6.91RA appear to be caused by significant atmospheric diffusive exchange, whilst He–Ne diffusive fractionation precludes correction using 20Ne. 40Ar/36Ar ratios of 295–310 are elevated above seawater, indicating the majority of argon is seawater derived but with a magmatic component. ?18O anhydrite ratios are 6.5‰ to 11‰ for Snowcap and 6.4‰ to 11.9‰ for Roman Ruins. Using oxygen isotope fractionation factors for the anhydrite–water system, the temperatures calculated assuming isotopic equilibrium at depth are up to 100 °C cooler than fluid inclusion trapping temperatures. It is likely that anhydrite is precipitated rapidly, preventing ?18O equilibration. By comparing new ?18O values for anhydrite with corresponding published 87Sr/86Sr ratios, seawater is inferred to penetrate deep into the Snowcap system with little conductive heating. A simple fluid mixing model has been constructed whereby the differing venting styles can be explained by a plumbing system at depth which favors delivery of end-member hydrothermal fluid to the high temperature sites.

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More information

Published date: 1 April 2011
Keywords: PACMANUS, hydrothermal, helium isotopes, fluid inclusions
Organisations: Geochemistry, Ocean and Earth Science

Identifiers

Local EPrints ID: 180197
URI: http://eprints.soton.ac.uk/id/eprint/180197
ISSN: 0012-821X
PURE UUID: f4046b6b-8787-4f02-9e8d-b6b79cbdedc9
ORCID for S. Roberts: ORCID iD orcid.org/0000-0003-4755-6703

Catalogue record

Date deposited: 05 Apr 2011 13:48
Last modified: 20 Jul 2019 01:26

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