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Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256

Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256
Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256
ODP/IODP Hole 1256D penetrates an in situ section of ocean crust formed at the East Pacific Rise, through lavas and sheeted dikes and ?100 m into plutonic rocks. We use mineralogy, oxygen isotopes, and fluid inclusions to understand hydrothermal processes. The lavas are slightly altered at low temperatures (<150°C) to phyllosilicates and iron oxyhydroxides, with a stepwise increase in grade downward to greenschist minerals in the upper dikes. This resulted from generally upwelling hydrothermal fluids in the dikes mixing with cooler seawater solutions in the lavas, also producing minor metal sulfide mineralization in the upper dikes. Alteration grade increases downward in the dikes, with increasing recrystallization to amphibole and loss of metals at higher temperatures (>350°C up to ?600°C). Intrusion of gabbro bodies into the lower dikes resulted in contact metamorphism to granoblastic hornfels at 850°C–900°C, representing a thermal boundary layer between the axial melt lens and the overlying hydrothermal system. Downward penetration of hydrothermal fluids led to rehydration of granoblastic dikes and plutonic rocks at ?800°C down to <300°C. Fluid inclusion and oxygen isotope data show that vein quartz formed at ?300°C to >450°C from hydrothermal fluids that were affected by supercritical phase separation. Fluids had variable salinities and were enriched in 18O (+0.4‰ to +3.5‰) relative to seawater, similar to seafloor vent fluids. Dike margins are brecciated and mineralized, suggesting hydrothermal activity coeval with magmatism. Anhydrite formed mainly in the upper dikes when partly reacted seawater fluids were heated as they penetrated deeper into the system. Low-temperature alteration of the volcanic section continued as cold seawater penetrated along fluid pathways, forming minor iron oxyhydroxides in the rocks. Hydrothermal processes at Site 1256 fit with current models whereby greenschist alteration of dikes at low water/rock ratios is overprinted by fracture-controlled alteration and mineralization by upwelling hydrothermal fluids, a conductive boundary layer above gabbroic intrusions, leaching of metals from dikes and gabbros in the deep “root zone,” and stepped thermal and alteration gradients in the basement. The Site 1256 section, however, is intact and retains recharge effects (anhydrite), allowing an integrated view of processes in the subsurface.
1525-2027
Q10010
Alt, Jeffrey C.
d2e22a46-a2e0-4d56-abbb-37199de80dbc
Laverne, Christine
f40796b6-e03a-4fea-b149-89130118129f
Coggon, Rosalind M.
09488aad-f9e1-47b6-9c62-1da33541b4a4
Teagle, Damon A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Banerjee, Neil R.
5d39afaa-a973-40f5-9aa1-91d68e50745a
Morgan, Sally
b090da90-e450-482d-a174-1da3c7c1483f
Smith-Duque, Christopher E.
72820fa7-dd47-4a64-8310-68d35b2973e8
Harris, Michelle
2ea5985e-614c-4d8a-9cb0-82d9590d4ebc
Galli, Laura
2c1b5862-8def-4ac3-b9a2-ecdc42b80bc8
Alt, Jeffrey C.
d2e22a46-a2e0-4d56-abbb-37199de80dbc
Laverne, Christine
f40796b6-e03a-4fea-b149-89130118129f
Coggon, Rosalind M.
09488aad-f9e1-47b6-9c62-1da33541b4a4
Teagle, Damon A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Banerjee, Neil R.
5d39afaa-a973-40f5-9aa1-91d68e50745a
Morgan, Sally
b090da90-e450-482d-a174-1da3c7c1483f
Smith-Duque, Christopher E.
72820fa7-dd47-4a64-8310-68d35b2973e8
Harris, Michelle
2ea5985e-614c-4d8a-9cb0-82d9590d4ebc
Galli, Laura
2c1b5862-8def-4ac3-b9a2-ecdc42b80bc8

Alt, Jeffrey C., Laverne, Christine, Coggon, Rosalind M., Teagle, Damon A.H., Banerjee, Neil R., Morgan, Sally, Smith-Duque, Christopher E., Harris, Michelle and Galli, Laura (2010) Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256. Geochemistry, Geophysics, Geosystems, 11 (10), Q10010. (doi:10.1029/2010GC003144).

Record type: Article

Abstract

ODP/IODP Hole 1256D penetrates an in situ section of ocean crust formed at the East Pacific Rise, through lavas and sheeted dikes and ?100 m into plutonic rocks. We use mineralogy, oxygen isotopes, and fluid inclusions to understand hydrothermal processes. The lavas are slightly altered at low temperatures (<150°C) to phyllosilicates and iron oxyhydroxides, with a stepwise increase in grade downward to greenschist minerals in the upper dikes. This resulted from generally upwelling hydrothermal fluids in the dikes mixing with cooler seawater solutions in the lavas, also producing minor metal sulfide mineralization in the upper dikes. Alteration grade increases downward in the dikes, with increasing recrystallization to amphibole and loss of metals at higher temperatures (>350°C up to ?600°C). Intrusion of gabbro bodies into the lower dikes resulted in contact metamorphism to granoblastic hornfels at 850°C–900°C, representing a thermal boundary layer between the axial melt lens and the overlying hydrothermal system. Downward penetration of hydrothermal fluids led to rehydration of granoblastic dikes and plutonic rocks at ?800°C down to <300°C. Fluid inclusion and oxygen isotope data show that vein quartz formed at ?300°C to >450°C from hydrothermal fluids that were affected by supercritical phase separation. Fluids had variable salinities and were enriched in 18O (+0.4‰ to +3.5‰) relative to seawater, similar to seafloor vent fluids. Dike margins are brecciated and mineralized, suggesting hydrothermal activity coeval with magmatism. Anhydrite formed mainly in the upper dikes when partly reacted seawater fluids were heated as they penetrated deeper into the system. Low-temperature alteration of the volcanic section continued as cold seawater penetrated along fluid pathways, forming minor iron oxyhydroxides in the rocks. Hydrothermal processes at Site 1256 fit with current models whereby greenschist alteration of dikes at low water/rock ratios is overprinted by fracture-controlled alteration and mineralization by upwelling hydrothermal fluids, a conductive boundary layer above gabbroic intrusions, leaching of metals from dikes and gabbros in the deep “root zone,” and stepped thermal and alteration gradients in the basement. The Site 1256 section, however, is intact and retains recharge effects (anhydrite), allowing an integrated view of processes in the subsurface.

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Published date: 2010
Organisations: Ocean and Earth Science

Identifiers

Local EPrints ID: 167879
URI: https://eprints.soton.ac.uk/id/eprint/167879
ISSN: 1525-2027
PURE UUID: 6695a47c-7765-4d74-8a89-3ccc83cc0ee5
ORCID for Damon A.H. Teagle: ORCID iD orcid.org/0000-0002-4416-8409

Catalogue record

Date deposited: 19 Nov 2010 15:32
Last modified: 06 Jun 2018 12:52

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