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Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D

Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D
Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D
ODP Hole 1256D in the eastern equatorial Pacific is the first penetration of a complete section of fast spread ocean crust down to the dike–gabbro transition, and only the second borehole to sample in situ sheeted dikes after DSDP Hole 504B. Here a high spatial resolution record of whole rock and mineral strontium isotopic compositions from Site 1256 is combined with core observations and downhole wireline geophysical measurements to determine the extent of basalt–hydrothermal fluid reaction and to identify fluid pathways at different levels in the upper ocean crust.

The volcanic sequence at Site 1256 is dominated by sheet and massive lava flows but the Sr isotope profile shows only limited exchange with seawater. However, the upper margins of two anomalously thick (>25 m) massive flow sequences are strongly hydrothermally altered with elevated Sr isotope ratios and appear to be conduits of lateral low-temperature off-axis fluid flow. Elsewhere in the lavas, high 87Sr/86Sr are restricted to breccia horizons. Mineralised hyaloclastic breccias in the Lava–Dike Transition are strongly altered to Mg-saponite, silica and pyrite, indicating alteration by mixed seawater and cooled hydrothermal fluids. In the Sheeted Dike Complex 87Sr/86Sr ratios are pervasively shifted towards hydrothermal fluid values (?0.705). Dike chilled margins display secondary mineral assemblages formed during both axial recharge and discharge and have higher 87Sr/86Sr than dike cores, indicating preferential fluid flow along dike margins. Localised increases in 87Sr/86Sr in the Dike–Gabbro Transition indicates the channelling of fluids along the sub-horizontal intrusive boundaries of the 25 to 50 m-thick gabbroic intrusions, with only minor increases in 87Sr/86Sr within the cores of the gabbro bodies.

When compared to the pillow lava-dominated section from Hole 504B, the Sr isotope measurements from Site 1256 suggest that the extent of hydrothermal circulation in the upper ocean crust may be strongly dependent on the eruption style. Sheet and massive flow dominated lava sequences typical of fast spreading ridges may experience relatively restricted circulation, but there may be much more widespread circulation through pillow lava-dominated sections. In addition, the Hole 1256D sheeted dikes display a much greater extent of Sr-isotopic exchange compared to dikes from Hole 504B. Because seawater-derived hydrothermal fluids must transit the dikes during their evolution to black smoker-type fluids, the different Sr-isotope profiles for Holes 504B and 1256D suggest there are significant variations in mid-ocean ridge hydrothermal systems at fast and intermediate spreading ridges, which may impact geochemical cycles of elements mobilised by fluid–rock exchange at different temperatures.
mid-ocean ridge, IODP, hydrothermal circulation, fluid–rock reaction, Sr isotopes, fluid channelling
0012-821X
56-66
Harris, Michelle
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Coggon, Rosalind M.
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Smith-Duque, Christopher E.
72820fa7-dd47-4a64-8310-68d35b2973e8
Cooper, Matthew J.
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Milton, James A.
9e183221-d0d4-4ddb-aeba-0fdde9d31230
Teagle, Damon A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Harris, Michelle
2ea5985e-614c-4d8a-9cb0-82d9590d4ebc
Coggon, Rosalind M.
09488aad-f9e1-47b6-9c62-1da33541b4a4
Smith-Duque, Christopher E.
72820fa7-dd47-4a64-8310-68d35b2973e8
Cooper, Matthew J.
54f7bff0-1f8c-4835-8358-71eef8529e7a
Milton, James A.
9e183221-d0d4-4ddb-aeba-0fdde9d31230
Teagle, Damon A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286

Harris, Michelle, Coggon, Rosalind M., Smith-Duque, Christopher E., Cooper, Matthew J., Milton, James A. and Teagle, Damon A.H. (2015) Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D. Earth and Planetary Science Letters, 416, 56-66. (doi:10.1016/j.epsl.2015.01.042).

Record type: Article

Abstract

ODP Hole 1256D in the eastern equatorial Pacific is the first penetration of a complete section of fast spread ocean crust down to the dike–gabbro transition, and only the second borehole to sample in situ sheeted dikes after DSDP Hole 504B. Here a high spatial resolution record of whole rock and mineral strontium isotopic compositions from Site 1256 is combined with core observations and downhole wireline geophysical measurements to determine the extent of basalt–hydrothermal fluid reaction and to identify fluid pathways at different levels in the upper ocean crust.

The volcanic sequence at Site 1256 is dominated by sheet and massive lava flows but the Sr isotope profile shows only limited exchange with seawater. However, the upper margins of two anomalously thick (>25 m) massive flow sequences are strongly hydrothermally altered with elevated Sr isotope ratios and appear to be conduits of lateral low-temperature off-axis fluid flow. Elsewhere in the lavas, high 87Sr/86Sr are restricted to breccia horizons. Mineralised hyaloclastic breccias in the Lava–Dike Transition are strongly altered to Mg-saponite, silica and pyrite, indicating alteration by mixed seawater and cooled hydrothermal fluids. In the Sheeted Dike Complex 87Sr/86Sr ratios are pervasively shifted towards hydrothermal fluid values (?0.705). Dike chilled margins display secondary mineral assemblages formed during both axial recharge and discharge and have higher 87Sr/86Sr than dike cores, indicating preferential fluid flow along dike margins. Localised increases in 87Sr/86Sr in the Dike–Gabbro Transition indicates the channelling of fluids along the sub-horizontal intrusive boundaries of the 25 to 50 m-thick gabbroic intrusions, with only minor increases in 87Sr/86Sr within the cores of the gabbro bodies.

When compared to the pillow lava-dominated section from Hole 504B, the Sr isotope measurements from Site 1256 suggest that the extent of hydrothermal circulation in the upper ocean crust may be strongly dependent on the eruption style. Sheet and massive flow dominated lava sequences typical of fast spreading ridges may experience relatively restricted circulation, but there may be much more widespread circulation through pillow lava-dominated sections. In addition, the Hole 1256D sheeted dikes display a much greater extent of Sr-isotopic exchange compared to dikes from Hole 504B. Because seawater-derived hydrothermal fluids must transit the dikes during their evolution to black smoker-type fluids, the different Sr-isotope profiles for Holes 504B and 1256D suggest there are significant variations in mid-ocean ridge hydrothermal systems at fast and intermediate spreading ridges, which may impact geochemical cycles of elements mobilised by fluid–rock exchange at different temperatures.

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Accepted/In Press date: 29 January 2015
e-pub ahead of print date: 25 February 2015
Published date: 15 April 2015
Keywords: mid-ocean ridge, IODP, hydrothermal circulation, fluid–rock reaction, Sr isotopes, fluid channelling
Organisations: Geochemistry

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Local EPrints ID: 375603
URI: http://eprints.soton.ac.uk/id/eprint/375603
ISSN: 0012-821X
PURE UUID: 74393771-10cd-4eb2-82cf-ba9fa26f093a
ORCID for Rosalind M. Coggon: ORCID iD orcid.org/0000-0002-9228-9707
ORCID for Matthew J. Cooper: ORCID iD orcid.org/0000-0002-2130-2759
ORCID for James A. Milton: ORCID iD orcid.org/0000-0003-4245-5532
ORCID for Damon A.H. Teagle: ORCID iD orcid.org/0000-0002-4416-8409

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Date deposited: 30 Mar 2015 15:34
Last modified: 15 Mar 2024 03:45

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Contributors

Author: Michelle Harris
Author: Christopher E. Smith-Duque
Author: James A. Milton ORCID iD

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