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Hydrothermal cooling of the ocean crust: insights from ODP Hole 1256D

Hydrothermal cooling of the ocean crust: insights from ODP Hole 1256D
Hydrothermal cooling of the ocean crust: insights from ODP Hole 1256D
The formation of new ocean crust at mid-ocean ridges is a fundamental component of the plate tectonic cycle and involves substantial transfer of heat and mass from the mantle. Hydrothermal circulation at mid-ocean ridges is critical for the advection of latent and sensible heat from the lower crust to enable the solidification of ocean crust near to the ridge axis. The sheeted dike complex (SDC) is the critical region between the eruptive lavas and the gabbros through which seawater-derived recharge fluids must transit to exchange heat with the magma chambers that form the lower ocean crust.

ODP Hole 1256D in the eastern equatorial Pacific Ocean provides the only continuous sampling of in-situ intact upper ocean crust formed at a fast spreading rate, through the SDC into the dike-gabbro transition zone. Here we exploit a high sample density profile of the Sr-isotopic composition of Hole 1256D to quantify the time-integrated hydrothermal recharge fluid flux through the SDC. Assuming kinetically limited fluid-rock Sr exchange, a fluid flux of 1.5-3.2x106 kgm-2 34 is required to produce the observed Sr-isotopic shifts. Despite significant differences in the distribution and intensity of hydrothermal alteration and fluid/rock Sr-isotopic exchange between Hole 1256D and SDC sampled in other oceanic environments (ODP Hole 504B, Hess Deep and Pito Deep), the estimated recharge fluid flux at all sites are similar, suggesting that the heat flux extracted by the upper crustal axial hydrothermal system is relatively uniform at intermediate to fast spreading rates.

The hydrothermal heat flux removed by fluid flow through the SDCs, is sufficient to remove only ~20 to 60% of the available latent and sensible heat from the lower crust. Consequently, there must be additional thermal and chemical fluid-rock exchange deeper in the crust, at least of comparable size to the upper crustal hydrothermal system. Two scenarios are proposed for the potential geometry of this deeper hydrothermal system. The first requires the downward expansion of the upper crustal hydrothermal system ~800 m into the lower crust in response to a downward migrating conductive boundary layer. The second scenario invokes a separate hydrothermal system in the lower crust for which fluid recharge bypasses reaction with the sheeted dikes, perhaps via flow down faults
Ocean crust, hydrothermal, Sr isotopes, heat flux
0012-821X
110-121
Harris, Michelle
2ea5985e-614c-4d8a-9cb0-82d9590d4ebc
Coggon, Rosalind M.
09488aad-f9e1-47b6-9c62-1da33541b4a4
Wood, Martin
d391ae85-c836-4339-8b56-f444577e191c
Smith-Duque, Christopher
f1385032-8c67-40ca-bc6d-6dd3e6c5b013
Henstock, Timothy
27c450a4-3e6b-41f8-97f9-4e0e181400bb
Teagle, Damon
396539c5-acbe-4dfa-bb9b-94af878fe286
Harris, Michelle
2ea5985e-614c-4d8a-9cb0-82d9590d4ebc
Coggon, Rosalind M.
09488aad-f9e1-47b6-9c62-1da33541b4a4
Wood, Martin
d391ae85-c836-4339-8b56-f444577e191c
Smith-Duque, Christopher
f1385032-8c67-40ca-bc6d-6dd3e6c5b013
Henstock, Timothy
27c450a4-3e6b-41f8-97f9-4e0e181400bb
Teagle, Damon
396539c5-acbe-4dfa-bb9b-94af878fe286

Harris, Michelle, Coggon, Rosalind M., Wood, Martin, Smith-Duque, Christopher, Henstock, Timothy and Teagle, Damon (2017) Hydrothermal cooling of the ocean crust: insights from ODP Hole 1256D. Earth and Planetary Science Letters, 462, 110-121. (doi:10.1016/j.epsl.2017.01.010).

Record type: Article

Abstract

The formation of new ocean crust at mid-ocean ridges is a fundamental component of the plate tectonic cycle and involves substantial transfer of heat and mass from the mantle. Hydrothermal circulation at mid-ocean ridges is critical for the advection of latent and sensible heat from the lower crust to enable the solidification of ocean crust near to the ridge axis. The sheeted dike complex (SDC) is the critical region between the eruptive lavas and the gabbros through which seawater-derived recharge fluids must transit to exchange heat with the magma chambers that form the lower ocean crust.

ODP Hole 1256D in the eastern equatorial Pacific Ocean provides the only continuous sampling of in-situ intact upper ocean crust formed at a fast spreading rate, through the SDC into the dike-gabbro transition zone. Here we exploit a high sample density profile of the Sr-isotopic composition of Hole 1256D to quantify the time-integrated hydrothermal recharge fluid flux through the SDC. Assuming kinetically limited fluid-rock Sr exchange, a fluid flux of 1.5-3.2x106 kgm-2 34 is required to produce the observed Sr-isotopic shifts. Despite significant differences in the distribution and intensity of hydrothermal alteration and fluid/rock Sr-isotopic exchange between Hole 1256D and SDC sampled in other oceanic environments (ODP Hole 504B, Hess Deep and Pito Deep), the estimated recharge fluid flux at all sites are similar, suggesting that the heat flux extracted by the upper crustal axial hydrothermal system is relatively uniform at intermediate to fast spreading rates.

The hydrothermal heat flux removed by fluid flow through the SDCs, is sufficient to remove only ~20 to 60% of the available latent and sensible heat from the lower crust. Consequently, there must be additional thermal and chemical fluid-rock exchange deeper in the crust, at least of comparable size to the upper crustal hydrothermal system. Two scenarios are proposed for the potential geometry of this deeper hydrothermal system. The first requires the downward expansion of the upper crustal hydrothermal system ~800 m into the lower crust in response to a downward migrating conductive boundary layer. The second scenario invokes a separate hydrothermal system in the lower crust for which fluid recharge bypasses reaction with the sheeted dikes, perhaps via flow down faults

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Accepted/In Press date: 11 January 2017
e-pub ahead of print date: 30 January 2017
Published date: 15 March 2017
Keywords: Ocean crust, hydrothermal, Sr isotopes, heat flux
Organisations: Geochemistry, Geology & Geophysics

Identifiers

Local EPrints ID: 405239
URI: http://eprints.soton.ac.uk/id/eprint/405239
ISSN: 0012-821X
PURE UUID: 69ffcad4-ad19-41a5-9715-0578ff95d89f
ORCID for Rosalind M. Coggon: ORCID iD orcid.org/0000-0002-9228-9707
ORCID for Timothy Henstock: ORCID iD orcid.org/0000-0002-2132-2514
ORCID for Damon Teagle: ORCID iD orcid.org/0000-0002-4416-8409

Catalogue record

Date deposited: 30 Jan 2017 14:12
Last modified: 26 Nov 2021 03:00

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Contributors

Author: Michelle Harris
Author: Martin Wood
Author: Christopher Smith-Duque
Author: Damon Teagle ORCID iD

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