Hydrothermal alteration of the ocean crust: insights from Macquarie Island and drilled in situ ocean crust


Coggon, Rosalind Mary (2006) Hydrothermal alteration of the ocean crust: insights from Macquarie Island and drilled in situ ocean crust. University of Southampton, School of Ocean and Earth Science, Doctoral Thesis , 339pp.

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Description/Abstract

Hydrothermal circulation is a fundamental process in the formation and aging of the ocean crust,
influencing its structure, physical and chemical properties, and the composition of the oceans and
the mantle. The impact of hydrothermal circulation on mid-ocean ridge processes depends on the
composition and volume of circulating hydrothermal fluids, and the extent of partitioning
between high temperature axial- and low temperature ridge flank- systems, but these processes
remain poorly constrained. This study uses whole rock and secondary mineral chemistries of
altered ocean crust to (i) assess the extent of fluid-rock exchange during hydrothermal circulation,
and (ii) determine the compositions of axial and ridge flank hydrothermal fluids.
Sub Antarctic Macquarie Island is a unique sub-aerial exposure of a complete section of ocean
crust in the ocean basin in which it formed. Sr and O isotope analyses from Macquarie Island,
combined with stratigraphic reconstructions provide the first isotopic profiles through a complete
section of normal ocean crust. Tracer transport mass balance calculations indicate that a timeintegrated
fluid flux of 4 ± 1 x 106 kg/m2 is required to produce the observed shift in Sr-isotopic
composition. This can be supported by the available mid-ocean ridge magmatic heat and is
similar to estimates for sections of in situ ocean crust, but a factor of 10 lower than estimates for
ophiolites indicating a fundamental difference between the hydrothermal cooling of mid-ocean
ridge and supra-subduction zone ocean crust.
Heat flow studies indicate that hydrothermal circulation persists for tens of millions on the ridge
flanks, with approximately two-thirds of hydrothermal heat loss occurring off-axis at significantly
lower-temperatures than in axial hydrothermal systems. Consequently a much larger volume of
fluid is required and only small deviations in fluid compositions may result in significant
contributions to ocean chemical budgets. Direct sampling of in situ basement fluids is extremely
difficult, and can only be applied to active systems. Here, methods to calculate the compositions
of ridge flank fluids from the compositions of secondary mineral precipitates are presented and
applied to basalt-hosted calcium carbonate veins. Veins from the eastern flank of the Juan de
Fuca Ridge record a temperature dependent fluid evolution, similar to that of near-basement pore
fluids sampled by borehole studies. Carbonate veins from the Juan de Fuca Ridge and Ocean
Drilling Program Site 1256 record a sufficient decrease in the fluid Sr-isotopic composition with
temperature to balance the global ocean Sr budget, however, this result cannot be reconciled with
the observation of Davis et al. (2003) that the studied ocean crust has exchanged insufficient Sr
with the oceans to balance the global Sr budget. This suggests that these areas cannot be typical
of the ocean crust as a whole.

Item Type: Thesis (Doctoral)
Additional Information: 339p & appendices A-F
Subjects: G Geography. Anthropology. Recreation > GC Oceanography
Q Science > QE Geology
Divisions: University Structure - Pre August 2011 > School of Ocean & Earth Science (SOC/SOES)
ePrint ID: 66355
Date Deposited: 05 Jun 2009
Last Modified: 27 Mar 2014 18:47
URI: http://eprints.soton.ac.uk/id/eprint/66355

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