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