Summary
ODP hole 504B in the eastern Pacific intersected a thick zone of 1-5% sulfide sulfur at the contact between basalts and underlying sheeted dikes. Sulfur isotopic enrichment above typical igneous values in this zone has been cited as evidence by Jeff Alt and others for the transfer of seawater sulfate into ocean crust during off-axis hydrothermal alteration. The process is likely to be common to any aquifer contact between high-temperature reduced fluids in low permeability rocks, and lower temperature sulfate-bearing fluids in adjacent more porous rocks. Assuming that the process operates uniformily everywhere, it has been calculated to remove 2.1x1012g/yr of S from the oceans. However, the homogeneity of the process is far from established and is evaluated here. At Macquarie Island (Southern Ocean), extensive cross-sections through typical ocean crust lithologies have been studied, providing an insight into the mechanisms of hydrothermal alteration during ocean spreading. This crust formed between 8 and 12 Ma on the Proto-Macquarie Spreading Ridge during slow spreading, prior to the margin evolving to its current transpressive state. The island is now ~5.5 km east of the main plate boundary within the Pacific plate. This study has focussed on a dike-basalt transition in a 7.5 km long, paleo-ridge-parallel section, where the initiating edge of a sheeted dike swarm is exposed, as well as its upper basalt contact. This area is here termed the Sandell Bay Sheeted Dikes (SBSD). The area was mapped in detail, and sampled along two E-W transects 1.5 km apart. The crust consists of a western strongly rotated basaltic sequence, that is intruded by sub-vertical SBSD. These are in turn overlain by a second shallowly east-dipping basalt sequence. The site was subject to at least two major episodes of magmatism and extension prior to modern uplifting transpression. The first produced and rotated the western basalts around an axis to the east, and predated the SBSD.