THE EFFECT OF INCREASED SLOW SPREADING EXTENSION ON THE OCEAN CRUST DIKE/BASALT HYDROTHERMAL SULFUR ANOMALY (MACQUARIE ISLAND, SOUTHERN OCEAN)

Sulfur isotopic enrichment above typical igneous values occurs within sulfide-rich Transition Zone rocks of oceanic crust in ODP Hole 504B, and on the island of Troodos. This has been cited as evidence for the transfer of seawater sulfate into ocean crust during off-axis hydrothermal alteration. The process likely involves mixing of high-temperature reduced fluids (sourced from low permeability sheeted dikes) with lower temperature sulfate-bearing waters sourced from the overlying basalts. However, the homogeneity of the process is far from established, or the influence of other factors such as degree of syn-mixing extension.

At Macquarie Island, extensive exposures of typical slow spread ocean crust (8-12 Ma) contain a significant disseminated and vein-controlled sulfur anomaly on the dike-basalt contact. We have studied this phenomenon in a 7.5 km long, paleo-ridge-parallel section, where the initiating edge of a sheeted dike swarm (Sandell Bay Sheeted Dikes; SBSD) is exposed, as well as its upper basalt contact.

Background pyrite S contents are high across the SBSD-basalt contact for 5-50 m. However, there is also a strong structural control that produces significant along-strike variations in sulfide abundances, and an association with quartz-cemented sphalerite-bearing fault breccias, with evidence of turbulent flow regimes. These differences in the size of the sulfur anomaly are attributed to the active extensional slow spreading setting. With increasing extension, diffuse mixing across the dyke-basalt contact was replaced by channelized flow and dynamic mixing in extensional fault arrays. The most intense alteration occurred in fault splays sub-parallel to the ridge axis. The size of the Transition Zone sulfur-sink must be reassessed to take account of this heterogeneity.