Sulfur isotope patterns of oceanic crust on Macquarie Island-implications for global sulfur cycling G

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Metadata record for data from ASAC Project 2212 See the link below for public details on this project.\n\nFrom the abstracts of two of the references:\n\n----------\nOcean Drilling Program hole 504B revealed an ocean crust hydrothermal sulphur anomaly on the dyke-lava transition, with implications for global sulphur sinks. Here we confirm the presence of the anomaly sporadically along 7.5km of dyke-basalt contact on the Macquarie Ridge at Macquarie Island, a 39-9.7 Ma slow-spreading setting. Background contact-zone pyrite S contents average 1845 ppm across ~50 m. However zones of small-scale brittle faulting that commonly occur on and above the dyke-basalt contact average between 5000 and 11000 ppm S (20-30 m widths). These consist of steep ridge-parallel faults and fault splays on the contact, overlain by up to 50m of linked pyritic fault trellis. The contact zone faults are haloed by disseminated pyrite-chlorite, cross-cut by quartz-chlorite-sphalerite and epidote-cemented breccias, containing evidence of turbulent flow. The structural control on sulphur deposition is attributed to the active extensional slow spreading setting. With increasing extension, diffuse mixing across the contact was replaced by channelised flow and dynamic mixing in fault arrays. The magnitude of the dyke-lava transition sulphur sink must be reassessed to take account of this heterogeneity.\n----------\nThere are only a handful of known hydrothermal sulfate occurrences from the mid-ocean ridge crust sub-surface, despite predictions that they should be common because of the imbalance between sulfur concentrations in venting MOR hotsprings, and that of recharging seawater. This deficit indicates that sub-surface sulfate deposition could be a globally important sulfur cycle sink. Therefore any new occurrences add considerably to the information base on sulfate in this environment.\n\nAn important hydrothermal sulfate occurrence is preserved in ~10 Ma MOR crust on the east coast of Macquarie Island, formed during very slow oblique spreading prior to transition to a magmatic strike-slip plate boundary. The sulfate occurs mainly as white gypsum veins and breccia cement associated with a major fault zone 400m south of Nuggets Point. The site is in the amphibolite facies sheeted dyke and gabbro screen hanging wall of a major northwest-trending graben, itself filled with sub-ziolite facies basaltic breccias and lava flows.\n\nThe sulfate veins occur as several 2-5m wide vein complexes, with surrounding vein networks over several hundred metres. Veins are strongly associated with oblique-sinistral jogs on a N to NNE-trending fault zone, here termed the Nuggets fault. This fault is intruded by thin, weakly metamorphosed, vesicular sheeted dykes (forming a greater than 30m wide zone) interpreted to have developed contemporaneously with the nearby graben, Gypsum is mainly in the amphibolite facies rocks and the younger dykes, but also occurs as thin gape-fill in dykes of the younger volcanic graben. These field relations indicte that the sulfate veins were emplaced contemporaneously with graben formation and infill, approximately 200m below the sea floor. The host fault zone is contiguous with mapped graben offsets, and is interpreted as an oblique transfer fault.\n\nSecondary epidote and quartz-chalcopyrite veining, together with subsequent chlorite-pyrite alteration, predate sulfate, and suggest early hydrothermal upflow conditions. These are cut by vein complexes which display anhydrite relics within foliated gypsum plus or minus pyrite veins surrounded by marginal vein networks of zeolite-gypsum-calcite. These assemblages require central temperatures of greater than 150 degrees C, with a rapid gradation in outer veins to cooler conditions, perhaps less than 100 degrees C. These features imply general cooler recharge conditions; our previous work has shown that this involved a complex history of sub-surface microbial interaction. These field and mineralogical relations provide one predictive tectonic context for the deposition and style of hydrothermal sulfate in extending MOR crust. Sulfate fluid is strongly fault channeled, and rather than occurring in graben boundary faults, deposits precipitates preferentially in transfer faults under-going limited magmatic activity on the graben edge.\n\nA description of the fields in this dataset:\n\nm from start: metres measured over the ground between sample points.\n\nEasting mE, and Northing mE: estimated position relative to the AMG grid used in the 1;10000 mapping series, Mineral Resources Tasmania, using a horizontal datum of WGS 1984. Map date of production, August 1997.

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