A profile of multiple sulfur, oxygen, strontium isotopes and related chalcophile elements through lower slow-spreading crust (IODP Expedition 360, Atlantis Bank, Southwest Indian Ridge)

IODP Expedition 360 to Atlantis Bank, at Southwest Indian Ridge (SWIR), ended at 30.01.2016, and is Leg 1 of the SloMo Project, a Multi Phase Drilling Program to drill through the crust/mantle boundary at a slow spreading ocean ridge. The primary objective of the SloMo Project is to test competing hypotheses on the nature of the Moho at the slow spreading oceanic lithosphere. Expedition 360 Hole U1473A was drilled 789.7 m through massive olivine gabbro, gabbro, Fe Ti rich oxide gabbro, gabbronorite, cut by numerous felsic veins. Numerous long intervals of porphyroclastic to ultramylonitic, gabbro, representing a 600 meter thick shear zone, demonstrate that this occurred in a dynamic environment, beginning while the gabbros were partially molten, and continuing as they cooled and were emplaced diapirically and tectonically upward into the zone of diking beneath the rift valley floor. With the help of the Hole U1473U core drilled during Expedition 360, we aim at quantifying the cycling of sulfur and related chalcophile elements in typical slow spreading oceanic crust. The analytical approach will include measuring of multiple sulfur isotopes and trace elements mostly in gabbros and felsic rocks from the drilled core, supported by a few samples from the extrusive crust and from mantle rocks from that area, recovered conventionally by other ship cruises. Downhole concentration profiles of sulfur and chalcophile elements will enable us to identify the main trends of their enrichment and depletion. Multiple sulfur isotopes, in combination with silicate oxygen isotopes and bulk rock Sr isotopes will help to identify whether sulfides observed in various paragenetic mineral assemblages have been formed by primary magmatic or secondary processes, i.e. by seawater derived fluids resulting in water rock interaction. Optical and reflected light microscopy as well as in situ analysis with electron microprobe (EMPA), and laser ablation inductively coupled plasma mass spectrometry (LA ICPMS) will reveal fundamental understanding of the characteristics of related processes involving sulfur mineral formation in the deep slow spreading oceanic crust.