INVESTIGATION OF SULFIDE AND AMPHIBOLE MELTING PROCESSES IN GABBRO, ATLANTIS BANK, SOUTHWEST INDIAN RIDGE

The Atlantis Bank is an oceanic core complex (OCC) that formed along the ultraslow-spreading Southwest Indian Ridge (SWIR). This OCC is located where the SWIR intersects the Atlantis II transform and is composed of a gabbro core. OCCs are characterized by intrusion and exhumation along a detachment shear zone with a record of crystal-plastic deformation >1 km below seafloor. Sulfides, amphibole, and/or Fe-Ti oxides can subsequently remobilize, forming dikelets that cross-cuts crystal-plastic foliations; however, the timing and role of these melts is not well constrained especially considering sulfide and Fe-Ti-rich melts have very low viscosity and thereby would have a large impact on crustal rheology. Thin sections from Atlantis Bank gabbro were observed with a petrographic microscope to determine mineral assemblages and deformation textures with a focus on dikelets. The Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS) was used to construct chemical maps. Sulfide dikelets often are short in length and show strong sulfur and iron and/or copper backscatter from the SEM. Possible sulfides contained in the dikelets include pyrite, pyrrhotite, and pentlandite. Amphibole dikelets are often long, cutting across bands of recrystallized plagioclase. Mineral assemblages in these dikelets could contain minerals such as magnetite, ilmenite, amphibole, and apatite. Based on previous work, the estimated melting temperature for sulfide mineral assemblages is around 700˚ C while the estimated melting temperature for amphibole is 900˚ C. Based on these temperatures, these dikelets are likely to form during granulite grade deformation when plagioclase recrystallization was the main deformation mechanism. As a result, zones with partial melt rich in Fe-Ti-P with very low viscosity preferentially partitioned strain which thereby make the footwall of this OCC weaker helping to explain the large distribution in crystal-plastic fabrics.