Hydrous partial melting in gabbros drilled at the Southwest Indian Ridge (ODP Hole 735B): evidence from microstructures at grain boundaries

Abstract. The Atlantis Bank at the Southwest Indian Ridge (SWIR) is probably the most thoroughly investigated oceanic core complex from a typical slow-spreading ridge. Here, ODP (Ocean Drilling Program) conducted two drilling expeditions at Drill Site 735B. The gabbro massif is characterized by the presence of hundreds of felsic veins, which are rocks consisting of evolved, silica-rich material showing the composition of typical “oceanic plagiogranites”. Two models have been suggested for their generation: (1) the extreme differentiation of a MORB magma, resulting in a highly differentiated melt, and (2) the hydrous remelting of gabbroic rocks in the deep oceanic crust. Detailed petrographical and microanalytical investigations of gabbroic rocks from ODP drill core 735B reveal that many of these gabbros exhibit microstructures characteristic of hydrous partial melting of gabbro. Key features that led to this conclusion are zones of anorthite-enriched plagioclase on grain boundaries, which form parageneses with interstitial brown amphibole and orthopyroxene. These are interpreted as the residual phase assemblage left after hydrous partial melting. These events are triggered by water-rich fluids migrating along grain boundaries within the cooling gabbro complex at temperatures above the wet gabbro solidus. This conclusion is supported by two further observations: (1) trace element concentrations in the An-enriched plagioclase (Ti, Mg, and K) are strongly impoverished, in line with experiments simulating the hydrous partial melting of gabbro. (2) The bulk TiO2 content of the felsic veins shows characteristically low concentrations, consistently with an origin through the melting of TiO2-impoverished cumulate gabbro. This contrasts with melt compositions derived through fractional crystallization of MORB, where TiO2 concentrations are significantly higher. Our results show that hydrous partial melting of gabbro played a role in the formation of the felsic melts, alongside fractional crystallization, which is the standard model for felsic vein formation evidenced by several papers.