Unique chemical compositions of cumulates from 53E Southwest Indian Ridge reveal distinct evolution and cooling history of gabbro sills in amagmatic accretionary segments

Ultra-slow spreading ridges are unique in global ridges in its highly heterogeneous crustal thickness, numerous economically significant hydrothermal vents despite of extremely limited melt supply, and exposure of lower crust gabbro and mantle peridotite at seafloor particularly at amagmatic spreading segments. Although amagmatic accretionary segments are considered to be the key component of ultraslow-spreading ridges (Dick et al., 2003), how limited melt supply accommodates such a tectonically-dominated spreading system and heats the hydrothermal system remain unclear. And the key resides in the evolution of gabbro sills which preserves the history of frozen melt in the cold lithosphere and also provides the heat necessary for hydrothermal systems at such a condition. Therefore, for a better understanding of the lithosphere accretion history of amagmatic segments, we conduct systematic petrographic and geochemical analyses on a variety of samples collected from so far the best sampled and mapped amagmatic segment -- 53E Southwest Indian Ridge, including abyssal peridotite, primitive to evolved cumulates (olivine-rich troctolite, gabbro, and oxide gabbro) and MORB. We identify several unique chemical compositions of the minerals which was never reported in ocean ridges before, especially in the olivine-rich troctolite located to the south of rift valley within a massive exposure of peridotite up to ~3200 km2 (Zhou and Dick, 2013). 1. Unique NiO vs. Fo for olivine in the olivine-rich troctolite record the reaction between highly evolved magma (with Mg#~20) and dunitic mush. 2. Highly evolved trace element signatures with high Mg# for clinopyroxene and orthopyroxene in the gabbro vein cutting the troctolite confirm that the highly evolved magma intruding into the dunitic mush is felsic in composition. 3. The occurrence of oxide gabbro in the gabbro core complex (~380 km2) to the north of the rift valley indicates the presence of highly evolved gabbro sill in the north, which is most likely the parental magma of the evolved felsic melt invading the primitive troctolite in the south. 4. The occurrence of small volume of primitive troctolite with a crystallization temperature of ~1192C in the massive peridotite in the south, and large volume of variably differentiated gabbro in the gabbro core complex with crystallization temperature as low as 998C in the north reveal the unique cooling history of gabbro sills with different size in the newly-formed lithosphere in the amagmatic spreading segment. The unique chemical compositions and 200C variation in temperature suggests two juxtaposed gabbro sills in the lithosphere in amagmatic segments can vary greatly with different cooling and crystallization history. We propose that bigger gabbro sills in amagmatic spreading systems would more likely have a prolonged cooling history to crystallize more evolved lithologies, which would provide the necessary heat supply for potential hydrothermal systems. Future exploration on the occurrence of gabbro sills beneath hydrothermal systems is recommended to better understand how varied cooling history of individual gabbro sill control the formation and evolution of hydrothermal systems at ultraslow-spreading ridges. References: Dick, H. J. B., et al. (2003). Nature 426(6965): 405-412. Zhou, H. Y. and H. J. B. Dick (2013). Nature 494(7436): 195-200.