Crust-scale reactive porous flow revealed by the brown amphibole in the IODP hole U1473A gabbros, Southwest Indian Ridge

Abstract

Mid-ocean ridge magma reservoirs are mush-dominated systems, where reactive porous flow is a dominant post-cumulus process. However, the length scale of this process is poorly constrained, especially in ultraslow-spreading ridges. Here, we seek to address this question by presenting a comprehensive brown amphibole dataset of gabbros in IODP Hole U1473A drilled on the Atlantis Bank ocean core complex, Southwest Indian Ridge (SWIR). The results suggest a magmatic origin of the brown amphiboles. Intra-grain chemical variations and compositional zoning of these amphibole grains suggest they were crystallized in a closed system. Nevertheless, geochemical modelings indicate the remarkable Zr/Nd ratio variations in brown amphibole can only be explained by assimilation-fractional crystallization processes triggered by the reactive porous flow. Thus, the brown amphibole in olivine gabbros was crystallized from trapped melts residual after earlier melt-mush interaction. In this light, the reactive porous flow should have ceased prior to the brown amphibole precipitation. This, in turn, suggests the stratigraphic variations in amphibole trace elements reflect the final-stage reactive melt migration. The stratigraphic variations in amphibole trace elements reflect a major episode of reactive melt migration through an ∼800-m section upwardly. This implies that the porosity of the entire gabbroic section remained connected, providing a minimum estimate of the size of the melt-present zones in the lower oceanic crust during the spreading of the 57-60°E magmatic segment of SWIR. Our study reveals that large-scale reactive porous flow can occur in the magmatic segments of ultraslow-spreading ridges, and has important implications for the thermal and rheological evolution of the lower oceanic crust.