Evidence from oceanic gabbros for porous melt migration within a crystal mush beneath the Mid‐Atlantic Ridge

The evolution of a gabbroic crystal mush beneath the Mid‐Atlantic Ridge has been investigated using evidence from gabbros recovered from Ocean Drilling Program (ODP) Hole 923A (Leg 153). Lithological variations occur on a vertical scale of meters and correlate with mineral compositions. This defines a detailed chemical stratigraphy in which variations in olivine, plagioclase, and clinopyroxene solid‐solution component compositions correlate with each other. The volumetrically dominant lithology (plagioclase + clinopyroxene ± olivine ± orthopyroxene gabbros) has variable grain size, grain shape, and mineral compositions. These variations correlate, such that coarser grained samples have more granular textures and lower mafic phase Mg/Fe ratios than adjacent finer grained samples. Clinopyroxene trace element systematics, determined by ion probe, cannot be explained by growth from a melt that evolved along either an equilibrium or a fractional crystallization trend. Clinopyroxene crystals are strongly zoned and enriched in Zr with respect to rare earth elements (more to less incompatible elements). These textural and geochemical characteristics are not expected from simple crystal accumulation processes or the crystallization of trapped melt. Instead, melt migration within a crystal mush is suggested as the most likely process to explain them. The meter‐scale mineral compositional variations, which correlate between phases (e.g., olivine forsterite content and plagioclase anorthite content), suggest that the porous melt flux after the formation of this layering was insufficient to destroy this correlation.