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Schematic cross-section of the oceanic crust at a spreading center showing subseafloor mixing of hydrothermal fluid and seawater in the extrusive layer. Porosity values in the axial pillow lavas/extrusives are taken from gravity measurements of axial pillow flows (37) and extrusive/dike transition and dike porosity taken from Ocean Drilling Program Hole 504B (38).
