Spatial distribution of melt conduits in the mantle beneath oceanic spreading ridges: Observations from the Ingalls and Oman ophiolites

Abstract

Ophiolites are on‐land exposures of igneous crust and residual upper mantle formed beneath submarine spreading ridges. Upper mantle outcrops in ophiolites provide insight into focusing of melt transport from a ∼100 km wide region of partial melting into an ∼5 km wide zone of igneous crustal accretion beneath the ridges. Dunite veins, composed of the minerals olivine and spinel, mark conduits for melt transport through at least the uppermost 30 km of the mantle. New data in this paper, on dunite veins in the Ingalls ophiolite, central Washington Cascades, show a power law relationship between frequency and width, in which frequency/m ≈0.02 width−3 over a size interval from ∼0.1 to 2 m. There may be several ways to generate this relationship, but we favor the hypothesis that the dunites represent a coalescing melt transport network. This conclusion is broadly consistent with the related hypothesis that mantle melt extraction occurs in a fractal, branching network, and with recent results on formation of a coalescing network of dissolution channels via flow of a solvent through a partially soluble, compacting porous medium.