Trapped Melt in the Josephine Peridotite: Implications for Permeability and Melt Extraction in the Upper Mantle

Tabular dunites in the Josephine peridotite of southwestern Oregon represent conduits for melt extraction from the upper mantle. The amount of melt trapped within these channels during cooling and exhumation of the peridotite massif was calculated using a mass-balance approach. These calculations suggest that no more than 0·1–0·2 vol. % melt could have been trapped within these dunites after crystallization. This observation indicates that the threshold porosity for melt extraction is ≤0·2 vol. %. We use this constraint to evaluate published permeability models for partially molten dunite. We then use constraints from these permeability models to calculate lower bounds on the bulk viscosity of partially molten dunite and conclude that the bulk viscosity is of the order of 1023 Pa s at melt contents of ∼0·1 vol. % and reasonable melt extraction column heights. The small amount of trapped melt in these dunites is consistent with geochemical evidence for near fractional melting in the upper mantle.