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Observations of dunite channels in ophiolites and uranium-series disequilibria in mid-ocean ridge basalt suggest that melt transport in the upper mantle beneath mid-ocean ridges is strongly channelized. We present experimental evidence that spatial variations in mineralogy can also focus melt on the grain-scale. This lithologic melt partitioning, which results from differences in the interfacial energies associated with olivine-melt and orthopyroxene-melt boundaries, may complement other melt focusing mechanisms in the upper mantle such as mechanical shear and pyroxene dissolution. We document here lithologic melt partitioning in olivine/orthopyroxene-basaltic melt samples containing nominal olivine to orthopyroxene ratio of 3 to 2 and melt fractions of 0.02 to 0.20. Experimental samples were imaged using synchrotron-based X-ray micro-computed tomography at a resolution of 700 nm per voxel. By analyzing the local melt fraction distributions associated with olivine and orthopyroxene (opx) grains in each sample, we found that the melt partitioning coefficient, i.e., the ratio of melt fraction around olivine to that around orthopyroxene grains, varies between 1.1 and 1.6. The permeability and electrical conductivity of our digital samples were estimated using numerical models and compared to those of samples containing only olivine and basaltic melt. Our results suggest that lithologic melt partitioning and preferential localization of melt around olivine grains might play a role in melt focusing, potentially enhancing average melt ascent velocities.

Basaltic lavas recovered in a single dredge haul from the Blanco Trough, in the western portion of the Blanco Fracture Zone, demonstrate that off-axis volcanism can produce significant compositional variability in lavas that are closely related spatially. Glass compositions range from depleted in incompatible elements (0.07 wt % K2O; 35 ppm Zr) to strongly enriched (0.43 wt % K2O; 214 ppm Zr) with SiO2 contents from 48. 1 to 50.9 wt % over a limited range of MgO (9.22 to 6.82 wt %). These basalts are distinct from mid-ocean ridge basalts sampled globally at spreading centers in that they are characterized by high FeO* and Na2O at a given MgO content and low normative SiO2, a compositional signature consistent with relatively low extents of partial melting at high pressures. This results from the presence of thick lithosphere that forces melting to stop at a significantly greater depth than is the case at ridge axes. Forward models of low-pressure liquid lines of descent demonstrate that the major and trace element variations observed in these lavas cannot be explained by crystal fractionation from a single parent magma. After correcting the glass compositions to 8 wt % MgO to eliminate the effects of low-pressure crystallization, the Fe8.0-Na8.0 and Si8.0-Na8.0 variations display characteristics of both Atlantic- and Pacific-type local trends. Variable extents of partial melting and interaction with the mantle as the magmas ascended through the lithosphere can explain these systematics. A subset of the samples shows evidence for crystallization of olivine + plagioclase + high-Ca clinopyroxene at upper mantle pressures (∼400 MPa). Modeling of incompatible element variations suggests that two distinct mantle sources were involved in producing the Blanco Trough lavas. The first resembles the primitive upper mantle of Hart and Zindler (1986) with a small initial depletion (∼0.1 wt % melt extraction). The second is characterized by a significant depletion in incompatible elements (K2O, TiO2, Zr, Sr).