Small effect of partial melt on electrical anomalies in the asthenosphere.

High conductivity anomalies in the shallow mantle are frequently attributed to minor partial melt (basalt or carbonatite) in the olivine-dominated peridotites. Conductivity of a melt-mineral mixture depends on the configuration of melt that could be affected by grain size of the constitutive mineral(s), but this has rarely been explored. Here, we provide experimental evidence using a conductive carbonatite analog and olivine that the bulk conductivity decreases systematically with increasing olivine grain size. The required amount of melt for producing the geophysically resolved high conductivities in the asthenosphere is much greater than previously assumed. We suggest that the effect of partial melt on many conductive regions in the asthenosphere is small. Instead, the electrical anomalies (especially those away from mid-ocean ridges) originate more likely from subsolidus solid assemblages in the upper mantle. This reconciles well the geochemical and petrological constraints of the shallow mantle with its geophysically determined electrical properties.