Enhanced mantle conductivity from sulfide minerals, southern Sierra Nevada, California

Petrographic studies of peridotitic xenoliths entrained in late Quaternary basalts from beneath the southern Sierra Nevada have revealed the presence of accessory sulfide minerals along grain boundaries and fractures. Equilib ration temperatures from the xenoliths are sufficiently high that the molte n sulfides coexist with the basaltic melt. Sulfides are extremely conductiv e relative to the solid matrix or the basaltic melt, so a small fraction ca n increase the bulk conductivity of the mantle appreciably. Previous estima tes of 2-5% partial melt from magnetotelluric measurements can be plausibly reduced to less than 1%. Such low melt percentages have longer residence t imes in the mantle and are more consistent with the volumetrically minor la te Quaternary basalt flows and the primitive basalt compositions.