HIGH-PRESSURE PHASE-RELATIONS OF PRIMITIVE HIGH-ALUMINA BASALTS FROM MEDICINE LAKE VOLCANO, NORTHERN CALIFORNIA

Anhydrous phase relations were determined at 1 atm and 10 to 15 kbar for primitive high-alumina basalts (79-35 g and 82-72f) from Giant Crater at Medicine Lake volcano. These compositions are multiply saturated with olivine + augite + plagioclase + spinel +/- orthopyroxene near the liquidus at about 11 kbar. Experiments on mixtures of sample 79-35 g with orthopyroxene and olivine determined the location of the multiple saturation boundaries where liquid coexists with the assemblage olivine + augite + orthopyroxene + plagioclase at 10 kbar and olivine + augite + orthopyroxene + spinel at 15 kbar. The mix experiments showed that primitive Medicine Lake high alumina basalts (HABs) are close in composition to liquids in equilibrium with a mantle lherzolite source containing olivine + augite + orthopyroxene + spinel + plagioclase at 11 kbar. Orthopyroxene observed as a near liquidus phase in an 11 kbar experiment on sample 82-72f supports this conclusion. The most primitive HABs from Medicine Lake are low in K2O (0.07 wt.%), high in MgO (> 10 wt.%) and Ni (231 ppm), and have light-rare earth element depletions and large ion lithophile element enrichments. A model for the origin of these near-primary high-alumina basalts is that they are partial melts of a MORB-like mantle lherzolite source that has been enriched by a fluid component derived from the subducted slab. The HAB magma segregated from its mantle residue just below the base of the crust near the crust-mantle boundary.