Ultra high pressure-temperature melting experiments on an SiO2-rich lamproite from Smoky Butte, Montana: Derivation of siliceous lamproite magmas from enriched sources deep in the continental mantle

Liquidus to near-solidus phase relations between 0.8 and 12.0 GPa and 900 and 1200 degrees C were determined for an SiO2-rich lamproite from Smoky Butte, Montana. At 7-10 GPa, assemblages between liquidus and solidus are clinopyroxene + garnet, garnet + clinopyroxene + coesite, garnet + clinopyroxene + coesite + rutile + K-Ti silicate, and garnet + clinopyroxene + K-Ti silicate + phlogopite+coesite + rutile. At 12 GPa, comparable assemblages are garnet, garnet + clinopyroxene + coesite, and garnet + clinopyroxene + K silicate + coesite + rutile. At 6-12 GPa, K-Ti silicates and IC-Ba phosphate compounds occur between 1200 and 1400 degrees C. At 12 GPa, K silicate replaces the K-Ti silicate compound present at lower pressures. At >6 GPa, clinopyroxene and garnet incorporate K in their structures, particularly at lower temperatures. Near-liquidus phase relations of the Smoky Butte lamproite are compared with those of previously investigated lamproite of a similar composition. It is considered unlikely that near-liquidus garnetite mineral assemblages at 7-12 GPa could reflect the source of these magmas. A more likely source for these SiO2-rich lamproitic magmas is an assemblage comparable with the near-solidus assemblage of garnet + clinopyroxene + phlogopite and minor K-, Ti-, Ba-enriched compounds. On partial melting such an assemblage would yield a composition comparable with that of the SiO2-rich magma from Smoky Butte. SiO2-rich lamproite magmas may be primary melts from an enriched source from deep in the lithospheric mantle.