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 ad 12.0 GPa and 900 and 1200°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 K–Ba phosphate compounds occur between 1200 and 1400°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.