GEOCHEMISTRY OF THE 1989-1990 ERUPTION OF REDOUBT VOLCANO .2. EVIDENCE FROM MINERAL AND GLASS CHEMISTRY

Early stages (December 1989) of the 1989-1990 eruption of Redoubt Volc ano produced two distinct lavas. Both lavas are high-silica andesites with a narrow range of bulk composition (58-64 wt.%) and similar miner alogies (phenocrysts of plagioclase, hornblende, augite, hypersthene a nd Fe-Ti oxides in a groundmass of the same phases plus glass). The tw o lavas are distinguished by groundmass glass compositions, one is dac itic and the other rhyolitic. Sharp boundaries between the two glasses in compositionally banded pumices, lack of extensive coronas on hornb lende phenocrysts, and seismic data suggest that a magma-mixing event immediately preceeded the eruption in December 1989. Textural disequil ibrium in the phenocrysts suggests both magmas (dacitic and rhyolitic glasses) had a mixing history prior to their interaction and eruption in 1989. Sievey plagioclase and overgrowths of magnetite on ilmenite a re textures that are at least consistent with magma mixing. The presen ce of two hornblende compositions (one a high-Al pargasitic hornblende and one a low-Al magnesiohornblende) in both the dacitic and rhyoliti c groundmasses indicates a mixing event to yield these two amphibole p opulations prior to the magma mixing in December 1989. The pargasitic hornblende and the presence of Ca-rich overgrowths in the sievey zones of the plagioclase together indicate at least one component of this e arlier mixing event was a mafic magma, either a basalt or a basaltic a ndesite. Eruptions in 1990 produced only andesite with a rhyolitic gro undmass glass. Glass compositions in the 1990 andesite are identical t o the rhyolitic glass in the 1989 andesite. Cognate xenoliths from the magma chamber (or conduit) are also found in the 1990 lavas. Magma mi xing probably triggered the eruption in 1989. The eruption ended when this rather viscous (rhyolitic groundmass glass, magma capable of entr aining sidewall xenoliths) magma stabalized within the conduit.