The 1996 Eruption of Karymsky Volcano, Kamchatka: Historical Record of Basaltic Replenishment of an Andesite Reservoir

Abstract

The simultaneous eruption in 1996 of andesite from Karymsky volcano and of basalt from the Academy Nauk vent 6km away appears to provide a case of mafic recharge of an andesite reservoir for which the time of recharge is exactly known and direct samples of the recharging magma are available. The explosive phreato-magmatic eruption of basalt was terminated in less than 24h, whereas andesite erupted continuously during the following 4 years. Detailed petrological study of volcanic ash, bombs and lavas of Karymsky andesite erupted during the period 1996–1999 provides evidence for basaltic replenishment at the beginning of the eruptive cycle, as well as a record of compositional variations within the Karymsky magma reservoir induced by basaltic recharge. Shortly after the beginning of the eruption the composition of the matrix glass of the Karymsky tephra became more mafic and then, within 2 months, gradually returned to its original state and remained almost constant for the following 3 years. Further evidence for basaltic replenishment is provided by the presence of xenocrysts of basaltic origin in the andesite of Karymsky. A conspicuous portion of the plagioclase phenocrysts in the Karymsky andesite has calcic cores, with compositions and textures resembling those of plagioclases in the Academy Nauk basalt. Similarly, the earlier portion of the andesite of the eruption sequence contains rare olivines, which occur as resorbed cores in pyroxenes. The composition of the olivine matches that of olivines in the Academy Nauk basalt. The sequence of events appears to be: (1) injection of basaltic magma into the Karymsky chamber with immediate, compensating expulsion of pre-existing chamber magma from the Karymsky central vent; (2) direct mixing of basaltic and andesitic magmas with dispersal of phenocrysts associated with the basalt throughout the andesite so that newly mixed magma appeared at the vent within 2 months; (3) re-establishment of thermal and chemical equilibrium within the reservoir involving crystallization in the new hybrid liquid, which returned the melt composition to ‘normal’, formed rims on inherited calcic plagioclase, and caused the resorption of dispersed olivine xenocrysts. Taken together, these findings indicate that the Karymsky magma reservoir was recharged by basalt at the onset of the 1996 eruptive cycle. The rapidity and thoroughness of mixing of the basalt with the preexisting andesite probably reflects the modest contrast in temperature, viscosity, and density between the two magmas.