The 1996 Eruption Of Karymsky Volcano, Kamchatka: Detailed Petrological Study Of A Single Basalt -Triggered Eruption Cycle

Abstract

The current activity at Karymsky Volcano, Kamchatka, began on January 2, 1996, with simultaneous eruptions from two vents located 6 km apart: Karymsky summit vent, which erupted andesite, and a newly formed vent within Academy Nauk caldera, which erupted basalt. Detailed petrologic study of volcanic ash, bombs, and lavas of Karymsky erupted during 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 eruption the composition of matrix glasses of Karymsky tephra became more mafic, and then, within two months, gradually returned to its original state and remained almost constant the following three years. Further evidence for basaltic replenishment includes the presence of xenocrysts of basaltic origin in andesites erupted from Karymsky. A conspicuous portion of plagioclase phenocrysts in Karymsky andesites contain calcic cores, the composition and texture of which mimic those in Academy Nauk basalt. The earlier portions of andesite also contain rare xenocrysts of olivine, which occur as relicts in plagioclase-pyroxene aggregates. The compositions of olivine xenocrysts match those of olivines in Academy Nauk basalt. Compositional variations of glass and the presence of xenocrysts indicate that Karymsky magma reservoir was recharged by basalt at the onset of the 1996 eruptive cycle. The mixing of basalt with host andesite was both thorough and rapid, perhaps due to a modest contrast in temperature, viscosity, and density between the magmas. Academy Nauk basalt contains granophyre xenoliths, the whole-rock compositions of which are identical to that of dacites erupted twice at 40,000 yr. BP and 7,900 yr. BP, and formed the neighboring Academy Nauk and Karymsky calderas. According to hydrothermal experiments and petrologic observations both dacites last equilibrated at 3--4 km depth. At the same depth granophyre phase assemblage is reproduced by isobaric crystallization of dacites, thus implying that the granophyres represent a crystallized silicic reservoir, which produced dacites 40,000 yr. BP and formed Academy Nauk caldera. In 1996 this crystallized body was sampled by ascending basalt, which erupted in the northern part of the caldera.