Investigating Magma Mixing By Using Chemical and Textural Observations of Plagioclase from Mutnovsky Volcano, Kamchatka

Abstract:

Magma mixing is recognized as a ubiquitous process at stratovolcanoes, however the importance of mixing in the generation of intermediate magmas remains controversial. Here we present plagioclase chemistry and textural observations lavas erupted from Mutnovsky volcano, Kamchatka, which are consistent with the mixing of basaltic and dacitic components to form intermediate compositions basaltic andesite and andesite. These data are paired with whole rock chemical data, coexisting iron-titanium oxide temperature data, viscosity models, and MELTS modeling, all of which together elucidate magma mixing beneath Mutnovsky.

Mutnovsky is an arc-front volcano consisting of four superimposed eruptive centers, Mutnovsky I-IV, which has erupted lavas ranging in composition from basalt to dacite. Volumetrically, basalt and basaltic andesite dominate with only minor eruptions of andesite and dacite. Previous model results indicate that the chemistry of the Mutnovsky basaltic andesites is consistent with formation by the mixing of a basaltic and dacitic component. The chemistry of the erupted dacites is consistent with formation by partial melting of underplated basaltic material.

Plagioclase compositions for Mutnovsky basalts, basaltic andesites, and dacites fall into two distinct populations, An_80±10 and An_50±15. The basaltic samples contain only the anorthite rich plagioclase population while the dacites contain only the more albitic plagioclase population. All erupted basaltic andesites contain both plagioclase populations, with a distinct gap between populations. The chemistry of plagioclase microlites in the basaltic andesites is more calcic than the albitic plagioclase population, whereas the plagioclase microlites in the basalts and dacites are the most sodic plagioclase observed for their respective samples. Sodic plagioclase in the basaltic andesites exhibit dissolution/resorption textures, indicating disequilibrium. These new data are consistent with mixing of a basaltic and dacitic component to generate the erupted intermediate lavas at Mutnovsky.