Magma chamber processes at Mutnovsky Volcano, Russia.

摘要

This dissertation is composed of two manuscripts about the evolution of Mutnovsky Volcano in Kamchatka, Russia. Mutnovsky has been active for the past approximately 80,000 years, during which there was the formation and subsequent eruption of four major stratocones. These four eruptive centers, named Mutnovsky I, II, III, and IV from oldest to youngest, have a range of erupted product compositions from basalt to dacite. The first major goal of this project was to investigate the melt source for Mutnovsky. Whole rock trace element and Nd, Sr, and Pb isotopic data were used to determine that the melt source was fluid flux melting of the mantle wedge with very little crustal or slab surface sediment-melt input. The second major goal of this project was to determine the cause of the compositional heterogeneity of erupted products at Mutnovsky. Whole rock geochemical modeling demonstrated that melts were generated both above and below the garnet/spinel transition at approximately 67 km depth in the mantle wedge. Different degrees of partial melting of these two mantle compositions, followed by fractional crystallization, was initially determined to be the cause of the range of compositions found at Mutnovsky. However, melt inclusion data collected later provided new insight into magmatic differentiation processes. Melt inclusion data have a much wider range of values for the major and trace elements than the whole rock data; whole rock compositions fall along mixing lines between the most and least evolved melt inclusion compositions. This observation, combined with mafic enclaves found in more felsic hosts in whole rock samples and a variety of types of zoning found in plagioclase, indicate that magma mixing is the major cause of the compositional heterogeneity at Mutnovsky. The third major goal of this study was to determine the structure of the preeruptive magma storage system for Mutnovsky. Two thermobarometers, orthopyroxene-liquid (Putirka, 2008) and clinopyroxene-liquid (Putirka et al., 2003), were used to determine the depths of pyroxene-melt equilibrium and thus the depths of magma stagnation. Both thermobarometers indicated that the depth of magma storage chambers was increasing with time, from Mutnovsky I to IV.