Abstract

Disequilibrium textures, particularly the coarse sieve texture of plagioclase, are common in orogenic volcanic rocks. The textures are usually interpreted as resulting from magma mixing, but they may occur by rapid decompression, where heat loss is minor relative to the ascent rate. We conducted high-pressure piston-cylinder experiments on an andesite to test this hypothesis. Experiments starting at 12 kbar, followed by isothermal pressure release in increments of 2, 4, and 6 kbar, produce sieve textures in plagioclase very much like those in many volcanic rocks. Therefore, the presence of sieve-textured plagioclase should not be taken as a priori evidence for magma mixing. Many volcanic systems probably experience conditions of decompression similar to those simulated in this study, and decompression is considered to be a simple mechanism to produce such textures, as it requires no addition of heat or mass. Rapid decompression may also operate in conjunction with magma mixing.

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