Mixed Magmas, Mush Chambers and Eruption Triggers: Evidence from Zoned Clinopyroxene Phenocrysts in Andesitic Scoria from the 1995 Eruptions of Ruapehu Volcano, New Zealand

Abstract

Juvenile ejecta from the September and October 1995 eruptions of mush network. Hence, eroded crystals from the mush were captured Ruapehu volcano, New Zealand, indicate that mixing occurred by the high-T magma. Repeated reverse zonations in the crystal between relatively higherand lower-temperature (high-T and lowmantles, and variable mantle widths, indicate that injection of highT) andesitic magmas. Compositional zonations in clinopyroxene T magma intermittently continued, both to form and to enlarge phenocrysts provide direct evidence for a pre-eruption crystal–melt magma pockets in the mush. In the process, melt mixing occurred mush chamber containing low-T magma, and elucidate the processes owing to convection and/or conjunction and coalescence of the of magma mixing and eruption, following the injection of high-T magma pockets. The mixing between the magma pockets and magma. Many phenocrysts with Fe-rich cores derived from low-T interstitial melt of the mush gave rise to normal zoning in the magma have extremely reverse zoned mantles around slightly resorbed mantles of contained phenocrysts. Zoning in the outermost rims of cores. Mg-value [100Mg/(Mg + Fe)] increases from 65–70 to pyroxenes differs between the September and October scorias. Almost >85 over a short width ( 85) is the same as or coalescence of magma pockets. These differences in presumed the core compositions of phenocrysts derived from the high-T magma, triggering process are consistent with geophysical observations before suggesting that these extremely zoned phenocrysts were surrounded and during the 1995 eruptions. immediately by invading high-T magma, as opposed to mixed melt. This does not indicate a simple melt-mixing process between crystalrich magmas, but is interpreted as evidence for mixing in a mush chamber. It is suggested that high-T magma was injected into a mushy low-T magma, then the denser and hotter high-T magma