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Journal of Petrology | Volume 43 | Number 12 | Pages 2279-2303 | 2002
© Oxford University Press 2002
Mixed Magmas, Mush Chambers and Eruption Triggers: Evidence from Zoned Clinopyroxene Phenocrysts in Andesitic Scoria from the 1995 Eruptions of Ruapehu Volcano, New Zealand
1DIVISION OF EARTH AND PLANETARY SCIENCES, GRADUATE SCHOOL OF SCIENCES, HOKKAIDO UNIVERSITY, N10 W8 KITA-KU, SAPPORO, 060-0810, JAPAN
2DEPARTMENT OF EARTH SCIENCES, HOKKAIDO UNIVERSITY OF EDUCATION, ASAHIKAWA, JAPAN
3INSTITUTE OF GEOLOGICAL AND NUCLEAR SCIENCES, WAIRAKEI RESEARCH CENTRE, TAUPO, NEW ZEALAND
Juvenile ejecta from the September and October 1995 eruptions of Ruapehu volcano, New Zealand, indicate that mixing occurred between relatively higher- and lower-temperature (high-T and low-T) andesitic magmas. Compositional zonations in clinopyroxene phenocrysts provide direct evidence for a pre-eruption crystal–melt mush chamber containing low-T magma, and elucidate the processes of magma mixing and eruption, following the injection of high-T magma. Many phenocrysts with Fe-rich cores derived from low-T magma have extremely reverse zoned mantles around slightly resorbed cores. Mg-value [100Mg/(Mg + Fe)] increases from 65–70 to 
85 over a short width (<20 µm) at the inner edge of the mantle, usually accompanied by an abrupt change of Cr2O3 and Wo contents. After the first extreme reverse zoning, high Mg-value in the crystal mantle usually continues over several tens to 200 µm in width, with intermittent reversals, and then Mg-value decreases towards the outer rim. The high Mg-value (85) is the same as the core compositions of phenocrysts derived from the high-T magma, suggesting that these extremely zoned phenocrysts were surrounded immediately by invading high-T magma, as opposed to mixed melt. This does not indicate a simple melt-mixing process between crystal-rich 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 displaced the lighter and cooler interstitial melt in the mush. In addition, the high-T magma could thermally erode crystals in the mush network. Hence, eroded crystals from the mush were captured by the high-T magma. Repeated reverse zonations in the crystal mantles, and variable mantle widths, indicate that injection of high-T magma intermittently continued, both to form and to enlarge magma pockets in the mush. In the process, melt mixing occurred owing to convection and/or conjunction and coalescence of the magma pockets. The mixing between the magma pockets and interstitial melt of the mush gave rise to normal zoning in the mantles of contained phenocrysts. Zoning in the outermost rims of pyroxenes differs between the September and October scorias. Almost all of the October phenocrysts, but few of the September phenocrysts, have thin (several micrometres) reverse zoned rims. Therefore we suggest that the October eruptions may have been triggered by intensive injection just before the eruption, whereas the September eruptions may have been triggered by either smaller-scale injections or coalescence of magma pockets. These differences in presumed triggering process are consistent with geophysical observations before and during the 1995 eruptions.
KEY WORDS: magma mixing; magma chamber; mush chamber; compositional mineral zonation; Ruapehu volcano
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