Crystallization of calc-alkaline andesite under controlled high-pressure hydrous conditions |
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Authors: | Trevor H Green |
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Institution: | (1) School of Earth Sciences, Macquarie University, 2113 North Ryde, N.S.W., Australia |
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Abstract: | A series of experimental runs has been conducted on a glass prepared from a natural island are calc-alkaline andesite from
Fiji. The crystallization sequence was determined for the pressure interval 9–36 kb under anhydrous conditions and with 2,
5, and 10% by weight of water carefully added.
Addition of water markedly lowers the liquidus, depresses the appearance of quartz and plagioclase in the crystallization
sequence, and greatly enlarges the field of garnet-clinopyroxene crystallization above 25 kb. Amphibole crystallizes in hydrous
runs up to 25 kb.
Electron microprobe analyses of critical phases allows calculation of controls on crystal fractionation trends. For hydrous
conditions at 5–15 kb amphibole-clinopyroxene dominate fractionation and a moderate decrease in Mg/Fe and a slight increase
in K/Na occurs. At 15–25 kb garnet also affects the fractionation and a moderate decrease in Mg/Fe and an increase in K/Na
results. Above 25 kb garnet-clinopyroxene control the fractionation and there is a slight decrease in Mg/Fe but a significant
increase in K/Na and a pronounced silica enrichment.
In terms of major element chemistry, the derivation of the Fijian dacites in the second period of eruption may be satisfactorily
explained by the fractionation of hydrous andesite at pressures >25 kb. Alternatively the dacites may result from lower degrees
of melting of the down-going hydrous lithosphere. Similarly other members of this eruptive period may be derived according
to a model of eclogite-controlled fractional melting or crystallization. Models involving amphibole fractionation at lower
pressures are less satisfactory for explaining compositions in the Fijian second period of eruption, but in other environments
models including amphibole-controlled fractionation may form part of a continuum of melting processes in subduction zones. |
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