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- Title
Petrology of the Younger Andesites and Dacites of Iztaccíhuatl Volcano, Mexico: I. Disequilibrium Phenocryst Assemblages as Indicators of Magma Chamber Processes.
- Authors
NIXON, GRAHAM T.
- Abstract
Disequilibrium phenocryst assemblages in the Younger Andesites and Dacites of Iztaccíhuatl, a major Quaternary volcano in the Trans-Mexican Volcanic Belt, provide an excellent record of episodic replenishment, magma mixing, and crystallization processes in calc-alkaline magma chambers. Phenocryst compositions and textures in ‘mixed’ lavas, produced by binary mixing of primitive olivine-phyric basalt and evolved hornblende dacite magmas, are used to evaluate the mineralogical and thermal characteristics of end-members and the physical and chemical interactions that attend mixing. Basaltic end-members crystallized olivine (FO) and minor chrome spinel during ascent into crustal magma chambers. Resident dacite magma contained phenocrysts of andesine (An), hypersthene (En67−61), edenitic-pargasitic hornblende, biotite, quartz, .titanomagnetite, and ilmenite. On reaching high-level reservoirs, basaltic magmas were near their liquidi at temperatures of about 1250–1200°C according to the olivine-liquid geothermometer. Application of the Fe-Ti-oxide geothermometer-oxygen barometer indicates that hornblende dacite magma, comprising phenocrysts (<30 vol. per cent) and coexisting rhyolitic liquid, had an ambient temperature between 940 and 820°C at fs approximately 0·3 log units above the nickel-nickel oxide buffer assemblage. Mixing induced undercooling of hybrid liquids and rapid crystallization of skeletal olivine (Fo), strongly-zoned clinopyroxene (endiopside-augite), calcic plagioclase (An); and orthopyroxene (bronzite), whereas low-temperature phenocrysts derived from hornblende dacite were resorbed or decomposed by hybrid melts. Quartz reacted to form coronas of acicular augite and hydroxylated silicates were heated to temperatures above their thermal stability limit (˜940°C for amphibole, according to clinopyroxene-orthopyroxene geothermometry, and ˜880°C for biotite). Calculations of phenocryst residence times in hybrid liquids based on reaction rates suggest that the time lapse between magma chamber recharge and eruption was extremely short (hours to days). It is inferred that mixing of magmas of diverse composition is driven by convective turbulence generated by large differences in temperature between end-members. The mixing mechanism involves:(1) rapid homogenization of contrasting residual liquid compositions by thermal erosion and diffusive transfer (liquid blending); (2) assimilation of phenocrysts derived from the low-temperature end-member; and (3) fractional crystallization of rapidly evolving hybrid liquids in a turbulent boundary layer separating basaltic and dacitic magmas. The mixed lavas of lztaccíhuatl represent samples of this boundary layer quenched by eruption.
- Publication
Journal of Petrology, 1988, Vol 29, Issue 2, p213
- ISSN
0022-3530
- Publication type
Article