Argon isotopes and geochemistry of El Chichón Volcano, Chiapas, Mexico

Abstract

Argon isotope systematics can be highly insightful for the investigation of magmatic processes, as the isotopic ratios of different terrestrial reservoirs vary greatly and argon diffuses rapidly at magmatic temperatures. In an effort to understand the dynamics of argon systematics in volcanic systems, the major phases of the eruptive products of EI Chichón volcano were subjected to argon isotopic and bulk chemical analysis. While glass analyses were typically atmospheric in composition, phenocrysts display variation in both concentration and isotopic composition, both within and between the eruptive units, with a major change in behavior noted around 1500 YBP. This same event is recorded in trace element variation, and while little variation is seen in major oxide composition, fractionation trends are apparent. Correlations among argon isotopes, Ca and Cl reveal that the bulk of the argon resides within melt inclusions, but different reservoirs within phenocrysts contain differing isotopic ratios. Variations in the amount and isotopic composition of argon within a phenocryst are likely a function of crystal residence and the isotopic signature of the magma chamber during phenocryst growth. ⁴⁰Ar/³⁹Ar geochronology shows pre-Holocene eruptive activity had an approximate 100 ka interval and spanned at least 370 ka.