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

Jones, Daniel A.

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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.

Description

Thesis (M.S.) University of Alaska Fairbanks, 2007

1. Introduction -- 2. Previous work -- 2.1. Introduction -- 2.2. Van den Bogaard and Schirnick, 1995 -- 2.3. Esser et al., 1997 -- 2.4. Singer et al., 1998 -- 2.5. Layer and Gardner, 2001 -- 2.6. Gardern et al., 2002 -- 2.7. Summary -- 3. A 3100-year history of Argon Isotopic and compositional variation at El Chichón Volcano -- Abstract -- 3.1. Introduction 3.2. Geologic background -- 3.3. Samples and methods -- 3.3.1. Samples -- 3.3.2. Mass spectrometry -- 3.3.3. Microprobe -- 3.3.4. X-ray fluorescence (XRF) -- 3.4. Results -- 3.4.1. Petrography -- 3.4.2. Plagioclase compositions -- 3.4.3. Glass compositions -- 3.4.4. Ar data -- 3.5. Discussion -- 3.5.1. Plagioclase compositions -- 3.5.2. Argon distribution -- 3.5.3. Argon isotopic variations -- 3.5.4. Rock and mineral compositional variations in El Chinchón plagioclase -- 3.6. Conclusions -- 3.7. Acknowledgements -- 4. Glassy matrix analyses -- 4.1. Introduction -- 4.2. Samples and methods -- 4.3. Results and discussion -- 4.4. Conclusions -- 5. Geochronology -- 5.1. Introduction -- 5.2. Samples and methods -- 5.3. Results -- 5.4. Discussion and conclusions -- 6. Conclusions -- References -- Appendices.

Date

2007-12

Type

Thesis

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Geosciences