El Chichon Phenocrysts: A Study in the Variability of Excess Argon Within Plagioclase and Hornblende

El Chichon volcano has produced 11 eruptions in the last 8,000 years, all of which display a relatively constant trachyandesitic composition. The constancy of the eruptive products attests to the tapping of a long-lived magma chamber and suggests a system held in steady state between the influx of basaltic magma and differentiation of resident magma. What is poorly understood about such systems, though, is the interval between such injections and eruptions. We have sampled pyroclastic and lava flows from six of these eruptions, subjecting their dominant phenocryst phases (plagioclase and hornblende) to argon isotopic analyses in order to investigate the dynamics of a steady state magmatic system. Plagioclase from the 1982, 550 ybp, 1500 ybp and 3100 ybp eruptions display variable enrichment of 40Ar ("excess" argon), whereas hornblende from the same eruptions shows little or no enrichment. Modeling of argon diffusion in these mineral phases indicates variable but brief intervals between injection and eruption (on the order of years) and rules out xenocrystic contamination as the source of excess argon. Mafic enclaves were found within these eruptive units, and the source of this argon is most likely the same as the source of this basaltic magma injected prior to eruption. In contrast, both plagioclase and hornblende phenocrysts from the 1250 ybp and 900 ybp eruptions have argon isotopic ratios near atmospheric values. This lack of excess argon is indicative of a longer time interval between injection of new material and eruption, which allowed the phenocrysts to re-equilibrate with atmospheric argon.