Magma mixing, recharge and eruption histories recorded in plagioclase phenocrysts from El Chichon Volcano, Mexico

Consistent core-to-rim decreases of Sr-87/Sr-86 ratios and coincident increases in Sr concentrations in plagioclase phenocrysts of varying size (similar to 1 cm to 2 mm) are reported from samples of the 1982 and pre-1982 (similar to 200 ka) eruptions of El Chichon Volcano. Maximum 87Sr/86Sr ratios of similar to 0.7054, significantly higher than the whole-rock isotopic ratios (similar to 0.7040-0.7045), are found in the cores of plagioclase phenocrysts, and minimum 87Sr/86Sr ratios of similar to 0.7039 are found near some of the rims. Plagioclase phenocrysts commonly display abrupt fluctuations in An content (up to 25 mol %) that correspond to well-developed dissolution surfaces. The isotopic, textural and compositional characteristics suggest that these plagioclase phenocrystals grew in a system that was periodically recharged by higher-temperature magma with a lower 87Sr/86Sr ratio and a higher Sr concentration. Rim 87Sr/86Sr ratios in plagioclase phenocrysts of rocks from the 200 ka eruption indicate that, at that time, the magma had already attained the lowest recorded 87Sr/86Sr value of the system (similar to 0.7039). In contrast, cores from plagioclase phenocrysts of the 1982 eruption, inferred to have grown in the past few thousand years, have the highest recorded 87Sr/86Sr ratios of the system. Collectively, the Sr isotopic data (for plagioclase and whole rock), disequilibrium textural features of the phenocrysts, known eruption frequencies, and inferred crystal-residence times of the plagioclases are best interpreted in terms of an intermittent magma chamber model. Similar processes, including crustal contamination, magma mixing, periodic recharge by addition of more mafic magma to induce plagioclase disequilibrium (possibly triggering eruption) and subsequent re-equilibration, apparently were operative throughout the 2000 ky history of the El Chichon magma system.