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 incre ases in Sr concentrations in plagioclase phenocrysts of varying size (simil ar to 1 cm to 2 mm) are reported from samples of the 1982 and pre-1982 (sim ilar to 200 ka) eruptions of El Chichon Volcano. Maximum 87Sr/86Sr ratios o f similar to 0.7054, significantly higher than the whole-rock isotopic rati os (similar to 0.7040-0.7045), are found in the cores of plagioclase phenoc rysts, and minimum 87Sr/86Sr ratios of similar to 0.7039 are found near som e of the rims. Plagioclase phenocrysts commonly display abrupt fluctuations in An content (up to 25 mol %) that correspond to well-developed dissoluti on surfaces. The isotopic, textural and compositional characteristics sugge st that these plagioclase phenocrystals grew in a system that was periodica lly 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 ha d already attained the lowest recorded 87Sr/86Sr value of the system (simil ar 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 h ighest recorded 87Sr/86Sr ratios of the system. Collectively, the Sr isotop ic data (for plagioclase and whole rock), disequilibrium textural features of the phenocrysts, known eruption frequencies, and inferred crystal-reside nce times of the plagioclases are best interpreted in terms of an intermitt ent 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 r e-equilibration, apparently were operative throughout the 2000 ky history o f the El Chichon magma system.