Repeated recharge, assimilation, and hybridization in magmas erupted from El Chichon as recorded by plagioclase and amphibole phenocrysts

Compositional and isotopic zoning patterns in plagioclase and amphibole phenocrysts from El Chichon record multiple cycles of country rock assimilation, magma injection, hybridization, and mixing. Laser ablation ICP-MS and electron microprobe analyses of plagioclase crystals from 7 eruptions spanning 3100 years reveal four types of zoning. These compositional and isotopic zones are often associated with textural changes observed in the crystals in thin section (e.g. sieved or patchy regions). Amphiboles are frequently zoned in Al and Si, and, in two magmas, may have clinopyroxene rims. Interestingly, most plagioclase show multiple and repeated zoning patterns. Moreover, all magmas contain all zoning patterns and textures, and crystals with substantially different sequences of zones occur within mm of one another. The most reasonable explanation for the origin of these textures is a frequently recharged chamber. Plagioclase zones with increasing anorthite contents (An) and decreasing (87)Sr/(86)Sr ratios record injection by a hotter, possibly wetter, and more primitive magma (lower (87)Sr/(86)Sr ratio). Zones with decreasing An and increasing (87)Sr/(86)Sr ratios record assimilation of country rock and/or hybridization of the host and injected magmas; injection of hot magma may provide the heat for country rock assimilation. Changes in An without corresponding changes in (87)Sr/(86)Sr ratio likely record slight variations in pressure or temperature during crystallization, or the far-field thermal effects of magma injection. Variations in (87)Sr/(86)Sr ratio unaccompanied by Anzoning record assimilation of country rock. Amphibole zoning patterns also record periodic heating events: amphibole with clinopyroxene rims record episodes where the magma was heated beyond the amphibole stability field. Bulk compositional homogeneity and the juxtaposition of many crystals with disparate zoning patterns in single pumice require the magmatic system to be well mixed. Strontium diffusion rates indicate that the plagioclase zoning patterns cannot have persisted at magmatic pressures and temperatures for more than similar to 500 years, thus cycles of injection and assimilation occur on timescales equal to or shorter than the eruption recurrence interval. Long-term compositional and isotopic homogeneity indicate that there is a balance between recharge, assimilation, and crystallization. (C) 2008 Elsevier B.V. All rights reserved.