Long-term Magma Recharge Recorded in 87Sr/86Sr Zonation in Plagioclase Phenocrysts, El Chichon, Mexico

During the last 8,000 years, El Chichon volcano has erupted relatively similar trachyandesitic magma 11 times. This compositional trend likely indicates that long-term, steady state processes operate in the El Chichon magmatic system. Previous studies found that plagioclase crystals in magma erupted in 1982 A.D. are zoned in anorthite content (An) and 87Sr/86Sr ratio, recording magma recharge, differentiation, and changes in volatile concentration; we extend those studies to examine plagioclase crystals from six eruptions dating back to 3100BP. 87Sr/86Sr ratios and major-element compositions were measured with laser ablation ICP-MS and electron microprobe, respectively, along core-to-rim transects of crystals. 87Sr/86Sr ratios vary from 0.7040 to >0.7060 ±0.0002 and An contents range from ~An35 to ~An70. In some crystals, An content and 87Sr/86Sr ratio correlate negatively. Not all An zonations are, however, coincident with 87Sr/86Sr zonations; this is particularly true for An zones smaller than 30 um. Interestingly, crystals from the same eruption (even the same pumice) do not record the same zonation patterns. Indeed, some crystals from the 1250 BP pumice show an overall trend of increasing 87Sr/86Sr and decreasing An, whereas other crystals show 87Sr/86Sr ratios and An content that, although variable, maintain approximately constant baseline values. Our results contrast with Sr isotopic analyses of 1982 El Chichon plagioclases that showed systematic, core-to-rim decreases in 87Sr/86Sr independent of An zonation. Multiple magmatic processes are required to produce the different plagioclase zonation patterns. Negatively correlated Sr and An zones probably reflect replenishment by mafic magmas with low 87Sr/86Sr ratios, followed by magma differentiation and possible assimilation of high-87Sr/86Sr material. Zonations in An content unaccompanied by 87Sr/86Sr variations result from either changes in temperature or volatile content of the magma. Different zonation patterns in phenocrysts from the same eruption may indicate incomplete mixing during magma recharge. Plagioclase phenocrysts thus record multiple episodes of magma recharge, differentiation, and possibly changes in temperature or volatile concentration that have maintained a relatively uniform long-lived magmatic system at El Chichon volcano.