Variable regimes in magma systems documented in palgioclase zoning patterns:: El Misti stratovolcano and Andahua monogenetic cones

Magmas may experience closed-or open-system evolution during their ascent and evolution in the upper crust. Chemically closed-system processes can involve heat transfer from underplating mafic magmas or decompression due to magma ascent. On the other hand, newly injected magmas can mix with the residing magma and hybridize. Plagioclase zonation patterns and electron microprobe major and trace element analysis constrain these contrasting scenarios for andesites from the 80 km 3 El Misti stratovolcano (5822 m) and a field of relatively small monogenetic Andahua scoria cones, both in southern Peru. In particular, Fe concentration in plagioclase reveals valuable information within a magmatic system to explore the dominance of closed- vs. open-system processes. Large and correlated Anorthite-Fe contrasts at resorption zones identify compositional mixing related to recharge. Subhorizontal An-Fe variations record thermal or water-induced effects. El Misti volcano, a large but still relatively youthful stratovolcano, is dominated during most of its approx. 120 ka history by closed-system effects during its evolution. Recharge and mixing mostly occur-red cryptically as it is not recorded in drastic compositional changes in magmas or their phenocrysts. A small volume erupted at El Misti, however, is affected by strong open-system behaviour as reflected in strongly correlated variations in An and Fe contents of plagioclase compositions. Its crystal population is texturally and chemically very similar to lavas of the monogenetic cinder cones of the Andahua Volcanic Field. For the El Misti magma reservoir, resorption events caused by thermal effects or magma ascent are more frequent in comparison to the actual eruption frequencies. Many cycles of closed-system evolution (heat transfer and decompression) therefore occurred without resulting in an eruption. Plagioclase zonations from lavas of the monogenetic Andahua field indicate that any mafic batch that intruded the much smaller reservoirs immediately triggered an eruption. By contrast, a large magma system that feeds El Misti volcano can easily accommodate a new batch without causing eruption. (c) 2007 Elsevier B.V. All rights reserved.