DISCOVERY OF INHERITED AND NEOFORM ZIRCON IN VOLCANIC ROCKS OF EL CHICHÓN VOLCANO (CHIAPAS, MEXICO)

El Chichón volcano is the only active volcano located within the Chiapanecan Volcanic Arc in southern Mexico, which lies between the Trans-Mexican Volcanic Belt and the Central American Volcanic Arc. Previous studies have shown that at least eleven eruptions have occurred in El Chichón within the last 8000 years, forming a complex of lava domes with a central crater and surrounding pyroclastic deposits. Here, we report the discovery of zircon in El Chichón rocks, which we analyzed by high spatial resolution imaging (cathodoluminescence) and isotopic (secondary ionization mass spectrometry) methods to resolve core and rim crystallization ages. Pumice samples from proximal pyroclastic flow and fall-out deposits (n = 5) were collected based on published stratigraphy. Two of these were further classified by new ¹⁴C dates (n = 2). In addition, we sampled lavas (n = 2) from the 1982 eruption and from remnants of the older Somma dome. Zircon crystals were dated using ²³⁰Th/²³⁸U disequilibrium (U-Th) and U-Pb geochronology. U-Th zircon ages dominantly fall between near eruption ages to ca. 84 ka, with overlapping ages in all samples. Zircon core U-Pb ages range between ca. 290 Ma and 1.9 Ga, and are consistent with xenocrystic origins from hidden basement rocks. The U-Th and U-Pb ages indicate arrested assimilation of crustal rocks in a cold crystal mush from which zircon crystals were recycled. Neoform zircon is significantly more long-lived than major minerals whose ages and chemical zonation suggest crystallization within a few ka before eruption. Accessory minerals such as zircon uniquely reveal that mixing and recycling briefly before eruption exert significant control over the chemical properties of arc magmas.