The 1982 eruptions of El Chichón Volcano, Chiapas, Mexico: Mineralogy and petrology of the anhydritebearing pumices

Abstract

The 1982 eruptions of El Chichón Volcano produced three major pumice- and ash-fall layers. Fresh pumices from the three layers are indistinguishable porphyritic trachyandesites with 55.9 wt. % SiO 2, 2.2% MgO, 2.8% K 2O, 0.4% P 2O 5, and over 1.2% SO 3. The pumices contain 58 wt. % crystals dominated by plagioclase (mode An 43) and hornblende, with lesser amounts of augite, titanomagnetite, anhydrite, apatite, sphene, pyrrhotite, and biotite enclosed in a vesiculated matrix glass with 68 wt. % SiO 2. Anhydrite forms subhedral to euhedral microphenocrysts without reaction coronas, as well as inclusions within the outer zones of major phenocrystic minerals. Inclusions of apatite and glass occur within anhydrite microphenocrysts. Anhydrite was crystallizing from the melt prior to eruption. Pumices resampled some ten months after the eruptions contain only 0.2 wt. % SO 3, and have only scattered remnants of anhydrite and traces of gypsum lining vesicles. A single rainy season in Chiapas (>4 m yr −1 rainfall) was sufficient to remove most primary anhydrite, which may be a relatively common igneous mineral, although rarely preserved in the geologic record. El Chichón trachyandesites are relatively enriched in K 2O, Rb, Sr, Th, U, Cs, and light rare elements compared to other andesites from Mexico and Central America. These enrichments may be related to the large distance from El Chichón to the Middle America Trench, or to subduction beneath Chiapas of the Tehuantepec Ridge, a major fracture zone of the Cocos Plate. Major- and trace-element analyses obtained by microprobe and instrumental neutron activation are given for all major minerals and glass, allowing trace-element partition coefficients to be calculated. Sphene and apatite contain high concentrations of rare earth elements, W, Th, and U. Sphene is also highly enriched in Hf and Ta. Prior to eruption, the El Chichón trachyandesite was at a relatively low temperature in the range 750° to 850°C, with f O 2 above the Ni-NiO buffer. The melt was very water rich with some 4 to 10 wt. % H 2O; the magmatic water content was 2–4 wt. %. The magma also contained an estimated 2.6 wt. % sulfur (as SO 3). Of this amount, 1.2 wt. % resided in anhydrite crystals and <0.1 wt. % in pyrrhotite. The melt appears to have contained <0.1 wt. % sulfur, consistent with experimentally determined sulfur solubility limits. The remaining 1.2 wt. % sulfur was probably present as a discrete vapor phase with H 2S > SO 2.