James A. Stimac

Geochemical surveillance of magmatic volatiles at Popocatépetl volcano, Mexico

Surveillance of Popocatepetl volcanic plume geochemistry and SO 2 flux began in early 1994 after fumarolic and seismic activity increased significantly during 1993. Volatile traps placed around the summit were collected at near-monthly intervals until the volcano erupted on December 21, 1994. Additional trap samples were obtained in early 1996 before the volcano erupted again, emplacing a small dacite dome in the summit crater. Abundances of volatile constituents (ppm/day of Cl, S total , F, CO 2 , Hg, and As) varied, but most constituents were relatively high in early and late 1994. However, ratios of these constituents to Cl were highest in mid-1994. δ 34 S-S total in trap solutions ranged from 1.5‰ to 6.4‰; lowest values generally occurred during late 1994. δ 13 C-CO 2 of trap solutions were greatly contaminated with atmospheric CO 2 and affected by absorption kinetics. When trap data are combined with SO 2 flux measurements made through November 1996, Popocatepetl released about 3.9 Mt SO 2 , 16 Mt CO 2 , 0.75 Mt HCl, 0.075 Mt HF, 260 t As, 2.6 t Hg, and roughly 200 Mt H 2 O. Near-vent gas concentrations in the volcanic plume measured by correlation spectrometer (COSPEC) and Fourier transform infrared (FTIR) commonly exceed human recommended exposure limits and may constitute a potential health hazard. Volatile geochemistry combined with petrologic observations and melt-inclusion studies show that mafic magma injection into a preexisting silicic chamber has accompanied renewed volcanism at Popocatepetl. Minor assimilation of Cretaceous wall rocks probably occurred in mid-1994.

Mafic-felsic magma interaction at Satsuma-Iwojima volcano, Japan: Evidence from mafic inclusions in rhyolites

Geochemical and petrographic studies of the rhyolites and mafic inclusions from Satsuma-Iwojima volcano were carried out in order to investigate evolution of a silicic, bimodal magma system during the post-caldera stage. Abundant mafic inclusions, which are fine-grained with vesicles in their cores, are present in the Showa-Iwojima rhyolitic lava. Inclusions with similar textures are found in Iwodake volcanic bombs but are less common than in the Showa-Iwojima lava. The major and trace element compositions of the inclusions plot along mixing lines connecting the host rhyolites with spatially and temporally associated basaltic to basaltic andesite magmas. Plagioclase phenocrysts in the inclusions have a large variation in core compositions (An42 to An96), and exhibit various zoning profiles and reaction textures, indicating they coexisted with melts ranging from basaltic to rhyolitic composition. Pyroxenes also exhibit a wide range in composition and a variety of zoning patterns consistent with multiple sources. These results suggest that a stratified magma chamber exists beneath the volcano, consisting of a lower basaltic layer, an upper rhyolitic layer and an episodically-present, thin middle layer of andesite. Variations in the chemistry of the Iwodake and Showa-Iwojima mafic inclusions suggest that multiple injections of very similar basaltic magma have occurred since the growth of the Iwodake dome. More extensive textural disequilibrium shows that the Showa-Iwojima rhyolites formed through more extensive interaction with mafic magma. The mafic-felsic interaction is consistent with degassing model of a magma chamber estimated by other researchers, which consists of degassing of upper rhyolitic magma by convection in a conduit and supply of a CO2-rich volatile phase from underlying basaltic magma to the rhyolitic magma.