The Geochemistry of Central American Volcanic Gas Condensates

摘要

Results of analyses for Cl, F, SO₄, Ca, Mg, Na, and K in 132 condensates from fumaroles of the Central American volcanoes Santiaguito, Fuego, Pacaya, Izalco, Cerro Negro, and Arenal are given. Each fumarole is of high temperature, greater than 150°C, and is located within 500 m of an active volcanic vent. Condensates were collected over a 5-yr period, August 1964-July 1969. The variation in chemistry of the condensates is shown to depend upon the fumarole location and the time of collection relative to the eruptive and cooling history of the volcano. Ground-water dilution also affects concentration because it is variable and cumulative. The data show that Cl and F in condensates decrease as the magma source is depleted and as the ground-water component in the fumarole increases. F is believed to be depleted by reaction with wallrock, as well. The alkalis, Na and K, are shown to come in part from the wallrock, and probably in part from the magma. Wallrock is suspected to be the chief source of Ca and Mg, as the Ca/Mg ratio in condensates is that of the wallrock. Fumaroles at main active vents are shown to be SO₄ rich, with comparatively low CI/SO₄ ratios. Fumaroles on flows are Cl-rich. The ratio CI/SO₄ decreases at fumaroles in or near a main vent at the beginning of an eruptive period. Regular monitoring of the condensate at such fumaroles should enable prediction of eruptive periods months in advance, although the exact timing would vary with location. The practicality of the method depends upon the existence of accessible fumaroles.