ABSTRACT

Oxygen, hydrogen, and sulfur isotope data for fluids and minerals associated with the crater lake of Poás Volcano, Costa Rica, are interpreted in the context of the chemical and hydrologic structure of the volcano. Oxygen and hydrogen isotope data were obtained for rain, spring, and river water, low-temperature fumarole condensates, and acid brines collected from the hot crater lake before its disappearance in April 1989. Flank river and spring waters whose solute compositions have been modified by volcanic and hydrothermal activity have, with one exception, isotopic compositions similar to local meteoric water. Acid chloride-sulfate brines of the summit crater lake are extremely enriched in ¹⁸O with respect to local meteoric water; in the most enriched brines ¹⁸O shifts are greater than 20‰. The ¹⁸O shift is related to a kinetic isotope effect associated with the intense evaporation at the surface of the lake. These same brines exhibit only minimal shifts in their D/H ratios. The apparent lack of deuterium fractionation in the brines is attributed to an increase in the flux of isotopically light steam into the crater lake and/or a decrease in the deuterium fractionation factor for evaporation that occurs at the surface of the lake. The decrease in deuterium fractionation is correlated with large increases in lake-brine acidity and dissolved solids concentration that preceded the disappearance of the lake. Sulfur isotope data are presented for H₂S and SO₂ gas collected from low temperature fumaroles; dissolved sulfate in spring, river, and crater lake waters; and native sulfur and gypsum found in the acid lake and active crater area. Δ_SO2-H2S for low temperature gases is approximately 24‰ indicating an equilibration temperature of 165°C. Δ_SO2-H2S for low temperature H₂S and lake brine sulfate is approximately 23‰, all indicating subsurface equilibration occurred at 265°C. The H₂S and native sulfur are both highly depleted in ³⁴S (δ³⁴S = –8 to –11‰). δ³⁴S values of ³⁴S-depleted H₂S and ³⁴S-enriched sulfate in lake brine are produced by disproportionation of SO₂ released by the shallow magma body. Native sulfur is formed by the oxidation of ³⁴S-depleted H₂S by non-sulfur-bearing oxidants such as atmospheric oxygen and ferric iron. Mass-balance calculations indicate that sulfitolysis of polythionic acids could also result in the deposition of significant quantities of native sulfur. Implications of the isotopic composition of present-day fluids observed at Poás Volcano with respect to the isotope systematics of acid-sulfate ore deposits are considered.