OXYGEN, HYDROGEN, AND SULFUR ISOTOPE SYSTEMATICS OF THE CRATER LAKE SYSTEM OF POAS VOLCANO, COSTA-RICA

Oxygen, hydrogen, and sulfur isotope data for fluids and minerals asso ciated with the crater lake of Poas Volcano, Costa Rica, are interpret ed in the context of the chemical and hydrologic structure of the volc ano. 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 bee n modified by volcanic and hydrothermal activity have, with one except ion, isotopic compositions similar to local meteoric water. Acid chlor ide-sulfate brines of the summit crater lake are extremely enriched in O-18 with respect to local meteoric water; in the most enriched brine s O-18 shifts are greater than 20parts per thousand. The O-18 shift is related to a kinetic isotope effect associated with the intense evapo ration at the surface of the lake. These same brines exhibit only mini mal shifts in their D/H ratios. The apparent lack of deuterium fractio nation in the brines is attributed to an increase in the flux of isoto pically light steam into the crater lake and/or a decrease in the deut erium fractionation factor for evaporation that occurs at the surface of the lake. The decrease in deuterium fractionation is correlated wit h large increases in lake-brine acidity and dissolved solids concentra tion that preceded the disappearance of the lake. Sulfur isotope data are presented for H2S and SO2 gas collected from low temperature fumar oles; dissolved sulfate in spring, river, and crater lake waters; and native sulfur and gypsum found in the acid lake and active crater area . DELTA(SO2)-H2S for low temperature gases is approximately 24parts pe r thousand indicating an equilibration temperature of 165-degrees-C. D ELTA(SO4)-H2S for low temperature H2S and lake brine sulfate is approx imately 23parts per thousand, all indicating subsurface equilibration occurred at 265-degrees-C. The H2S and native sulfur are both highly d epleted in S-34 (delta(S-34) = -8 to -11parts per thousand). delta(S-3 4) values of S-34-depleted H2S and S-34-enriched sulfate in lake brine are produced by disproportionation of SO2 released by the shallow mag ma body. Native sulfur is formed by the oxidation of S-34-depleted H2S by non-sulfur-bearing oxidants such as atmospheric oxygen and ferric iron. Mass-balance calculations indicate that sulfitolysis of polythio nic acids could also result in the deposition of significant quantitie s of native sulfur. Implications of the isotopic composition of presen t-day fluids observed at Poas Volcano with respect to the isotope syst ematics of acid-sulfate ore deposits are considered.