Geochemistry of metallic trace elements in fumarolic condensates from Nicaraguan and Costa Rican volcanoes

Abstract

Metallic trace-element composition of volcanic gas from Cerro Negro, Momotombo, San Cristóbal, Telica and Masaya volcanoes in Nicaragua and Poás volcano in Costa Rica is indicated by analyses of fumarolic condensates for alkalis, alkaline earths, transition elements, precious metals, halogens and sulphate. The temperatures of the fumaroles varied from 85 to 852°C. Element concentrations range from 10 4 to 10 −3 ppm. Statistical tests indicate that the trace-element constituents of the condensates are log-normally distributed. Although there are variations in element concentrations in individual condensates, the general order of decreasing elemental abundances in Nicaraguan and Costa Rican fumarolic gas is: Cl > SO 4 > certain igneous rock-forming elements (Si, Al, Fe, Ti) > F and alkalis (Li, Na, K) > alkaline earths (Mg, Ca, Ba) > Br and transition elements (Cr, Mn, Co, Ni, Cu, Zn) > other ore-forming elements (Mo, Cd, Sn, Pb) > semi-metals (As, Sb) > Sc, Hg, and precious metals (Au, Ag). Li, Mn, Cr, and Pb were found to be near or below detection limits in many of the condensates analyzed. SO 4, Ti, Al, K, Na, Zn, Sn, Cr, and Br all tend to decrease in abundance as mean fumarolic temperature between volcanoes decreases. The remaining elements remain constant in abundance or behave erratically. The metallic trace-element composition of the Nicaraguan and Costa Rican fumarolic condensates is very similar to condensates from subduction zone volcanoes around the Pacific margin and the Mediterranean. Metals in volcanic gas are postulated to have been transported as halogen, sulphate, and oxide compounds. Chloride and fluoride compounds are considered to be the primary transporters of metals in volcanic gas. Correlations between metal and halogen content vary considerably between volcanoes. Statistically significant correlations between metal content and both chlorine and bromine are found in the Cerro Negro and between metal content and bromine and fluorine in the Momotombo condensates. Metal-bearing compounds in volcanic gas are postulated from correlation matrices. Bromine, though seldom reported in condensates, is a minor but important constituent of volcanic gas with concentrations similar to those of fluorine. Metal-bromide compounds have higher volatilities than fluoride compounds and volatilities similar to chloride compounds. Significant correlations between bromine content and metal concentration suggests that bromide compounds play a role in the transportation of metals in volcanic gas.