TWELVE YEARS OF ELEMENT FLUX MEASUREMENTS AT COPAHUE VOLCANO, ARGENTINA

Copahue Volcano (37.75^oS, 71.17^oW) had minor phreato-magmatic eruptions in 1992-1995 and a small magmatic eruption in 2000. An active volcano-hydrothermal system (pH~0, T~300 ^oC) at depth scrubs all volcanic volatiles, and SO₂ disproportionates into bi-sulfate and liquid elemental Sulfur. The acid fluids are injected into a crater lake and acid hot springs, which feed a glacial meltwater river that drains into the large glacial Lake Caviahue. We have measured the water fluxes and river water compositions for 12 years, including the 2000 eruptive period. This has provided a flux record of volcanic elements (VE: F, Cl, S, toxic trace elements like Li, B, As) and major Rock Forming Elements (RFE: Al, Si, Fe, Mg, Ca, K, Na). The annual river flux measurements are complemented with analytical data from vertical water profiles through Lake Caviahue, which through its ~3.5 years water residence time has a ‘chemical memory’ of past element fluxes. A two-box non steady-state dynamic lake model was used to constrain the variations in element influxes into the lake for comparison with the annual influx measurements. The mean, time-weighted measured volcanogenic fluxes (tonnes/month) were 1510 S, 1180 Cl and 104 F, with peak values in 1999-2000 and very low yields in 2001-2002 after the eruption. The F-Cl-S concentrations in Lake Caviahue waters have dropped from their high values in 2000 as a result of the modest fluxes in more recent years. The RFE fluxes peaked during the 2000 eruption, and went through a minimum in 2001-2002. Precipitation of jarosite / alunite since 2000 has strongly reduced the flux of K and Al into the lake. The S flux rate was translated into a volumetric magma degassing rate of ~ 3.5 10⁸ m³/decade (using a high estimate of 200 ppm S released from the melt, based on glass inclusion and matrix glass analyses), whereas the mean RFE flux provides a rock dissolution rate of ~2 10⁵ m³/decade. The degassing ultimately leads to magma crystallization because of water loss, suggesting that the magma solidification rate is at least 1000 times larger than the rock dissolution rate, leading to net-volcano growth. Additional growth stems from the small eruptions that emplace magma at the surface. The rock dissolution probably leads to periodic flank collapses, which has given Copahue its rounded shape and modest elevation of 3000 m.