EXCESS VOLATILES AT COPAHUE VOLCANO, ARGENTINA

Many arc volcanoes emit more gas than can be accounted for by their erupted volumes (excess volatiles). Copahue is a 3 km high volcano in the Andes (37.5S, -71.5W), with Violent Strombolian eruptions in 2000 and late 2012. We have monitored the Cl discharge from Copahue’s hydrothermal system, which ranges from 250 – 1700 tonnes/month. The 2012 eruption produced basaltic andesite pumices, mixed with hydrothermal reservoir debris, and later denser scoria. The early deposits carry ‘plate-tephra’, with mm thick quenched rims and inflated core areas. These rocks have plagioclase, two pyroxenes, small olivine crystals and magnetite, with 60% glass in the pumices. Matrix glasses have at most 1165 ppm Cl, whereas melt inclusions range from 1100 to 1800 ppm Cl. We derived that the melt lost up to 500 ppm Cl during pre-eruptive degassing. Deposit isopachs, and eruption column height and duration suggest an erupted mass of 10¹² grams, which with pre-eruptive degassing could generate ~ 300 tonnes Cl. Between 2000 and 2012, Copahue released ~ 160,000 tonnes Cl, suggesting a ratio of degassing magma over erupted magma of about 500. Copahue is only the tip of the gasberg! Crystal-melt thermometry and barometry suggest a crystallization temperature of 1080 ^oC and a depth range of 3-10 km, indicating a 12 km tall degassing magma column. Nuclides from the ²³⁸U decay chain (²²⁶Ra-²²²Rn-²¹⁰Pb) show that almost all samples have ²¹⁰Pb deficits, strongly indicating large scale gas-loss from the magma into the overlying hydrothermal system. The maximum degassing time is ~ 10 years, in agreement with the eruption interval. We calculated bubble rise rates using Stokes Law and the intensive parameters calculated from the rock phases. If we assume that the bubbles in the rim of the plate tephra originated when erupting magma encountered hydrothermal fluids (quenching the original magmatic bubble size), such bubbles (200-300 micrometer diameter) would rise only a few tens to 100 meters per decade, and can not maintain the large observed Cl flux. Presumably, the magma was convecting upwards as well. The Copahue hydrothermal system is made of up to 60% degassed magmatic water, with abundant magmatic S and Cl. Copahue has substantial excess volatiles, but the mechanisms of gas transfer from great depth to the surface are not yet clear.