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Fumarole/plume and diffuse CO <inf>2</inf> emission from Sierra Negra caldera, Galapagos archipelago

Abstract:

Measurements of visible and diffuse gas emission were conducted in 2006 at the summit of Sierra Negra volcano, Galapagos, with the aim to better characterize degassing after the 2005 eruption. A total SO 2 emission of 11 ± 2 t day -1 was derived from miniature differential optical absorption spectrometer (mini-DOAS) ground-based measurements of the plume emanating from the Mini Azufral fumarolic area, the most important site of visible degassing at Sierra Negra volcano. Using a portable multigas system, the H 2S/SO 2, CO 2/SO 2, and H 2O/SO 2 molar ratios in the Mina Azufral plume emissions were found to be 0.41, 52.2, and 867.9, respectively. The corresponding H 2O, CO 2, and H 2S emission rates were 562, 394, and 3 t day -1, respectively. The total output of diffuse CO 2 emissions from the summit of Sierra Negra volcano was 990 ± 85 t day -1, with 605 t day -1 being released by a deep source. The diffuse-to-plume CO 2 emission ratio was about 1.5. Mina Azufral fumaroles released gasses containing 73.6 mol% of H 2O; the main noncondensable components amounted to 97.4 mol% CO 2, 1.5 mol% SO 2, 0.6 mol% H 2S, and 0.35 mol% N 2. The higher H 2S/SO 2 ratio values found in 2006 as compared to those reported before the 2005 eruption reveal a significant hydrothermal contribution to the fumarolic emissions. 3He/ 4He ratios measured at Mina Azufral fumarolic discharges showed values of 17.88 ± 0.25 R A, indicating a mid-ocean ridge basalts (MORB) and a Galapagos plume contribution of 53 and 47 %, respectively. © 2012 Springer-Verlag.