Ground Deformation Vs. Hydrothermal Activity at the Lastarria Volcano, Northern Chile: Preliminary Insights from the Geochemistry of Fluid Discharges

XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 S7_001

Ground deformation vs. hydrothermal activity at the Lastarria volcano, northern Chile: preliminary insights from the geochemistry of fluid discharges

Aguilera, F.1, Tassi, F.2, Darrah, T.3, Vaselli, O.2

(1) Departamento de Geología, Universidad de Atacama, Copayapu 485, Copiapó, Chile (2) Department of Earth Sciences, University of Florence, Via G. La Pira 4, 50121, Florence, Italy (3) Department of Earth and Environmental Sciences, 227 Hutchinson Hall, Rochester, NY 14627, U.S.A. felipe.aguilera@uda.cl

Introduction

Lastarria (25°10’S, 68°31’W; 5,697 m a.s.l.) is a composite stratovolcano located 250 km SE of the Antofagasta city (northern Chile), pertaining to the Lastarria-Cordón del Azufre (Lazufre) volcanic complex in the Central Andes Volcanic Zone (CAVZ). In the last century Lastarria volcano has been characterized by permanent fumarolic activity from both the main crater and the outer flanks of the edifice [1,2]. InSAR images recorded between 1992 and 2006 [3,4,5,6] showed that, since early 1998, a NNE-oriented elliptical area (45x37 km) of the Lazufre complex was affected by severe ground deformation, comparable to those recently measured at Yellowstone and Long Valley in USA, Uzon in Kamchatka and Hualca Hualca in Peru [3,7,8]. This phenomenon seems to be associated to the evolution of an over-pressured magmatic reservoir located at a depth 7-15 km [4,5]. Since 2003 a second event of ground deformation, affecting an area of about 6 km2 in correspondence of the Lastarria volcano, has also been measured [5]. The geomorphological evolution of this area was ascribed to pressurization of an aquifer at a certain depth [5]. This study concerns the first chemical and isotopic survey of fluid emissions discharging at the Lastarria volcano sampled during two campaigns carried out in May 2006 and March 2008. The main aim is to construct a geochemical model for defining the underground pathways of the fumarolic fluids and their possible relation with the geophysical signals recently recorded at this volcanic complex.

XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 Geological setting and volcanic activity of the Lastarria volcano

Lastarria volcano is part of a complex polygenetic structure (Lastarria complex) that, according to [9,10], is formed by: 1) the Negriales lava field (or Big Joe), situated SW of the main volcanic structure and composed by 0.6±0.3 My old andesitic-to-dacitic lava flow successions [11]; 2) the Spur volcanic edifice, located NE of Negriales; 3) the Lastarria volcano sensu stricto, formed by 5 NS-aligned nested craters and representing the main and youngest (<0.3 My) structure of the system [12]. The Lastarria volcanic edifice superimposed a basement mainly constituted by Upper Miocene-Lower Pleistocene andesitic-to-dacitic lava flows and domes, and Lower Pleistocene dacitic ignimbrites [11]. No historical eruptions were recorded. Fumarolic degassing is presently active at i) the north-westernmost craters and ii) a NW-SE trending fracture system along the NW external flank of the Lastarria edifice.

Chemical Composition of Gases

The outlet temperatures of the fumaroles were between 80.8 and 319 ºC. Water vapour ranged from 82.57 to 92.72 % by vol. The composition of the dry gas fraction was characterized by dominant CO2 (up to 989,969 μmol/mol) and relatively high concentrations of acidic compounds, such as SO2 (up to 27,921 μmol/mol), H2S (up to 27,196 μmol/mol), HCl (up to 4,200 μmol/mol) and HF (up to 583 μmol/mol). Nitrogen and H2 concentrations showed a large variability (from 3,210 to 34,917 and from 50.1 to 11,223 μmol/mol, respectively). Carbon monoxide in the gas discharges with temperature >120 °C ranged between 25 and 34.8 μmol/mol, whereas in the fumaroles having temperature < 96.1 °C its concentrations were significantly lower (< 3.6 μmol/mol). Helium concentrations did not exceed 9.6 μmol/mol. Atmospheric contribution was relatively low, being O2 and Ar concentrations up to 500 and 27.4 μmol/mol, respectively. The most abundant component of the organic gases, whose sum was <0.63 μmol/mol, was CH4, (up to 89.3 μmol/mol) followed by minor concentrations of light hydrocarbons, pertaining to the alkanes, alkenes, aromatics and heterocyclics groups.

Discussion and Conclusions

The fumarolic fluids of the Lastarria volcano were fed by conspicuous contribution from a high-temperature source, as indicated by the high concentrations of SO2 (up to 28,000 μmol/mol), a gas compound typical of direct magma degassing [13,14]. Gas species produced by gas-water-rock interactions at high temperatures (mainly H2 and CO), as well as the outlet fumarolic temperature, decreases toward the pheripery of the fumarolic fields, indicating the presence of a non-continuous shallow aquifer. Accordingly, δ18O and δD values measured in fumarolic condensates suggest a mixing between Andean Cordillera meteoric precipitation and “andesitic water”. Helium isotopic values (< 5.15

XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009

R/Rair) point to significant crustal gas contribution to magmatic-released fluids, likely occurring within the hydrothermal aquifer and/or caused by direct contamination of the magma related to melt of crustal rocks related to the evolution of the pre-caldera silicic 13 system [15]. The δ C-CO2 values, ranging from -4.1 to -0.4 ‰ V-PDB, suggest that marine sediments were involved in the subduction process producing the magmas feeding the CAVZ.

On the basis of this preliminary geochemical indications, the ground deformation recorded at the Lastarria volcanic system in the last years may be tentatively explained by the following mechanisms: the primary 1992-2006 episode could be related to an increase of heat input from depth causing an inflation of the extended magmatic-hydrothermal reservoir; the post-2003 ground deformation centred in the active system may be related to the evolution of hydrothermal fluid pressurization that have progressively extended toward the surface involving the shallow aquifers.

Referencias

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XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 [11] Naranjo, J., Cornejo, P. (1992) Hoja Salar de la Isla. Servicio Nacional de Geología y Minería, 72 [12] Naranjo, J. (1988) Coladas de azufre en los volcanes Lastarria y Bayo en el norte de Chile: Reología, génesis e importancia en geología planetaria. Revista Geológica de Chile, vol. 15, 3–12 [13] Gerlach, T., Nordlie, B. 1975. The C-O-H-S gaseous system, Part II: temperature, atomic composition and molecular equilibria in volcanic gases. American Journal of Science, Vol. 275, 377 – 394 [14] Giggenbach, W.F. (1987) Redox processes governing the chemistry of fumarolic gas discharges from White Island, New Zealand. Applied Geochemistry, vol. 2, 143–161 [15] Trumbull, R.B., Wittenbrink, R., Hahne, K., Emmermann, R., Büsch, W., Gerstenberger, H., Siebel, W. (1999) Evidence for late Miocene to recent contamination of arc andesites by crustal melts in the Chilean Andes (25–26°S) and its geodynamic implications. Journal of South American Earth Sciences, Vol. 12, 135–155