Fluid circulation and structural discontinuities inside Misti volcano (Peru) inferred from self-potential measurements

Abstract

One of the seven potentially active andesite stratovolcanoes in southern Peru, Misti (5822 m), located I 7 km northeast and 3.5 km above Arequipa, represents a major threat to the population (-900,000 inhabitants). Our recent geophysical and geochemical research comprises an extensive self-potential (SP) data set, an audio-magnetotelluric (AM1) profile across the volcano and CO 2 concentrations in the soil along a radial profile. The SP survey is the first of its kind in providing a complete mapping of a large andesitic stratovolcano 20 km in diameter. The SP mapping enables us to analyze the SP signature associated with a subduction-related active volcano. The general SP pattern of Misti is similar to that of most volcanoes with a hydrogeologic zone in the lower flanks and a hydrothermal zone in the upper central area. A quasi-systematic relationship exists between SP and elevation. Zones with constant SP/altitude gradients (Ce) are observed in both hydrogeologic (negative Ce) and hydrothermal (positive Ce) zones. Transition zones between the different Ce zones, which form a , concentric pattern around the summit, have been interpreted in terms of lateral heterogeneities in the lithology. The highest amplitudes of SP anomalies seem to coincide with highly resistive zones. The hydrothermal system 6 km in diameter, which extends over an area much larger than the summit caldera, may be constrained by an older, concealed collapse caldera. A sealed zone has apparently developed through alteration in the hydrothermal system, blocking the migration of CO 2 upward. Significant CO 2 emanations are thus observed on the lower flanks but are absent above the hydrothermal zone.

Magmatic gas percolation through the old lava dome of El Misti volcano

“…The high oxidation state preserved by the gases further indicates limited interaction with low temperature rock (Giggenbach, 1987). The absence of contamination of the dome gas composition by hydrothermal fluids is consistent with the idea of Finizola et al, (2004) who suggested that the hydrothermal system at El Misti is sealed by hydrothermal alteration. The clear magmatic signature and high equilibrium temperature of the emitted gases, together with their high exit temperature (270-310°C), suggest a relatively shallow magmatic source (see model from Stevenson, 1993).…”

A geochemical and geophysical investigation of the hydrothermal complex of Masaya volcano, Nicaragua

“…The first magmatic gas to exsolve from the magma, CO 2 , rises preferentially through magma and country rock along the main structural faults (e.g., Finizola et al, 2004;Chiodini et al, 2005;Bruno et al, 2007). While atmospheric CO 2 concentrations range from~350 to 500 ppm, magmatic gas can generate CO 2 concentrations from b1% to >95% within the soil pore spaces (e.g., Williams-Jones et al, 2000;Federico et al, 2010).…”

Electric potential anomaly induced by humid air convection within Piton de La Fournaise volcano, La Réunion Island

“…SP anomalies have been observed on active volcanoes with an amplitude ranging from a few hundreds millivolts up to several volts (Michel and Zlotnicki, 1998;Finizola et al, 2004;Aizawa, 2008;Barde-Cabusson et al, 2012;Brothelande et al, 2014). Saturated ground water flow is usually considered as the main process (so-called electrokinetic phenomenon) inducing the SP signal (Malengreau et al, 1994;Michel, 1998;Aubert et al, 2000;Finizola et al, 2004;Ishido, 2004;Bedrosian et al, 2007).…”

“…Saturated ground water flow is usually considered as the main process (so-called electrokinetic phenomenon) inducing the SP signal (Malengreau et al, 1994;Michel, 1998;Aubert et al, 2000;Finizola et al, 2004;Ishido, 2004;Bedrosian et al, 2007). Electrical lows located at the periphery of the volcano are interpreted to be related to saturated meteoric water downflow.…”