Induced polarization of the 1630-monogenetic dome, Furnas volcano, São Miguel Island, Azores archipelago

Highlights

• Induced polarization can be used to assess the water content and alteration of a monogenetic dome.

• A field application is done with the 1630-monogenetic dome at Furnas Volcano, Azores Islands.

• The work includes a petrophysical investigation using 6 core samples from the area of interest.

Abstract

Induced polarization is used to image the feeder dike of a monogenetic dome thanks to the signature of pronounced alteration around these conduits. We performed a 3D tomography of the electrical conductivity and normalized chargeability of the 1630-monogenetic dome located inside the caldera of Furnas, a quiescent stratovolcano in the Eastern side of São Miguel Island (Azores volcanic archipelago, Atlantic Ocean). A total of 2634 apparent resistivity and chargeability data were collected and inverted using a classical regularized least-squares inversion (based on a quasi-Gauss-Newton approach). The 3D tomograms display an area of both high electrical conductivity and normalized chargeability. This area corresponds to the altered subvolume associated with the magmatic feeding conduit of the monogenetic dome. The same conduit can be also observed on a 2D magnetotelluric (MT) section, at least for the first 200 m below the ground surface. Assuming that the in-situ pore water conductivity is the same as in Furnas Lake (∼0.016 S m⁻¹, 25 °C), the electrical conductivity and normalized chargeability are combined to obtain tomograms of the water content and cation exchange capacity of the monogenetic dome. The cation exchange capacity provides a proxy to image alteration associated with the formation of clay minerals and zeolites. This study highlights the usefulness of these techniques to image the structure of monogenetic domes.

Introduction

Geoelectric and electromagnetic methods can be used to image volcanoes (Smith et al., 1977; Bartel et al., 1983; Bolós et al., 2020; Revil et al., 2021). They can also be combined with seismicity to decipher their magmatic plumbing systems (e.g., Gresse et al., 2021). Induced polarization is a geophysical method extending the classical electrical conductivity tomography obtained with either galvanometric or electromagnetic methods (Vinegar and Waxman, 1984).

Electrical conductivity describes the ability of rocks to conduct electrical currents (e.g., Waxman and Smits, 1968). Normalized chargeability is a key property obtained through induced polarization measurements. It describes the ability of rocks to store reversibly electrical charge carriers when submitted to a primary (applied) electrical field (Vinegar and Waxman, 1984; Revil, 2013a, Revil, 2013b). These charge carriers accumulate at local polarization length scales associated with pore scale heterogeneities such as the pores and grains of the volcanic rocks (e.g., Vinegar and Waxman, 1984 for sandstones and Revil et al., 2017a, Revil et al., 2017b, for volcanic rocks). When the electrical field is suppressed, the charge carriers diffuse back in their concentration gradients generating a diffusion source current density (Vinegar and Waxman, 1984; Leroy et al., 2008; Soueid Ahmed and Revil, 2018). In some sense, time-domain induced polarization can be seen as a type of induced transient self-potential problem and can be treated as such (Soueid Ahmed and Revil, 2018; Soueid Ahmed et al., 2018).

Revil et al., 2017a, Revil et al., 2017b and Revil et al. (2019) developed a petrophysical model to explain the induced polarization properties of volcanic rocks. Their model connects the electrical conductivity and normalized chargeability as a function of salinity, water content, temperature and cation exchange capacity of the rocks. In addition, a large-scale induced polarization imaging method was developed to image volcanoes and their hydrothermal systems (Gross et al., 2021). Induced polarization was then used to image the temperature field below the caldera of Kilauea volcano in Hawaii (Revil et al., 2021) and to characterize clay caps in geothermal fields (Revil et al., 2019; Revil and Gresse, 2021). Two universal trends connecting temperature to both electrical conductivity and normalized chargeability, respectively, were obtained for volcanoes. The results were explained in terms of the presence of smectite as alteration product up to a critical temperature of 220 °C (Revil et al., 2021). Recently, Troiano et al. (2021) reported an induced polarization survey at la Solfatara in Italy, but their study was only qualitative with no associated petrophysical investigations of the rock properties or interpretation of the tomograms in terms of petrophysical properties of interest.

Monogenetic volcanoes are the result of a unique eruptive event, sometimes involving several phases with different eruptive styles (e.g., Murcia et al., 2019 and references therein). Result of small eruptive volumes, typically ≤1 km³ (e.g. Németh and Kereszturi, 2015), monogenetic activity creates relatively small but sometimes complex edifices often organized in few tens or hundreds square kilometers monogenetic fields. Despite a limited variation of the composition of the magma, various eruptive styles (producing scoria, tuff, or cinder cones, spatter cones, domes, or maars) can be observed in such fields. Another type of monogenetic activity develops in the form of parasitic volcanic cones, craters or domes on polygenetic edifices. They may be repeatedly emplaced and destroyed, from parasitic monogenetic domes usually resulting from a single eruption after which the magma supply stops and the feeding systems solidifies and cools down progressively. The alteration of magmatic domes was extensively discussed by Ball et al. (2015). Percolating fluids transforms primary minerals in dome lavas to weaker secondary products including clays and zeolites. The location and intensity of alteration in a dome depend on fluid preferential pathways in conjunction with heat supply. Regardless of their origin, monogenetic volcanoes and their alteration have been imaged with different techniques (see Cassidy et al., 2007; Mrlina et al., 2009; Gebhardt et al., 2011; Barde-Cabusson et al., 2013; Bolós et al., 2014; Gresse et al., 2017) but never using induced polarization, which has recently been proven to be a unique non-intrusive sensor of alteration as discussed above (e.g., Revil et al., 2021).

Furnas volcano is a quiescent volcano located in the Eastern side of São Miguel Island, in the Azores volcanic archipelago (Carmo et al., 2015). It comprises two nested calderas with a total size of 5 × 8 km² (Moore, 1990; Guest et al., 1999, Guest et al., 2015). Furnas volcanic activity has been characterized by several eruptive styles, ranging from mid-effusive activity to the caldera-forming explosive events (Guest et al., 1999, Guest et al., 2015). Nowadays, volcanic activity at Furnas volcano is characterized by secondary manifestations of volcanism, which comprise low-temperature fumarolic fields (95 to 100 °C), steaming ground, thermal and cold CO₂-rich springs, as well as soil diffuse degassing areas (e.g., Caliro et al., 2015; Viveiros et al., 2010; Silva et al., 2015). The caldera of Furnas volcano contains several monogenetic trachytic domes. Furnas trachytes are mainly derived from fractional crystallisation of alkali basalt parental magmas, at depths of 3 to 4 km. Ten intracaldera, moderately explosive, trachytic eruptions occurred at Furnas in the last 5000 years, two of which occurred in historical times with the formation of the 1439-43-Dome; and the 1630-Dome, two domes of ~700 m in diameter and ~100 m high (Cole et al., 1995; Guest et al., 2015). These historical subplinian/phreatomagmatic eruptions formed two tuff/pumice rings with central trachytic domes and its deposits mantle the caldera floor. The most recent 1630-eruption occurred in the southern flank of the volcano. It corresponds to the only historic eruption of Furnas volcano. This pumice-forming explosive eruption that lasted about a week, formed a lava dome responsible for pyroclastic flows together with 195 casualties. This dome was therefore selected because it is the most recent one in Furnas.

Our goal in this paper is to highlight how induced polarization can be applied to such monogenetic dome and to discuss the results in terms of a petrophyical model connecting alteration and induced polarization properties (conductivity and normalized chargeability). The petrophysical model will be compared with laboratory measurements obtained on core samples from Furnas to check the consistency between laboratory and field results.