Illitization sequence controlled by temperature in volcanic geothermal systems: The Tinguiririca geothermal field, Andean Cordillera, Central Chile

In this research, we have investigated the illitization process in Quaternary calc-alkaline volcanic and volcaniclastic rocks in the active Tinguiririca geothermal field (Andean Cordillera, central Chile). XRD, SEM, and HRTEM/AEM techniques have been used to establish the illitization sequence and evaluate the influence of the factors controlling the mineral reaction and kinetics at low temperature (T). Analysed samples were collected through a slimhole core up to 815 m deep in which the T was measured in situ (up to 230 °C at the bottom of the drill core). Textural information indicates that the dioctahedral clays have replaced most of the vitreous components. In contrast, plagioclase phenocrysts have only been partially and patchily albitized. The observed replacements imply dissolution-crystallization processes. The illitization sequence detected by XRD is apparently continuous from smectite to R3 I-S through R0 and R1, with a progressive increase in illite layers. HRTEM data show a similar illitization trend. However, the high-resolution images reveal that the clays are more heterogeneous than the XRD patterns suggest, with the coexistence of different types of dioctahedral clays at the sample level. They also indicate that the most abundant dioctahedral clays are smectite, R1 I-S, and illite. Therefore, the XRD patterns are probably the result of a mixture of these phases plus accessory I-S mixed layers with higher ordering (R > 1). Increasing T with depth would enhance the kinetic conditions necessary for illitization and also favour the dissolution of the vitreous K-rich component and, locally the albitization of plagioclases. Both processes release K, which, with the concomitant increase in T and K availability, enhances the crystallization of clays progressively richer in K. Thus, at T ≤ 85 °C smectite crystallizes, at T > 85 °C the conditions are appropriate for the crystallization of R1 I-S (with minor smectite + R0 I-S), up to T ≥ 175 °C, where illite is the most abundant and relatively stable phase.