Evolution of clay mineral assemblages in the Tinguiririca geothermal field, Andean Cordillera of central Chile: an XRD and HRTEM-AEM study
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AutorVázquez, Mercedes; Nieto, Fernando; Morata, Diego; Droguett, Bárbara; Carrillo-Rosúa, Javier; Morales-Ruano, Salvador
Geothermal alterationAlteración geotermalClay mineralsMinerales de la arcillaReaction progressXRDDRXHRTEM-AEMAndean cordilleraCordillera andinaChile
Vázquez, M.; Nieto, F.; Morata, D.; Droguett, B.; Carrillo-Rosúa, J.; Morales-Ruano, S. Evolution of clay mineral assemblages in the Tinguiririca geothermal field, Andean Cordillera of central Chile: an XRD and HRTEM-AEM study. Journal of Volcanology and Geothermal Research, 282: 43-59 (2014). [http://hdl.handle.net/10481/35468]
PatrocinadorDepartamento de Mineralogía y Petrología (Universidad de Granada).; Departamento de Didáctica de las Ciencias Experimentales (Universidad de Granada).; Instituto Andaluz de Ciencias de la Tierra (Universidad de Granada- Consejo Superior de Investigaciones Científicas); Department of Geology University of Chile Andean Geothermal Center of Excellence (CEGA); Energía Andina S.A.; Financial support was provided by the Chilean Research Projects Fondecyt-Regular-1140629 and FONDAP-CONICYT-15090013 “Andean Geothermal Center of Excellence (CEGA)”.
HRTEM textural evidence shows that clay minerals in the Tinguiririca geothermal field (Andean Cordillera, central Chile) are the result of direct alteration of former volcanic glass and minerals by hydrothermal fluids at similar temperatures to the present day. They show the classical pattern of diagenetic transformation from smectite at the top to illite at the bottom, with the progressive formation of corrensite and chlorite. The high fluid/rock ratio, disposability of necessary cations and absence of previous detrital phyllosilicates allow the consideration of this area as a natural laboratory to establish the extreme ideal conditions for very low-T reactions. Transformations from smectite to R1 illite–smectite (I–S) and from these to R3 mixed-layers occur respectively at 80–120 °C and 125–180 °C. In spite of ideal genetic conditions, the new-formed minerals show all the defective character and lack of textural and chemical equilibrium previously described in the literature for diagenetic and hydrothermal low-temperature assemblages. Chemistry of smectite–illite phases evolves basically through a diminution of the pyrophyllitic component toward a theoretical muscovite (Si4++□-> Al3++K+). However, a second chemical vector (Si4++Mg2+→Al3++Al3+), that is, decreasing of the tschermack component, also contributes to the evolution toward the less Si-more Al rich muscovite in relation to the original smectite. Residual Mg (and Fe) from the latter reaction is consumed in the genesis of chloritic phases. Nevertheless, as a consequence of the lack of chemical equilibrium (probably because of the short time-scale of the geothermal alteration processes), the composition of clay minerals is highly heterogeneous at the level of a single sample. Consequently, the respective fields of smectite, R1 I–S and R3 I–S overlap each other, making the distinction among these three phases impossible based exclusively on chemical data.