Evolution of clay mineral assemblages in the Tinguiririca geothermal field, Andean Cordillera of central Chile: an XRD and HRTEM-AEM study

Abstract

24HRTEM textural evidence shows that clay minerals in the Tinguiririca geothermal 25 field (Andean Cordillera, central Chile) are the result of direct alteration of former 26 volcanic glass and minerals by hydrothermal fluids at similar temperatures to the 27 present day. They show the classical pattern of diagenetic transformation from 28 * Corresponding author. Tel.: +56-2-29784539; Fax: +56-2-26963050 E-mail address: mvazquez@ing.uchile.cl (M.Vázquez) 2 smectite at the top to illite at the bottom, with the progressive formation of 1 corrensite and chlorite. The high fluid/rock ratio, disposability of necessary 2 cations and absence of previous detrital phyllosilicates allow the consideration of 3 this area as a natural laboratory to establish the extreme ideal conditions for very 4 low-T reactions. Transformations from smectite to R1 illite-smectite (I-S) and from 5 these to R3 mixed-layers occur respectively at 80-120ºC and 125-180ºC. In spite 6 of ideal genetic conditions, the new-formed minerals show all the defective 7 character and lack of textural and chemical equilibrium previously described in 8 the literature for diagenetic and hydrothermal low-temperature assemblages. 9Chemistry of smectite-illite phases evolves basically through a diminution of the 10 pyrophyllitic component toward a theoretical muscovite (Si 4+ +☐ -> Al 3+ +K + ). 11However, a second chemical vector (Si 4+ +Mg 2+ -> Al 3+ +Al 3+ ), that is, decreasing 12 of the tschermack component, also contributes to the evolution toward the less 13Si-more Al rich muscovite in relation to the original smectite. Residual Mg (and 14 Fe) from the latter reaction is consumed in the genesis of chloritic phases.

Clay Mineralogy of Coal-Hosted Nb-Zr-REE-Ga Mineralized Beds from Late Permian Strata, Eastern Yunnan, SW China: Implications for Paleotemperature and Origin of the Micro-Quartz

“…It has been suggested that the temperature that reflects the appearance of R1 I/S during the smectite illitization process is generally around 100 °C while the transition temperature from R1 to R3 I/S can be up to ca. 180 °C [48,50,[53][54][55]. In this study, most of the studied samples have the R1 ordered I/S, except for only one sample that has the R3 ordered I/S (Table 1) indicating that the paleo-diagenetic temperature for the studied samples could have been up to 180 °C but for most of the samples, this value ranges from 100 to 180 °C.…”

“…Factors affecting formation of ordered I/S include temperature, fluid chemistry, time, source material composition, and permeability of the host rock [48,51,52]. In the present study, the latter three factors are unlikely to be the main controls on the ordering of I/S because most of the studied samples are mudstones derived from the same phase of alkaline volcanism with same magma source in the earliest Late Permian [1,2].…”

“…The clay minerals in this study, particularly I/S, are sensitive to thermal conditions, thus their characteristics can be used to estimate the paleo-diagenetic temperatures during the burial process [31,38,[48][49][50]. Factors affecting formation of ordered I/S include temperature, fluid chemistry, time, source material composition, and permeability of the host rock [48,51,52].…”

“…Temperature and fluid chemistry thus should be the dominant factors that would have influenced the formation of I/S in this study. Many studies have focused on the diagenetic temperature during the burial process, using the evolution of smectite illitization (especially the percentages of smectite layers within I/S) [48,50,[53][54][55]. In the XRD patterns for the EG-saturated clay fractions, as smectite layers decrease in I/S, the peaks at 9 Å, as well as peaks at 5 Å, become sharp and narrow showing a trend towards illite and indicating a progressive increment in temperature (Figure 7).…”

Mineralogy and geochemical investigation of Cambrian and Ordovician–Silurian shales in South China: Implication for potential environment pollutions

“…The various composition of clay minerals occurs in different depositional conditions, input of terrestrial material, the palaeoclimate and diagenesis (Singer, ). For example, with the developing diagenesis, the contents of unstable clay minerals like kaolinite and smectite decrease at the expense of stable clay minerals like illite and chlorite, which increase gradually (Vázquez et al, ; Chen, Han, et al, ). And clay mineral index is commonly applied to unearth the original depositional environment (Singer, ).…”