The smectite–chlorite transition in drillhole NJ‐15, Nesjavellir geothermal field, Iceland: XRD, BSE and electron microprobe investigations

Abstract In well NJ‐15 of the Nesjavellir geothermal field, Iceland, the transition of discrete smectite into discrete chlorite has been studied from drill cuttings recovered at depths of less than 1714 m and over a continuous range of temperatures between 60 and 300° C. At temperatures below 180° C, the clay fractions contain mixtures of di‐ and trioctahedral smectites, whose layer charge increases with depth. Between 200 and 240° C, discrete smectites have transformed into smectite‐rich, randomly interstratified chlorite and smecite ( R 0 C/S). Because the abundance of chlorite interlayers in this C/S is generally <20%, its presence can be detected only by electron microprobe techniques and not by X‐ray diffraction. Between 245 and 265° C, both regularly ( R 1) and randomly interstratified C/S are the predominant layer silicates. Discrete chlorite first appears at approximately 270° C and coexists with minor amounts of R 0 C/S at higher temperatures. R 0 and R 1 C/S form a nearly complete compositional series between trioctahedral saponite and discrete chlorite end‐members. The interlayer cation and Si content of smectites and C/S decrease with increasing temperature. The Mg/(Mg + Fe) content of smectite, C/S, and chlorite is unrelated to temperature. The percentage of chlorite in C/S, as determined by electron microprobe analyses, increases continuously with increasing temperature, except for occurrences of smectite‐rich C/S in fresh basaltic dykes which have not thermally equilibrated with the higher grade country rocks.