Compositions of hydrothermal silicates and carbonates as indicators of physicochemical conditions in the Reykjanes geothermal system, Iceland

Abstract

This paper presents results of a quantitative investigation of mineral chemistry in the Reykjanes geothermal system, Iceland, designed to assess the relative influence of various hydrothermal minerals on the observed bulk-rock mass changes of elements. Detailed petrography and electron microprobe analyses (n=∼1400) were performed on samples of rock cuttings from eleven wells that intersect different parts of the hydrothermal system. An important finding was the strong partitioning of barium into adularia, which becomes incorporated through “capture” substitution into the 12-fold coordination site, coupled with Al3+ substitution into the tetrahedral site. Adularia has average and maximum Ba contents of 0.8 and 3.5wt.%, respectively. In addition, there is no relationship between Ba concentration in adularia and crystal size, occurrence type (replacement versus vein/vug), or rock composition. There is also no correlation between the Ba content of replacement adularia with that of the igneous plagioclase being altered, demonstrating that Ba concentration in the altered rocks of the Reykjanes geothermal system is being controlled by large-scale hydrothermal redistribution processes and not primary chemostratigraphic variability. Calcite exerts a major control on strontium proportions in the bulk rock, and contains up to 1wt.% Sr. Concentrations of many minor and trace elements and some major elements in hydrothermal minerals at Reykjanes correlate positively with measured downhole temperature (to varying degrees), e.g.: Ba and Rb in adularia; Al(iv) in chlorite; Fe, Mg, Mn, and Sr in calcite; and the andradite component in garnet. The Fe/(Fe+Mg) of chlorite is unrelated to the precursor host rock ratio. Chlorite geothermometry, based on Al(iv) substitution and Fe/(Fe+Mg) content, predicts temperatures of 94–313°C (average=236°C). These temperatures are similar to or less than the present-day temperatures measured for corresponding sample depths in all wells (with the exception of a shallow interval from RN10). This may indicate a prolonged and ongoing heating trend in the deep liquid-phase region of the Reykjanes reservoir.