Modification of Na-K-Mg triangular diagram and selection of Na-K geothermometer

Na-K-Mg triangular diagram is a preliminary method of determining the water-rock equilibrium state. It does not specifically consider the effect of geothermometer equations and mineral compositions on the equilibrium state. Therefore, using the Na-K-Mg triangular diagram modified by different Na-K geothermometer equations to judge water-rock equilibrium state, and using the suitable Na-K geothermometer equation to calculate the temperature of geothermal reservoirs, is of great significance to divide the genetic types of geothermal systems and develop or utilize geothermal resources. It is found that any point in the modified Na-K-Mg triangular diagram is fully equilibrated with the curve determined by one Na-K geothermometer equation and it is possibly partially equilibrated with the curve determined by the other Na-K geothermometer equation, and this phenomenon becomes obvious with the decrease of temperature. The most Mg-poor upper boundary and the most Mg-rich lower boundary are determined by different geothermometer equations. Using different Na-K geothermometer equations to draw the curve of logarithmic function of the ratio of Na to K with temperature, and drawing the Na-K-Mg triangular diagram based on all "fully equilibrated curves" corresponding to the intersection points of the curve arrives at the selection methods and Tibet Yanjing area was taken as an example to calculate the temperature of geothermal reservoirs.