Experimental study of shortite (Na2Ca2(CO3)3) formation and application to the burial history of the Wilkins Peak Member, Green River Basin, Wyoming, USA

The temperature at which shortite (Na2Ca2(CO3)(3)) forms from pirssonite (Na2Ca(CO3)(2)center dot 2H2O) and calcite using pure synthetic phases in the system Na2CO3-CaCO3-H2O has been experimentally determined. At similar to 1 atm pressure, shortite forms via the reaction Na2Ca(CO3)(2)center dot 2H(2)O vertical bar CaCO3= Na2Ca2(CO3)(3) vertical bar 2H2O above 55 perpendicular to 2 degrees C. This equilibrium temperature is lower than determined previously (90 +/- 25 degrees C) by Bradley and Eugster (1969). The solution in equilibrium with synthetic shortite, pirssonite, and calcite approximates a binary H2O-Na2CO3 brine with 1.1 m Na2CO3 (10.6 wt% Na2CO3). The equilibrium temperature is lowered to 52 +/- 2 degrees C with 5 m NaCl added to the system, which shows that this reaction is weakly dependent on the activity of H2O, a(H2O). This study suggests that shortite does not occur in surficial alkaline saline environments because temperatures are too low. Shortite is abundant in the Wilkins Peak Member (WPM) of the Eocene Green River Formation, Green River Basin, Wyoming, USA (>78 million tons/km(2)), where it occurs as diagenetic displacive crystals, fracture fills, and pseudomorphous replacements of a precursor Na-Ca-carbonate. The large area over which shortite occurs in the WPM indicates that saline pore fluids once existed in the buried lacustrine sediments, and thus, at times, large Na-CO3-rich saline alkaline lakes or laterally extensive saline groundwaters existed in the Green River Basin during WPM time. The thermal stability of shortite, together with vitrinite reflectance data and inferred regional geothermal gradients, establish that the shortite-bearing intervals of the WPM were buried to maximum depths of similar to 1000 m in the Green River Basin, and since experienced similar to 800 m of erosional exhumation. (C) 2013 Elsevier Ltd. All rights reserved.