The oxygen isotope and cation exchange chemistry of feldspars

Cation exchange experiments between alkali and alkaline-earth feldspars and corresponding 2–3 molal aqueous chloride solutions were performed at a fluid pressure of one kilobar over the temperature range 350°–800°C. Oxygen isotope analyses of the exchanged feldspar indicate that essentially complete oxygen isotope equilibration between solution and feldspars accompanies the cation exchange. Oxygen isotope fractionations obtained this way were proved to be equilibrium fractionations by their agreement with those obtained by true isotope exchange reactions between synthetic feldspars and pure water. The oxygen isotope fractionation factor (α) between alkali feldspar and water in the temperature range studied is given by the expression 10³ ln α = 2.91 (10⁶T⁻²)−3.41. No isotope fractionation was discernible between albite and potassium feldspar. However, the alkali feldspars were found to concentrate O¹⁸ relative to the alkaline-earth feldspars, indicating a relationship between the Al/Si ratio in feldspar and tendency to concentrate O¹⁸. The plagioclase-water fractionation follows the equation: 10³ In α = (2.91 − 0.76β)(10⁶T⁻²) − 3.41 − 0.41β where β is the An content of the plagioclase.