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Published October 1, 1967 | public
Journal Article

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.

Additional Information

We thank Paul Yanagisawa for carefully performing most of the fluorine extractions of the oxygen in the feldspars, Fritz Laves, Leon T. Silver, W. Barclay Kamb, Samuel Epstein, P. M. Orville, and S. M. Sheppard for stimulating discussions, Arthur Chodos for the electron microprobe analyses, Elizabeth Bingham for various analytical services, and G. J Wasserburg for supplying a sample of PE-2 perthite. Financial support for this research was provided by the National Science Foundation (Grants No. G23992, GP2254), by the Atomic Energy Commission Contract #AT (04-3)-427, CALT-427-15, and by the Alfred P. Sloan Foundation.

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October 24, 2023