Other
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.2595c&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 60, Issue 14, pp.2595-2608
Other
17
Scientific paper
Oxygen isotope fractionations in laboratory systems have been determined for each of the following minerals relative to calcite: muscovite, phlogopite, fluorophlogopite, and rutile. Statistical mechanical calculations following the method of Kieffer (1982) were fit to the experimental data and then used to extrapolate the experimental results to higher and lower temperatures. The calculations are represented by a series of equations which allow the reduced partition function ratios ( factors) for each of these minerals to be calculated at T > 400 K. These equations can be combined with corresponding equations for calcite, quartz, albite, anorthite, diopside, forsterite, and magnetite (Clayton and Kieffer, 1991) to give a large number of mineral-pair fractionations for use as isotopic thermometers. It was found that the high-frequency vibrations of OH bonds contribute such a small amount of the fractionation factors that they do not introduce significant nonlinearity to plots of vs. T -2 . The commonly used calibrations of quartz-muscovite and quartz-biotite fractionations are not in good agreement with the present experimental measurements. This probably reflects disturbance of the rock assemblages on which those calibrations were based, as a consequence of the high diffusivity of oxygen in micas. The experimental quartz-rutile fractionations are in good agreement with some earlier hydrothermal experiments and with an empirically determined calibration. The calculated rutile partition functions of Kieffer (1982) are not consistent with the experimental results, probably due in part to the neglect of the effect of cation mass on the vibrational energies. The large number of mineral systems with measured fractionation factors allows a test of various empirical relationships based on oxygen bond strengths. In general, these relationships are successful for anhydrous silicates, but do not adequately account for the behavior of hydrous minerals or metal oxides.
Chacko Thomas
Clayton Robert N.
Goldsmith Julian R.
Hu Xiangsheng
Mayeda Toshiko K.
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