Abstract
The channel H20, C02, and alkali contents of 33 natural cordierites have been analyzed using a combination of Fourier-transform infrared spectroscopy (ftir), electron-micro-probe analysis, and chemical and isotopic techniques. H20 and C02 were analyzed by infrared spectroscopy (ir); hydrocarbons such as methane were not detected. In the ir spectra, type-II water and total (Na + K + Ca) are strongly correlated and show a 2:1 ratio. Most cordierites from granulite-facies samples are characterized by low total volatile contents, low total water, low alkalis, and high but variable .
If a fluid phase exists during high-grade metamorphism, then the volatiles in cordierite potentially provide useful information about metamorphic-fluid conditions. Isopleths for estimating the equilibrium amounts of total volatiles in cordierite as a function of P, T, and have been derived using published experimental data. Many natural cordierites appear relatively volatile deficient by comparison. The degree of volatile deficiency correlates to the amounts of the channel cations (Na + K + Ca) and C02 and most likely results from postmetamorphic leakage. Such postmetamorphic volatile losses can significantly impact the applicability of using cordierite for geobarometric studies and must also be considered in any estimation of metamorphic-fluid compositions from . If volatile losses affect both H20 and C02 equally, then estimates of granulite-facies fluid compositions generally indicate values of > 0.7. These estimates could be too high by 0.15 or more if H20 is preferentially lost during retrogression. Such estimates of from volatile contents of cordierite do not, however, reliably reflect fluid saturation or quantities of fluid.
The C-isotope evidence for cordierites from a number of granulite-facies terranes is not consistent with theories that propose large amounts of C02 infiltration, and for this reason the cordierite data, taken together, are best explained by processes of dehydration by partial melting.
