Abstract
The H 2 O and CO 2 channel contents of cordierite in migmatites from the Bayerische Wald (Variscan Belt, Germany) were measured by infrared spectroscopy, directly from thin polished rock sections. Cordierite is characterized by low H 2 O and CO 2 contents of 0.38 to 0.74 and 0.04 to 0.10 wt%, respectively. Errors are mainly introduced by the varying orientations of the cordierite sections and are estimated to be 10–12% relative for the average total H 2 O content of a sample. The orientation effects are even more pronounced for CO 2 . Whereas the total cordierite H 2 O contents vary between the samples within the given range, the H 2 O contents of cordierite grains within a single sample show no variations, neither between cordierite formed by different reactions, nor between cores and rims. Only in one sample could an increase be observed in total H 2 O contents from cores to rims. The CO 2 contents vary between the samples and in some cases also within the samples. The low volatile contents are not consistent with equilibration of cordierite with H 2 O-CO 2 fluids. Most likely, they are due to equilibration with peraluminous water-undersaturated melts. The melt H 2 O contents calculated with the cordierite H 2 O contents of the Bayerische Wald, using recently published partition coefficients, are in the range of melt H 2 O contents from dehydration melting experiments. The variation of cordierite H 2 O contents between the studied samples can be explained by locally differing melting reactions and melt percentages. The lower range of the H 2 O concentrations of cordierite could also be due to dehydration, assuming the most unfavourable circumstances. However, the presence of Na, CO 2 and N 2 acting as channel blockers, the 1:1 ratio of H 2 O I: H 2 O II and rapid cooling of the rocks down to 500°C argue against significant dehydration. Therefore, in-situ FTIR spectroscopy on cordierite can be a useful tool to determine fluid/melting conditions in migmatites where clear evidence of partial melting is lacking, and calculated temperatures are between the H 2 O-saturated and the dehydration-melting solidus.
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