Cordierite channel volatiles as evidence for dehydration melting: an example from high-temperature metapelites of the Bayerische Wald (Variscan belt, Germany)
European Journal of Mineralogy Volume 12 Number 5 (2000), p. 987 - 998
published: Oct 4, 2000
manuscript accepted: Mar 30, 2000
manuscript received: May 15, 1999
ArtNo. ESP147051205007, Price: 29.00 €
Abstract The H2O and CO2 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 H2O and CO2 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 H2O content of a sample. The orientation effects are even more pronounced for CO2. Whereas the total cordierite H2O contents vary between the samples within the given range, the H2O 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 H2O contents from cores to rims. The CO2 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 H2O-CO2 fluids. Most likely, they are due to equilibration with peraluminous water-undersaturated melts. The melt H2O contents calculated with the cordierite H2O contents of the Bayerische Wald, using recently published partition coefficients, are in the range of melt H2O contents from dehydration melting experiments. The variation of cordierite H2O contents between the studied samples can be explained by locally differing melting reactions and melt percentages. The lower range of the H2O concentrations of cordierite could also be due to dehydration, assuming the most unfavourable circumstances. However, the presence of Na, CO2 and N2 acting as channel blockers, the 1:1 ratio of H2O I : H2O 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 H2O-saturated and the dehydration-melting solidus.