The cordierite fluid monitor: case studies for and against its potential application

The H₂O and CO₂ content of cordierite was analysed in thirty-eight samples from six different metamorphic terranes, using Fourier-transform infrared spectroscopy (FTIR). The measured volatile contents were used to calculate peak-metamorphic H₂O and CO₂ activities and estimate the H₂O content of coexisting melts. Total volatile contents are compared to recently modelled cordierite volatile saturation surfaces to assess the extent of fluid-present vs. fluid-absent conditions across the terranes. The data indicate that, for the most part, the cordierite fluid monitor can be applied to a wide P–T–X_(bulk) range yielding vital information on fluid heterogeneity within the crust. The localised and variable nature of the fluid phase emphasizes the need for a large sample suite, particularly when attempting to conclude that a certain fluid regime was developed across a wide terrane. As well as providing insight into fluid regimes, the cordierite fluid monitor may also provide additional thermobarometric constraints, whose margins of error may be less than those of conventional thermobarometers. The successful application of the monitor does require taking into account the potential effects of re-equilibration during decompression and/or cooling. If these effects are ignored, the cordierite fluid monitor will yield results that are erroneous. This emphasizes the need to apply the cordierite fluid monitor to rocks which are suitably well-characterized in terms of their P–T evolution. Whilst the cordierite fluid monitor yields reliable results in most of the case studies presented, there are examples in which the only viable explanation for the preserved volatile contents is that they lost some H₂O and/or CO₂. The problem of why some cordierites leak and others do not, is a problem that remains to be solved.