0022-3530/93 $3.00 The Significance of Channel and Fluid-Inclusion CO2 in Cordierite: Evidence from Carbon Isotopes

Carbon-isotope analysis of CO2 has been undertaken on pure mineral separates from six cordierite-bearing assemblages from Kerala, South India, to identify the significance of fluid inclusions and channel-trapped volatiles in cordierite. Field relations, phase-equilibria systematics, and fluid-inclusion studies suggest that cordierite megacrysts associated with orthopyroxene within charnockites have grown during an influx of CO2 post-dating regional metamorphism, in a process equivalent to incipient-charnockite formation in rocks of lower Mg/Fe ratios. In contrast, smaller cordierite grains associated with sillimanite and spinel, defining compositional bands in the metapelitic assemblages, have formed during the earlier regional metamorphism, although for one of these samples a subsequent period of local cordierite growth during channelized CO2 influx has been identified. CO2 has been extracted from fluid inclusions in quartz, garnet, and cordierite by a stepped-heating technique, and its abundance and carbon-isotope values determined in each case. Although unimodal release patterns generally characterize fluids released from quartz and garnet, for cordierite a strongly bimodal release profile is observed. At 500-700 °C volatiles are extracted from fluid inclusions in quartz, cordierite, and garnet, whereas above 800 °C CO2 release from cordierite is correlated with th