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 sep arates from six cordierite-bearing assemblages from Kerala, South Indi a, to identify the significance of fluid inclusions and channel-trappe d volatiles in cordierite. Field relations, phase-equilibria systemati cs, and fluid-inclusion studies suggest that cordierite megacrysts ass ociated with orthopyroxene within charnockites have grown during an in flux 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 s pinel, defining compositional bands in the metapelitic assemblages, ha ve formed during the earlier regional metamorphism. although for one o f these samples a subsequent period of local cordierite growth during channelized CO2 influx has been identified. CO2 has been extracted fro m fluid inclusions in quartz, garnet, and cordierite hy a stepped-heat ing technique, and its abundance and carbon-isotope values determined in each case. Although unimodal release patterns generally characteriz e fluids released from quartz and garnet, for cordierite a strongly bi modal release profile is observed. At 500-700-degrees-C volatiles are extracted from fluid inclusions in quartz, cordierite, and garnet, whe reas above 800-degrees-C CO2 release from cordierite is correlated wit h the expulsion of channel volatiles. Isotopic results confirm that ch annel-derived volatiles and fluid inclusions in cordierite retain simi lar deltaC-13 values in samples where cordierite is interpreted to hav e grown during CO2 influx(deltaC-13=-5 to -8 parts per thousand), wher eas pre-influx CO, can be retained in the channels of cordierite which predates the influx event. For cordierite-bearing metapelites that ha ve not been subjected to fluid influx, carbonic inclusions are virtual ly absent from cordierite. The results suggest that CO2 is trapped in cordierite channels during mineral growth, with little isotopic re-equ ilibration during subsequent events. Studies of fluid release from cor dierites can therefore yield important information on the evolution of metamorphic fluids provided that (1) fluid-inclusion release and chan nel-derived volatiles can be distinguished analytically, and (2) the c hronology of cordierite growth and fluid entrapment in both channels a nd inclusions can be constrained.