THE SIGNIFICANCE OF CHANNEL AND FLUID-INCLUSION CO2 IN CORDIERITE: EVIDENCE FROM CARBON ISOTOPES

Abstract

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 fluidinclusion 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 the expulsion of channel volatiles. Isotopic results confirm that channel-derived volatiles and fluid inclusions in cordierite retain similar ?l3C values in samples where cordierite is interpreted to have grown during CO2 influx (?13C= ?5 to ?8%), whereas pre-influx CO2 can be retained in the channels of cordierite which predates the influx event. For cordierite-bearing metapelites that have not been subjected to fluid influx, carbonic inclusions are virtually absent from cordierite. The results suggest that CO2 is trapped in cordierite channels during mineral growth, with little isotopic re-equilibration during subsequent events. Studies of fluid release from cordierites can therefore yield important information on the evolution of metamorphic fluids provided that (1) fluidinclusion release and channel-derived volatiles can be distinguished analytically, and (2) the chronology of cordierite growth and fluid entrapment in both channels and inclusions can be constrained.