CARBON ISOTOPE INVESTIGATION OF CHANNEL CARBON DIOXIDE IN RING SILICATES: CORDIERITE AND BERYL

Cyclosilicates are known to have the capacity to contain molecular water and carbon dioxide in their structural channels. In cordierite, channel volatile compositions have been used as a way to monitor metamorphic fluid compositions, and in beryl to understand gemstone formation. Carbon isotopes of channel carbon dioxide have for the most part not been analyzed except for a few studies of metamorphic cordierite. In this study we measured carbon isotopes of channel carbon dioxide to investigate the origin of carbon in metamorphic protoliths and igneous source materials. In Australian S-type granites δ¹³C(Crd)= -27 to -25‰, pointing to sedimentary organic carbon in the igneous source rocks. This is similar to the average δ¹³C(Crd)= -26‰ in metapelites (e.g. Vry et al., Am Min 1990). Cordierite-gedrite rocks have a distinctly different δ¹³C(Crd)= -13 to -9‰, which is consistent with seafloor oxidized organic carbon in the altered volcanic protoliths of these rocks.

Data from beryl are more preliminary. Step-heating experiments in beryl reveal the presence of a low-T reservoir with low δ¹³C (ca. -45‰) and a high-T reservoir that, with analogy to IR studies of cordierite, likely is the channel carbon dioxide. The δ¹³C of the high-T reservoir ranges from -26 to -6‰ (n= 22) with clusters around ca. -21‰ and -13‰ and an outlier with δ¹³C= 2.3‰, and does not clearly correlate with rock type or gemstone classification. Future work will focus on understanding the carbon isotope systematics of beryl and exploring other minerals with structures capable of holding molecular carbon dioxide, such as the feldspathoids.