SECONDARY CO2 INCLUSIONS IN GORE MOUNTAIN GARNET, NORTH CREEK, NY

Garnet from the Barton Mine, Gore Mountain, central Adirondacks, NY contains secondary CO₂fluid inclusions along healed conchoidal fractures. The inclusions are several 10’s to 100’s of micrometers in length and are highly irregular in shape. Some inclusions contain a birefringent solid that may be a daughter phase.

We obtained microthermometric data on forty-six inclusions. At room temperature, all of the inclusions are single phase and undergo phase separation upon cooling. Thirty-four out of forty-six inclusions had CO₂ triple-point temperatures of -56.6 ± 0.1°C. The remaining twelve occurred at -56.4°C. All forty-six inclusions homogenized to the liquid phase at temperatures between -15.1 and -1.1°C, but forty measurements (87%) occurred between -12.3 and -8.5°C, with a median of -10.6°C. Therefore, 87% of the inclusions have densities between 0.995 and 0.975 gm/cm³.

Isochores for the forty CO₂ inclusions plot in a narrow range between 5183 and 5463 bars at 700°C. These pressure conditions are less than those proposed for garnet formation and suggest that CO₂ could have been trapped along an initial clockwise retrograde path, as suggested by McLelland and Selleck (2011; Geosphere) assuming the trapping of pure CO₂. However, if the inclusions were originally trapped as a miscible H₂O+CO₂ phase, and lost H₂O through diffusion at high temperature, the bulk densities must have been higher and could have formed on a counter-clockwise path similar to that proposed by Spear and Markussen (1997; Jour. of Petrology), Lamb et al., (1991; CMP), and Darling and Bassett (2002; Am. Min.).