FLUIDS ACCOMPANYING TOURMALINE GROWTH WITHIN EARLY PALEOZOIC METASEDIMENTS OF THE MARYLAND PIEDMONT: THE FLUID INCLUSION EVIDENCE

Recent investigations of tourmaline geochemistry in rocks of the Maryland Piedmont have demonstrated several generations of tourmaline with variable compositions that depend in part on the host rock. Those in the Cockeysville Marble are dominantly magnesian (dravite and uvite), while those associated with the Setters Formation (schists and quartzites) are more iron-rich (schorl and feruvite) (Watt et al., 2003). The tourmalines contain sparse to abundant fluid inclusions, so the present study represents the first investigation of fluid inclusions in tourmalines from Maryland in an attempt to categorize and interpret the compositions and P-T conditions of fluids accompanying the tourmaline growth.

Cockeysville Marble tourmaline is relatively clear and has only sparse fluid inclusions, many of which contain three fluid phases at room temperature. In contrast, tourmaline from pegmatitic veins through the Setters Formation may contain, especially in their cores, abundant fluid inclusions and associated quartz. Three types of fluid inclusion have been recognized. Type I inclusions contain three fluid phases at room temperature. Warming on the microscope stage simply by removing the infrared filter results in homogenization of the two inner phases, showing that these inclusions initially contain liquid water, liquid CO₂, and CO₂ vapor. Type II inclusions consist of what appears to be a clear solid, possibly isotropic, and a small (<1 vol. %) shrinkage bubble. These are tentatively identified as frozen melt (glass) inclusions. The third common inclusion type (Type III) contains one or more crystalline solids plus a volatile phase, with the proportions highly variable. These are tentatively attributed to mixtures of trapped melt, now devitrified, and volatiles. A working hypothesis for the inclusions in pegmatite tourmaline involves the coexistence of silicate melt and an immiscible H₂O-CO₂ volatile phase. Trapping of either phase or both phases in combination might explain the whole assemblage of inclusions. In contrast, tourmaline in the Cockeysville Marble formed in the presence of a homogeneous volatile phase. Microthermometric data collection and detailed petrographic studies are in progress in order to obtain P-T constraints and compositional information on the fluids.