TRACE ELEMENT DIFFUSION IN HIGH GRADE METAMORPHIC GARNETS

Garnet crystals are often used to estimate pressure and temperature conditions of metamorphic and deformation events that the host rock has undergone throughout time, and at what point these events occurred. Particularly, this is achieved by examining the geochemistry within the garnet crystal as well as neighboring mineral phases. Under greenschist to upper amphibolite facies the major elements, such as Mg, Fe, Mn, and Ca, diffuse throughout the crystal and are therefore likely to be homogenous (e.g. Carlson, 2002; Chernoff and Carlson, 1997 and 1999; Spear and Daniel, 2001). Alternatively, it is thought that trace elements such as heavy rare earth elements (HREE), Cr, and Y found in garnet crystals do not diffuse under most pressure-temperature conditions and therefore do not re-equilibrate. This difference in the rates of movement in the garnet crystal can create or eliminate chemical zonation, a primary method used to decipher metamorphic pressure-temperature-time pathways. However, the ability of trace elements to preserve their original compositions is uncertain at high grade, even more so when anataxis is involved. The Central Maine Belt (CMB) formed 408 Ma to 404 Ma in the early Devonian, during the Acadian orogeny. Turbidites deposited during the Silurian and early Devonian were metamorphosed under greenschist to upper amphibolite facies at peak conditions of 700-830 °C and 4-4.5 kbar (Solar and Brown, 2001), forming metapelites. Synchronously, partial melting (anataxis) of the metapelites occurred, producing migmatites and emplacing granitic plutons. Samples from the Weld and Tumbledown Anatectic Domains were collected for this study to determine whether trace elements can in fact be re-equilibrated within garnets under upper amphibolite facies that have also undergone partial melting. When using trace elements to extrapolate pressure-temperature conditions of metamorphic and deformation events, it is essential to first determine if the elements in question represent the original composition or have re-equilibrated. Major and trace elements in the garnet crystals were analyzed using an electron microprobe to measure and map the chemical composition.