Eclogite-facies fluid infiltration: constraints from dO zoning in garnet

In situ analysis reveals that eclogite-facies garnets are zoned in dO with lower values in the core and rims that are *1.5 to 2.5 % higher. This pattern is present in 9 out of 12 garnets analyzed by SIMS from four orogenic eclogite terranes, and correlates with an increase in the mole fraction of pyrope and Mg/Fe ratio from core to rim, indicating prograde garnet growth. At the maximum temperatures and the time-scales experienced by these garnets, calculated intragranular diffusion distances for oxygen are small (\5 lm), indicating that dO records primary growth zoning and not diffusive exchange. The oxygen isotope gradients are larger than could form due to temperature changes during closed-system mineral growth. Thus, gradients reflect the compositions of fluids infiltrating during prograde metamorphism. Values of dO in garnet cores range from -1 to 15 %, likely preserving the composition of the eclogite protoliths. Two garnet cores from the Almenningen eclogite in the Western Gneiss Region, Norway, have dO *-1 % and are the first negative dO eclogites identified in the region. In contrast with orogenic eclogites, seven high dO garnets ([5 %) from two kimberlites are homogeneous in dO, possibly due to diffusive exchange, which is possible for prolonged periods at higher mantle temperatures. Homogeneity of dO in garnets outside the normal mantle range (5–6 %) may be common in kimberlitic samples.