Eclogite-facies fluid infiltration: constraints from delta O-18 zoning in garnet

In situ analysis reveals that eclogite-facies garnets are zoned in delta O-18 with lower values in the core and rims that are similar to 1.5 to 2.5 parts per thousand 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 mu m), indicating that delta O-18 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 delta O-18 in garnet cores range from -1 to 15 parts per thousand, likely preserving the composition of the eclogite protoliths. Two garnet cores from the Almenningen eclogite in the Western Gneiss Region, Norway, have delta O-18 similar to-1 parts per thousand and are the first negative delta O-18 eclogites identified in the region. In contrast with orogenic eclogites, seven high delta O-18 garnets (> 5 parts per thousand) from two kimberlites are homogeneous in delta O-18, possibly due to diffusive exchange, which is possible for prolonged periods at higher mantle temperatures. Homogeneity of delta O-18 in garnets outside the normal mantle range (5-6 parts per thousand) may be common in kimberlitic samples.