Oxygen isotope speedometry in granulite facies garnet recording fluid/melt‐rock interaction (Sør Rondane Mountains, East Antarctica)

In situ analysis of a garnet porphyroblast from a granulite facies gneiss from Sor Rondane Mountains, East Antarctica, reveals discontinuous step-wise zoning in phosphorus and large delta O-18 variations from the phosphorus-rich core to the phosphorus-poor rim. The gradually decreasing profile of oxygen isotope from the core (delta O-18 = similar to 15 parts per thousand) to the rim (delta O-18 = similar to 11 parts per thousand) suggests that the O-18/O-16 zoning was originally step-wise, and modified by diffusion after the garnet rim formation at similar to 800 degrees C and 0.8 GPa. Fitting of the O-18/O-16 data to the diffusion equation constrains a duration of the high-T event (similar to 800 degrees C) to c. 0.5-40 Ma after the garnet rim formation. The low delta O-18 value of the garnet rim, together with the previously reported low delta O-18 values in metacarbonates, indicates regional infiltration, probably along a detachment fault, of low delta O-18 fluid/melt possibly derived from meta-mafic to ultramafic rocks.