A tectonic block of garnet quartzite in the amphibolite-facies melange of the Catalina Schist (Santa Catalina Island, California, USA) records the metasomatic pre-treatment of high-delta O-18 sediments as they enter the subduction zone. The block is primarily quartz, but contains two generations of garnet that record extreme oxygen isotope disequilibrium and inverse fractionations between garnet cores and matrix quartz. Rare millimeter-scale garnet crystals record prograde cation zoning patterns, whereas more abundant similar to 200-mu m-diameter crystals have the same composition as rims on the larger garnets. Garnets of both generations have high-delta O-18 cores (20.8 parts per thousand-26.3 parts per thousand, Vienna standard mean ocean water) that require an unusually high-delta O-18 protolith and lower-delta O-18, less variable rims (10.0 parts per thousand-11.2 parts per thousand). Matrix quartz values are homogeneous (13.6 parts per thousand). Zircon crystals contain detrital cores (delta O-18 = 4.7 parts per thousand-8.5 parts per thousand, 124.6 + 1.4/-2.9 Ma) with a characteristic igneous trace element composition likely sourced from arc volcanics, surrounded by zircon with metamorphic age (115.1 +/- 2.5 Ma) and trace element compositions that suggest growth in the presence of garnet. Metamorphic zircon decreases in delta O-18 from near-core (24.1 parts per thousand) to rim (12.4 parts per thousand), in equilibrium with zoned garnets. Collectively, the data document the subduction of a mixed high-delta O-18 siliceous ooze and/or volcanic ash protolith reaching temperatures of 550-625 degrees C prior to the nucleation of small garnets without influence from external fluids. Metasomatism was recorded in rims of both garnet and zircon populations as large volumes of broadly homogeneous subduction fluids stripped matrix quartz of its extremely high oxygen isotope signature. Thus, zoned garnet and zircon in high-delta O-18 subducted sediments offer a detailed window into subduction fluids
