EXTREME OXYGEN ISOTOPE ZONING IN METASEDIMENTARY GARNET FROM CATALINA ISLAND, CALIFORNIA REVEALS PROGRESSIVE METASOMATISM DURING SUBDUCTION

The Catalina Schist subduction complex is well known for pervasive fluid-rock interaction. A narrow range in the oxygen isotope composition of quartz at the km scale was the first indication of massive fluid flow in the area (Bebout and Barton, 1989, Geology). High-grade mélange units on Catalina contain rare blocks of garnet quartzite (likely remnants of the subducting slab’s sedimentary veneer) in addition to more common mafic blocks. Quartz from these blocks preserves similar bulk δ¹⁸O (13.2-14.7‰, VSMOW) to published data, consistent with resetting by regional fluids, but bulk analysis of high-δ¹⁸O garnets has revealed disequilibrium in most samples. New in situ analysis of garnet from these blocks by ion microprobe reveals a complementary record of relict high-δ¹⁸O cores and lower δ¹⁸O rims in equilibrium with quartz.

The δ¹⁸O of garnet from quartzite was measured by ion microprobe for 4 blocks in mélange and 2 coherent outcrops. Garnets are small (<1mm) and record a range in oxygen isotope zoning, but are homogenous in cations. Grains from sample 05C-09 record the greatest zoning, with δ¹⁸O = 25.6‰ in cores and 10.5‰ in rims, while sample 17B1 records the smallest, with 11.1‰ cores and 9.3‰ rims. Overall, core values are high and variable in δ¹⁸O, from 11-25‰, whereas rims record lower and less variable values of 9.3-11.2‰. Garnet cores likely preserve the variable and elevated δ¹⁸O values of the sedimentary protolith, and subsequent changes from core to rim are due to progressive metasomatism by subduction fluids.

The garnet record of these fluids differs from the quartz record because of garnet’s slow intracrystalline oxygen diffusion rates and resistance to recrystallization. Preliminary ion probe analyses of quartz are similar to previous bulk analyses suggesting it fully equilibrated with the final fluid composition. All mélange garnets record δ¹⁸O in their rims in equilibrium with matrix quartz at published T. Coherent garnet quartzite samples present a different record, with higher matrix quartz δ¹⁸O (15.1-17.7‰) and more variable cation zoning. The differences can be explained by a less intense fluid flux through coherent samples. These zoned garnets provide some of the first evidence of evolving fluid conditions in Catalina subduction zone metamorphic rocks.