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