ISOTOPIC DISEQUILIBRIUM AND A GARNET RECORD OF SUBDUCTION METASOMATISM IN METACHERT FROM CATALINA ISLAND, CALIFORNIA

Sedimentary veneers on subducting plates are important contributors to mélange zones in subduction systems, but detailed work on fluid flow through these lithologies is lacking. Metacherts from the Catalina Schist subduction complex (CA) provide opportunities to study these interactions.

Garnet quartzite on Catalina Island is found as blocks in mélange or more rarely in coherent units, and is predominantly quartz (55-80%) layered with garnet, chlorite, mica (ms ± bt ± stp), and rutile. Some samples have minor (<5%) apatite, amphibole, pyrite, and zircon. Garnet and quartz were analyzed for δ¹⁸O by laser fluorination from 11 blocks in mélange and 2 coherent outcrops from the Catalina amphibolite unit.

Quartz from mélange blocks is relatively homogeneous in δ¹⁸O (13.2‰-14.7‰ VSMOW), consistent with previous work demonstrating wide-scale isotopic homogenization by fluids. δ¹⁸O values of garnet (homogeneous in cations, Alm₅₀Pyp₂₉Sps₁₀Grs₁₁) are more varied (10.5‰-16.0‰). Most garnet-quartz fractionations are reversed, and all represent disequilibrium except one sample that yields 685˚C, which is consistent with existing temperature estimates, but may be fortuitous. The samples from coherent layers have higher δ¹⁸O quartz (16.4,16.9‰) disequilibrium garnet-quartz fractionations (δ¹⁸O grt = 15.1,17.7‰), the garnets are Mn rich & Mg poor relative to mélange samples and show strong cation zoning (Alm_41-44Pyp_12-13Sps_30-34Grs_16-9).

The likely cause for the observed disequilibrium is variable isotopic exchange with fluids migrating up the subduction channel. Quartz recrystallizing in high fluid/rock ratios would homogenize to δ¹⁸O values in equilibrium with the fluid. Garnet, which is refractory to recrystallization and has slow oxygen diffusion rates, preserves earlier, higher δ¹⁸O signatures in their cores while rims record later growth with δ¹⁸O in equilibrium with the fluid. Preliminary ion microprobe work on one sample records δ¹⁸O = 25‰ cores, 18‰ mantles and 10‰ rims, suggesting that variable bulk δ¹⁸O of garnet represents variable zoning histories. Lower permeability of coherent layers may have resulted in lower fluid/rock ratios explaining differing fluid histories. A detailed study of isotopic zoning in metasedimentary garnets will provide a fuller picture of fluid flow in subduction zones.