HIDDEN SUBDUCTION FLUID RECORD FROM FRANCISCAN ECLOGITE: AN INTEGRATED OXYGEN ISOTOPE AND TRACE ELEMENT STUDY OF RUTILE AND SPHENE

The garnet-bearing tectonic blocks of blueschist, eclogite, and hornblendite hosted by the lower-grade Franciscan Formation (California, U.S.A) commonly have evidence of a blueschist-facies overprint in the form of texturally late chlorite, glaucophane and sphene (titanite) rims on rutile. Many blocks preserve evidence of fluid metasomatism in the form of oxygen isotope disequilibrium at the mineral grain scale and oxygen isotope zoning within garnet. To assess the relative timing of fluid metasomatism and tie it to a P-T history, we analyzed the oxygen isotope (by ion microprobe) and trace element (by laser ablation ICPMS) composition of rutile with sphene rims, in samples with garnets zoned in ẟ¹⁸O.

Zirconium concentrations in rutile yield temperatures of ~600˚C for eclogite from three localities (Junction School, Tiburon, Ward Creek). Rutile trace element concentrations are low and consistent with a mafic protolith. Sphene surrounding rutile inherited much of its trace element content from pre-existing rutile, and Zr-in-sphene temperatures are spuriously high. Oxygen isotope ratios from rutile and sphene compound grains that formed at different times and temperatures are fortuitously indistinguishable at the 2S.D.-level. This similarity in ẟ¹⁸O indicates disequilibrium between these minerals. However, both rutile and sphene could be in equilibrium with the same oxygen reservoir if sphene formed at much lower temperatures, consistent with both existing thermobarometry and disequilibrium trace element compositions.

Blocks from the Franciscan contain garnets with a diversity of cation and oxygen isotope zoning patterns, indicating that blocks did not undergo a uniform P-T-t-fluid history. Rutile in blocks with garnets that are zoned in oxygen isotopes are in equilibrium with the rims rather than the cores. Slow oxygen diffusion in rutile and the low temperatures of formation require that rutile crystallized (or recrystallized) after metasomatism and before blueschist-facies overprinting. Differing garnet zoning patterns suggest than metasomatism and subsequent rutile (re)crystallization took place during prograde subduction metamorphism for some samples, but also in a second episode of burial and heating for at least one sample.