THE REDOX STATE OF THE FRANCISCAN TRENCH

The oxidation state of the subducting slab, its fluids and the mantle wedge are of great interest in the study of arc volcanics, but are difficult to constrain. Garnet-epidote equilibria can be used as oxygen barometers in eclogite and related rocks to address part of this question. Garnets in eclogite and banded eclogite - blueschist blocks from the Franciscan Complex, California, USA, (locations: Tiburon, Ward Creek, Junction School, Jenner) contain inclusions of epidote that formed during subduction and record the redox state of subducting mafic rocks. Epidote inclusions (Cz_70-84 Ps_16-30) and their host garnets (Alm_66-54 Pyp_9-2 Sps_25-4 Grs_27-16) from each locality yield oxygen fugacities within 0.25 log units of the hematite-magnetite buffer between 400 and 600°C. Epidote inclusions from one Ward Creek sample record a fugacity of +3 log units relative to the hematite-magnetite buffer, which may represent a metasedimentary component. These calculated fugacities confirm the oxidized state of subducting oceanic crust.

Garnet-epidote oxygen barometry was also applied to each thin section using matrix epidote (Cz_80-84 Ps_16-20) texturally equilibrated with outermost garnet rims. These formed during exhumation and record fluid interactions in the subduction channel after blocks parted ways with the subducting slab. Garnet rims in these Franciscan blocks display complex and oscillatory zoning patterns in Ca, Fe and Mn that may record fluid or tectonic processes. Garnets in epidote-rich (Cz_85-79 Ps_14-21) patches of each thin section, exhibit less pronounced cation zoning and different rim composition (Alm_63-58 Pyp_9-2 Sps_12-4 Grs_27-23), which differ from garnets in areas richer in garnet (Alm_60-54 Pyp_7-5 Sps_25-9 Grs_26-12) with smaller amounts of epidote. However, calculations with matrix phases in both textures yield redox conditions at the hematite-magnetite buffer, even in the Ward Creek sample with more oxidized prograde conditions, suggesting similar redox conditions in the fluid-rich mélange of the subduction channel to the slab itself.

Oscillatory cation zoning in Franciscan garnets is not controlled exclusively by changes in fluid composition recorded by oxygen isotope ratios or oxygen fugacity. Oscilations in P and T such as those caused by yo-yo subduction may instead be responsible.