A new chronology for graphite formation related to subduction-zone metamorphism in Laytonville Quarry (California, USA) of the Franciscan Complex using the Re-Os decay system yields a precise graphite-pyrite Re-Os age of 161.39 ± 2.08 Ma (MSWD = 0.15; n = 5). Combined with Raman thermometry estimates (461−506 °C), this age links graphitization to the early stages of Franciscan subduction, synchronous with deep (blueschist-eclogite) rather than shallow subduction processes. Initial 187Os/188Os (0.195) of Laytonville graphite suggests that radiogenic 187Os/188Os signatures (0.130−0.158) preserved in some Franciscan forearc mantle peridotites (ca. 160−150 Ma) were plausibly sourced from graphitic carbon in the subducted slab, with graphitization being the primary mechanism for Os release. This conclusion implies that graphitic carbon is an important mineralogical host for Os recycling in subduction settings.

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