Record of forearc devolatilization in low‐T, high‐P/T metasedimentary suites: Significance for models of convergent margin chemical cycling

The Franciscan Complex (Coast Ranges and Diablo Range, California) and the Western Baja Terrane (WBT; Baja California, Mexico) were metamorphosed along high‐P/T paths like those experienced in many active subduction zones, recording peak conditions up to ∼1 GPa and 300°C. Franciscan and WBT metasedimentary rocks are similar in lithology and geochemistry to clastic sediments outboard of many subduction zones. These metamorphic suites provide evidence regarding devolatilization history experienced by subducting sediments, information that is needed to mass‐balance the inputs of materials into subduction zones with their respective outputs. Analyzed samples have lower total volatile contents than their likely protoliths. Little variation in LOI among similar lithologies at differing metamorphic grades, suggests that loss of structurally bound water occurred during early clay‐mineral transformations. Finely disseminated carbonate is present in the lowest‐grade rocks, but absent in all higher‐grade rocks. δ 13 C VPDB of reduced‐C is uniform in the lower‐grade Franciscan samples (mean = −25.1‰, 1σ = 0.4‰), but varies in higher‐grade rocks (−28.8 to −21.9‰). This likely reflects a combination of devolatilization and C‐isotope exchange, between organic and carbonate reservoirs. Nitrogen concentration ranges from 102 to 891 ppm, with δ 15 N air of +0.1 to +3.0‰ (n = 35); this organic‐like δ 15 N probably represents an efficient transfer of N from decaying organic matter to reacting clay minerals. The lowest‐grade rocks in the Coastal Belt have elevated carbonate contents and correlated N‐δ 15 N variations, and exhibit the most uniform δ 13 C and C/N, all consistent with these rocks having experienced less devolatilization. Most fluid‐mobile trace elements are present at concentrations indistinguishable from protoliths. Suggesting that, despite apparent loss of much clay‐bound H 2 O and CO 2 from diagenetic cements (combined, <5–10 wt. %), most fluid‐mobile trace elements are retained to depths of up to ∼40 km. Organic‐like δ 15 N, lower than that of many seafloor sediments, is consistent with some loss of adsorbed N (perhaps as NO 3 − ) during early stages of diagenesis. The efficient entrainment of fluid‐mobile elements to depths of at least 40 km in these relatively cool subduction zone settings lends credence to models invoking transfer of these elements to the subarc mantle.