In the Catalina Schist subduction-zone metamorphic complex (California), metasedimentary rocks show a decrease in N concentration and an increase in δ15Nair with increasing metamorphic grade. Lowest-grade lawsonite-albite rocks contain 632 ± 185 ppm N with δ15N = +1.9 ± 0.6‰, whereas highgrade amphibolite equivalents contain 138 ± 76 ppm N with δ15N = +4.3 ± 0.8‰. Loss of N accompanied devolatilization reactions that evolved H2O-rich C-O-H-S-N fluids through consumption of chlorite and phengitic white mica and production of mineral assemblages containing muscovite, biotite, garnet and kyanite. Whole-rock N concentrations of up to 200 ppm in veins and pegmatites produced during high- P T metamorphism reflect the redistribution of N during devolatilization and partial melting of the metasedimentary rocks. Bulk fluid-rock N-isotope fractionations (Δ15N = δ15Nfluid - δ15Nrock) of -1.5 ± 1‰ were calculated with the Rayleigh distillation equation, taking into account variability in rock composition by comparison of samples with similar K2O concentrations. These fractionations are similar to but slightly lower than published calculated fractionations for N2-NH4+ exchange at the temperature range of 350-600°C over which most of the devolatilization occurred in the Catalina Schist (approximately -3.4 to -2.25‰). The N systematics appear to be explained by N2-NH4+ exchange and a devolatilization process intermediate in behavior to batch volatilization and Rayleigh distillation. The observed shifts in N concentration and δ15N cannot be explained by NH3-NH4+ exchange at these temperatures using the equilibrium models. The distillation devolatilization process implicated in this study may govern the behavior of other trace elements partitioned into hydrous fluids during devolatilization (e.g., B, Cs, U). Similarity of the calculated fluid δ15N( ~ -1.5 to +5.5‰) with compositions of natural gases inferred to be derived from metasedimentary sources indicates the possibility of using N as a tracer of large-scale volatile transport.
ASJC Scopus subject areas
- Geochemistry and Petrology