Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.2839b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 7, pp.2839-2849
Other
30
Scientific paper
In the Catalina Schist subduction-zone metamorphic complex (California), metasedimentary rocks show a decrease in N concentration and an increase in 15 N air with increasing metamorphic grade. Lowest-grade lawsonite-albite rocks contain 632 ± 185 ppm N with 15 N = +1.9 ± 0.6 , whereas highgrade amphibolite equivalents contain 138 ± 76 ppm N with 15 N = +4.3 ± 0.8 . Loss of N accompanied devolatilization reactions that evolved H 2 O-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 ( 15 N = 15 N fluid - 15 N rock ) of -1.5 ± 1 were calculated with the Rayleigh distillation equation, taking into account variability in rock composition by comparison of samples with similar K 2 O concentrations. These fractionations are similar to but slightly lower than published calculated fractionations for N 2 -NH 4 + 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 N 2 -NH 4 + exchange and a devolatilization process intermediate in behavior to batch volatilization and Rayleigh distillation. The observed shifts in N concentration and 15 N cannot be explained by NH 3 -NH 4 + 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 15 N ( ~ -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.
Bebout Gray E.
Fogel Marilyn L.
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