Nitrogen isotope tracers of high-temperature fluid-rock interactions: Case study of the Catalina Schist, California

Nitrogen isotope data for metasomatized rocks, veins, and pegmatites in the Catalina Schist subduction zone metamorphic complex allow further characterization of complex, high-P/T metasomatic processes and evaluation of the scales of isotopic equilibration and fluid transport during subduction-zone metamorphism. Throughout the Catalina Schist, N resides predominantly as NH⁺₄ in white mica, which occurs in nearly all bulk compositions (i.e., metasedimentary, metamafic and, to a lesser extent, metaultramafic mélange) at all grades. Within each metamorphic unit of the Catalina Schist (ranging in grade from lawsonite-albite to amphibolite facies), δ¹⁵N values of mica in metasomatized metamafic and metaultramafic rocks are consistent with the metasomatic addition of N from nearby, devolatilizing metasedimentary rocks into the initially N-poor mafic and ultramafic rocks. Within each unit, uniformity of mica δ¹⁵N in metasomatized rocks relative to the δ¹⁵N of metasedimentary rocks in the same unit implies mixing of N from nearby, heterogeneous metasedimentary sources, perhaps producing fluids with uniform δ¹⁵N at up to the kilometer scale. However, the trend in δ¹⁵N of metasomatic mica with increasing metamorphic grade is inconsistent with larger scale up-temperature transfer of fluid (in this case, N₂-bearing) in the Catalina Schist paleosubduction zone; such flow (at scales of up to tens of kilometers) has been inferred through previous oxygen isotope study. Nitrogen isotope compositions are instead believed to have been controlled at a more local scale than the O isotope systematics, due to the more rock-dominated fluid-rock mass balance for N. The δ¹⁵N of muscovite in leucosomes and pegmatites in amphibolite-grade metasedimentary exposures matches that of muscovite in metasedimentary hosts, implying minimal N-isotope fractionation during migmatization processes and possible transfer of metasedimentary N-isotope signatures in silicate melts. These results illustrate the potential of the N-isotope system to yield valuable information regarding fluid-rock interactions in the crust and mantle. The data for the Catalina Schist demonstrate the ability of N isotopes to trace the transfer of sediment-derived C-O-H-S-N fluids and silicate melts, and show the expected benefit of the N-isotope system in having a differing fluid-rock mass balance, relative to the more commonly used stable isotope systems, that can yield unique constraints in quantitative models of crustal fluid processes.