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

Abstract

Nitrogen isotope data for metasomatized rocks, veins, and pegmatites in the Catalina Schist subduction zone metamorphic complex allow futher characterization of complex, high-P/T metasomatic proceses 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), δ 15N 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 δ 15N in metasomatized rocks relative to the δ 15N of metasedimentary rocks in the same unit implies mixing of N from nearby, heterogeneous metasedimentary sources, perhaps producing fluids with unifrom δ 15N at up to the kilometer scale. However, the trend in δ 15N of metasedimentary sources, with increasing metamorphic grade is inconsistent with larger scale up-temperature transfer of fluid (in this case, N 2-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 δ 15N 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.