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Nitrogen isotope fractionation, Nitrogen cycling, Crustal anatexis, Partial melting, Migmatites
Abstract:
We investigated nitrogen (N) partitioning and isotope fractionation during crustal anatexis, in three metapelitic migmatite suites: the Anatectic Complex of Toledo and Tormes Dome Anatectic Region in Spain, and the Adamello Pluton Contact Aureole (APCA) in Italy. Restites have N concentrations higher than those in corresponding leucosomes and pegmatites, related to higher modal abundances of biotite in restitic domains. In general, restites and migmatites (whole-rocks consisting of both leucosome and restite) have higher δ15N values, by up to 5.3 ‰, than the leucosomes and pegmatites. During partial melting, the mobility and isotope fractionation of N are strongly influenced by the stability and breakdown of muscovite and biotite. Differences in the δ15N of restites and leucosomes are inconsistent with the expectation of minimal fractionation at high temperatures among NH4+-bearing micas and feldspars in equilibrium in closed-systems. These differences may instead reflect, open-system behavior during melting and conceivably also during partial retrograde reactions (back-reaction) between partial melts and restites. Possible additional evidence for open-system behavior during melting exists for the APCA, where shifts to lower δ15N values resulting from a series of up-grade melting reactions contrast with the shifts to higher values expected for Rayleigh N loss involving N2, NH3, and NH4+. In general, the partial melts did not inherit the isotope compositions of their source rocks, but seem to record variably lower δ15N values, challenging the assumption that granitic melts directly reflect the isotope composition of their metasedimentary sources. The results of our study of these metapelitic migmatites indicate great compatibility of N in residues of partial melting, largely in biotite, and thus implicate highly devolatilized and partially melted sedimentary rocks as important sites of long-term N storage at mid- to deep-levels of the continental crust.
