Nitrogen partitioning and isotope fractionation during crustal Anatexis: New constraints from across-grade traverses and centimeter-scale Leucosome-Restite Pairs

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 δ 15 N 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 δ 15 N of restites and leucosomes are inconsistent with the expectation of minimal fractionation at high temperatures among NH 4 + -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 δ 15 N values resulting from a series of up-grade melting reactions contrast with the shifts to higher values expected for Rayleigh N loss involving N 2 , NH 3 , and NH 4 + . In general, the partial melts did not inherit the isotope compositions of their source rocks, but seem to record variably lower δ 15 N 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 continental crust.