Nitrogen loss and isotopic fractionation during granulite-facies metamorphism in the lower crust (Ivrea Zone, NW Italy)

Abstract

The Ivrea Zone, exposed in the Southern Alps, affords an examination of nitrogen mobilization and storage in deep continental crust, in rocks spanning the amphibolite to granulite facies transition and that experienced multiple devolatilization reactions and partial melting. Such information is important for considerations of whole-Earth N cycling, as continental crust is thought to contain up to ~15% of global N, much of it organic in origin and thus tied to ancient-Earth biogeochemical processes.In these rocks, N is redistributed among silicate phases during devolatilization and both H2O-saturated and dehydration melting, showing particular affinity for potassic phases such as biotite, muscovite and alkali feldspar. In metasedimentary rocks, N concentrations decrease strongly with increasing grade, from the Kinzigite Zone (amphibolite facies) with 29 to 181 μg/g to the Stronalite Zone (granulite facies) containing only 10 to 15 μg/g. The decrease in N is intermediate in magnitude to that of Cs, showing somewhat greater loss, and that of Rb that shows smaller degrees of loss. The loss of N with increasing grade is due to breakdown of biotite, which is the main mineral host of N. It also reflects a strong preference of N for the fluid during dehydration and a moderate preference of N for the melt during partial melting. Accompanying this loss of N is a shift by up to 9‰ in δ15N towards lower values. The direction of this shift contrasts with what would be expected for loss of N into fluids speciated as either N2 or NH3 and likely reflects complex isotope fractionation effects during equilibration between melt, fluid, and rock, perhaps during evolving redox conditions. The lowest δ15N values for metabasaltic rocks in this suite are near that of the mantle (−5‰) and the array of values across the traverse could represent initial protolith heterogeneities and superimposed effects of magmatic degassing of the protolith. The leucosomes broadly overlap in N concentrations and δ15N values with their host rocks and lack any systematic relationships with metamorphic grade or type of host lithology.As has been noted in previous work, estimation of the size and isotopic composition of the N reservoir in continental crust is complicated its great lithologic heterogeneity and by the paucity of data for metamorphic rocks representing greater depths. Superimposed on the lithologic heterogeneity associated with varying protoliths is the ability of highly devolatilized and partially melted deep crustal rocks to store N over long time periods. We used our new data of deep crustal rocks from the Ivrea Zone, combined with published N data and based on lithological proportions in the crust, to provide a first estimate for the N content in the middle continental crust of 56 ± 4 μg/g. For the lower continental crust, we estimate a N content of 26 ± 4 μg/g N but note that the data base of granulite-facies samples is still small. Calculation of the total continental crust N content yields 74 ± 4 μg/g, which is in between previous estimates of 56, 60 and 88 μg/g N. Finally, we estimate the N isotopic composition of the bulk continental crust as δ15N = 4.3 ± 0.5.