| Nitrogen isotopes can be used to trace organic components into and within the solid Earth. Study of the processes of this incorporation and redistribution is essential to understanding long-term Earth degassing and atmosphere evolution. In this study, the ability of N isotopes to trace pathways of organic components into the crust was assessed, focusing on the extent to which N is retained through partial melting reactions experienced in the mid-continental crust. The Mt. Stafford region, central Australia, provides well-exposed metasedimentary rocks of greenschist to granulite facies (metamorphosed at 1.8 Ga), with the higher-grade rocks showing evidence of partial melting at 2-4 kbars, 650-850°C. These rocks, divided into zones based on grade and extents of partial melting, were analyzed for N contents and delta15N using carrier-gas mass spectrometry methods.;delta15Nair increases from +2.4 to +6.2‰ for greenschist facies rocks to about +1.7 to +9‰ for granulite facies rocks, with a subtle trend of decreasing N concentration with increasing grade (50-160 ppm at greenschist facies; 20-180 ppm at the higher grades). Retention of N as a function of grade, perhaps as NH4+ in K-feldspar or as N2 in the channels of cordierite, may indicate that N is held in peritectic products of partial melting rather than in the melt. Analysis of the whole-rock major and trace element geochemistry of these samples demonstrates that N behavior is most similar to the behavior of K, Rb, Cs, Li, B, and U which show whole-rock losses with increasing grade and less similar to Ba and Th, which increase in concentration at high grades. The rocks also contain 0.1 to 0.2 wt % reduced C, likely as graphite. delta 13CV-PDB values are relatively uniform, and have organic compositions, at greenschist facies (mean = -25.3‰; 1sigma = 1.6‰; n = 6), but increase to values as high as -15.4‰ at higher-grades (overall range of -28.3 to -15.4‰). These data have implications for deep-Earth C-N cycling, bearing on the size of the N reservoir in the mid-continental crust, and support the use of the delta15N of upper-crustal intrusive rocks as a tracer of their deep crustal sources. |
