Storage and mobility of organic nitrogen and carbon in the continental crust: Evidence from partially melted metasedimentary rocks, Mt. Stafford, Australia.

摘要

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.