Behavior and cycling of volatile elements in the crust and mantle: Evidence from field-based studies of nitrogen-isotope geochemistry and high-P/T low-grade metamorphism.

摘要

Advances in understanding the geochemical behavior of nitrogen (N) have been limited by the relative difficulty of techniques for the analysis of δ15N in rocks, due to both the relatively low N-concentration in most rocks and the strong possibility of atmospheric contamination. Full and rapid extraction of N from biotite, muscovitic white mica, and NH ₄+-bearing feldspar mineral separates by sealed-tube combustion requires heating to 1200°C. Study of N behavior in regionally metamorphosed rocks from Townshend Dam (Vermont) and Western Maine provides the opportunity to assess intermineral NH₄+ partitioning and N-isotope fractionation among coexisting micas and affords application of the N system as a tracer of fluid-rock interactions. Analyzed mica samples range in δ15N_air from +3.3 to +11.9‰ and contain 9–1820 ppm N. There is no apparent equilibrium N-isotopic fractionation among coexisting micas, and NH₄+ appears to partition systematically among coexisting biotite and white mica (average N_w.mica/N_bt = 0.46). Samples most unlike the mean in both Δ15N_bt-w.mica and N_w.mica/N _bt contain abundant chlorite. Thus, retrograde replacement of biotite by chlorite may have been accompanied by fluid-mineral N-exchange.; High-P/T metamorphism of the Franciscan Complex represents subduction-zone processes at depth in an evolving accretionary prism. Local-scale isotopic buffering of CaCO₃ veins by adjacent host rocks obscures assessment of the contributions of fluid sources (most vein δ13C = −11.0 to −3.0‰, δ18O = +12.0 to +18.5‰). Some veins with elevated δ18O could reflect up-dip flow of H₂O released by devolatilization at greater depths. Calculated fluid-δ18O for lower-T veins spans the range of values for low-salinity fluids venting in active accretionary prisms. Metasedimentary rocks from the Franciscan and the Western Baja Terrane (Mexico) retain abundant C_reduced (up to 1.3%), N (102–891 ppm) and other fluid-mobile trace elements (B, As, Cs, Rb, Sb, U, Pb). δ13C _PDB ranges from −28.8 to −21.9‰, with some increased variability in the more highly metamorphosed rocks of the WBT. δ 15N_air ranges from +0.2 to +3.0‰, values that are similar to those expected for organic sediments. There is some mobility of N and B during early metamorphism, but these rocks record retention of protolith-like major and trace element compositions despite subduction-zone metamorphism.