The Fe-C-O-H-N system at 6.3-7.8 GPa and 1200-1400 degrees C: implications for deep carbon and nitrogen cycles

摘要

Interactions in a Fe-C-O-H-N system that controls the mobility of siderophile nitrogen and carbon in the Fe-0-saturated upper mantle are investigated in experiments at 6.3-7.8 GPa and 1200-1400 A degrees C. The results show that the gamma-Fe and metal melt phases equilibrated with the fluid in a system unsaturated with carbon and nitrogen are stable at 1300 A degrees C. The interactions of Fe3C with an N-rich fluid in a graphite-saturated system produce the epsilon-Fe3N phase (space group P6(3)/mmc or P6(3)22) at subsolidus conditions of 1200-1300 A degrees C, while N-rich melts form at 1400 A degrees C. At IW- and MMO-buffered hydrogen fugacity (fH(2)), fluids vary from NH3- to H2O-rich compositions (NH3/N-2 1 in all cases) with relatively high contents of alkanes. The fluid derived from N-poor samples contains less H2O and more carbon which mainly reside in oxygenated hydrocarbons, i.e., alcohols and esters at MMO-buffered fH(2) and carboxylic acids at unbuffered fH(2) conditions. In unbuffered conditions, N-2 is the principal nitrogen host (NH3/N-2 aecurrency 0.1) in the fluid equilibrated with the metal phase. Relatively C- and N-rich fluids in equilibrium with the metal phase (gamma-Fe, melt, or Fe3N) are stable at the upper mantle pressures and temperatures. According to our estimates, the metal/fluid partition coefficient of nitrogen is higher than that of carbon. Thus, nitrogen has a greater affinity for iron than carbon. The general inference is that reduced fluids can successfully transport volatiles from the metal-saturated mantle to metal-free shallow mantle domains. However, nitrogen has a higher affinity for iron and selectively accumulates in the metal phase, while highly mobile carbon resides in the fluid phase. This may be a controlling mechanism of the deep carbon and nitrogen cycles.