TY - JOUR

T1 - The Fe–C–O–H–N system at 6.3–7.8 GPa and 1200–1400 °C

T2 - implications for deep carbon and nitrogen cycles

AU - Sokol, Alexander G.

AU - Tomilenko, Anatoly A.

AU - Bul’bak, Taras A.

AU - Kruk, Alexey N.

AU - Zaikin, Pavel A.

AU - Sokol, Ivan A.

AU - Seryotkin, Yurii V.

AU - Palyanov, Yury N.

PY - 2018/6/1

Y1 - 2018/6/1

N2 - Interactions in a Fe–C–O–H–N system that controls the mobility of siderophile nitrogen and carbon in the Fe0-saturated upper mantle are investigated in experiments at 6.3–7.8 GPa and 1200–1400 °C. The results show that the γ-Fe and metal melt phases equilibrated with the fluid in a system unsaturated with carbon and nitrogen are stable at 1300 °C. The interactions of Fe3C with an N-rich fluid in a graphite-saturated system produce the ε-Fe3N phase (space group P63/mmc or P6322) at subsolidus conditions of 1200–1300 °C, while N-rich melts form at 1400 °C. At IW- and MMO-buffered hydrogen fugacity (fH2), fluids vary from NH3- to H2O-rich compositions (NH3/N2 > 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 fH2 and carboxylic acids at unbuffered fH2 conditions. In unbuffered conditions, N2 is the principal nitrogen host (NH3/N2 ≤ 0.1) in the fluid equilibrated with the metal phase. Relatively C- and N-rich fluids in equilibrium with the metal phase (γ-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.

AB - Interactions in a Fe–C–O–H–N system that controls the mobility of siderophile nitrogen and carbon in the Fe0-saturated upper mantle are investigated in experiments at 6.3–7.8 GPa and 1200–1400 °C. The results show that the γ-Fe and metal melt phases equilibrated with the fluid in a system unsaturated with carbon and nitrogen are stable at 1300 °C. The interactions of Fe3C with an N-rich fluid in a graphite-saturated system produce the ε-Fe3N phase (space group P63/mmc or P6322) at subsolidus conditions of 1200–1300 °C, while N-rich melts form at 1400 °C. At IW- and MMO-buffered hydrogen fugacity (fH2), fluids vary from NH3- to H2O-rich compositions (NH3/N2 > 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 fH2 and carboxylic acids at unbuffered fH2 conditions. In unbuffered conditions, N2 is the principal nitrogen host (NH3/N2 ≤ 0.1) in the fluid equilibrated with the metal phase. Relatively C- and N-rich fluids in equilibrium with the metal phase (γ-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.

KW - Carbon

KW - Fluid

KW - Gas chromatography–mass spectrometry

KW - Hydrocarbons

KW - Mantle

KW - Metal

KW - Nitrogen

KW - TERRESTRIAL CARBON

KW - HIGH-PRESSURE

KW - OXYGEN FUGACITY

KW - EARTHS MANTLE

KW - MAGMA OCEAN

KW - Gas chromatography-mass spectrometry

KW - SILICATE MELTS

KW - METALLIC IRON

KW - CORE FORMATION

KW - PHASE-RELATIONS

KW - DIAMOND CRYSTAL-GROWTH

UR - http://www.scopus.com/inward/record.url?scp=85047405082&partnerID=8YFLogxK

U2 - 10.1007/s00410-018-1472-3

DO - 10.1007/s00410-018-1472-3

M3 - Article

AN - SCOPUS:85047405082

VL - 173

JO - Contributions to Mineralogy and Petrology

JF - Contributions to Mineralogy and Petrology

SN - 0010-7999

IS - 6

M1 - 47

ER -