TY - JOUR

T1 - Composition of reduced mantle fluids

T2 - Evidence from modeling experiments and fluid inclusions in natural diamond

AU - Sokol, A. G.

AU - Tomilenko, A. A.

AU - Bul'bak, T. A.

AU - Sokol, I. A.

AU - Zaikin, P. A.

AU - Sobolev, N. V.

PY - 2020/5/1

Y1 - 2020/5/1

N2 - Experimental modeling in the C-O-H, C-O-H-N, and peridotite-C-O-H-N systems, combined with analyses of fluid inclusions in natural diamonds, is used to reconstruct the compositions of fluids that can be stable in the reduced mantle. Hydrocarbons (HCs) in the upper mantle can form either by reactions of carbonates with iron/wüstite and water or by direct hydrogenation of carbon phases (graphite, diamond, and amorphous carbon) interacting with reduced fluids. Carbon required for the formation of HCs can come from diamond, graphite, or carbonates. Mainly light alkanes are stable at the mantle pressures and temperatures in the C-O-H and C-O-H-N systems as well as in the peridotite-fluid system under ultrareduced to moderately reduced redox conditions at the oxygen fugacity from -2 to +2.5 lg units relative to the IW (Fe-FeO) buffer. Some oxygenated HCs can be stable in fluids equilibrated with carbonate-bearing peridotite. Ammonia and, to a lesser degree, methanimine (CH3N) are predominant nitrogen species in reduced fluids in the conditions of the subcratonic lithosphere or the Fe0-bearing mantle. The presence of HCs as common constituents of reduced mantle fluids is supported by data on inclusions from natural diamonds hosted by kimberlites of the Yakutian province and from placer diamonds of the northeastern Siberian craton and the Urals. Fluid inclusions have minor amounts of H2O, methane, and other light alkanes but relatively high concentrations of oxygenated hydrocarbons, while the H/(H + O) ratio varies from 0.74 to 0.93. Hydrocarbon-bearing fluids in some eclogitic diamonds have high CO2 concentrations. Also, the fluid inclusions have significant percentages of N2 and N-containing species, Cl-containing HCs, and S-containing compounds. Both the experimental results and the analyses of fluid inclusions in natural diamonds indicate that HCs are stable in the upper mantle conditions. The set of hydrocarbons, mainly light alkanes, might have formed in the mantle from inorganic substances. Further research should focus on the causes of the difference between experimental and natural fluids in the contents of methane, light alkanes, oxygenated hydrocarbons, and water and on the stability of N-, S-, and Cl-containing fluid components.

AB - Experimental modeling in the C-O-H, C-O-H-N, and peridotite-C-O-H-N systems, combined with analyses of fluid inclusions in natural diamonds, is used to reconstruct the compositions of fluids that can be stable in the reduced mantle. Hydrocarbons (HCs) in the upper mantle can form either by reactions of carbonates with iron/wüstite and water or by direct hydrogenation of carbon phases (graphite, diamond, and amorphous carbon) interacting with reduced fluids. Carbon required for the formation of HCs can come from diamond, graphite, or carbonates. Mainly light alkanes are stable at the mantle pressures and temperatures in the C-O-H and C-O-H-N systems as well as in the peridotite-fluid system under ultrareduced to moderately reduced redox conditions at the oxygen fugacity from -2 to +2.5 lg units relative to the IW (Fe-FeO) buffer. Some oxygenated HCs can be stable in fluids equilibrated with carbonate-bearing peridotite. Ammonia and, to a lesser degree, methanimine (CH3N) are predominant nitrogen species in reduced fluids in the conditions of the subcratonic lithosphere or the Fe0-bearing mantle. The presence of HCs as common constituents of reduced mantle fluids is supported by data on inclusions from natural diamonds hosted by kimberlites of the Yakutian province and from placer diamonds of the northeastern Siberian craton and the Urals. Fluid inclusions have minor amounts of H2O, methane, and other light alkanes but relatively high concentrations of oxygenated hydrocarbons, while the H/(H + O) ratio varies from 0.74 to 0.93. Hydrocarbon-bearing fluids in some eclogitic diamonds have high CO2 concentrations. Also, the fluid inclusions have significant percentages of N2 and N-containing species, Cl-containing HCs, and S-containing compounds. Both the experimental results and the analyses of fluid inclusions in natural diamonds indicate that HCs are stable in the upper mantle conditions. The set of hydrocarbons, mainly light alkanes, might have formed in the mantle from inorganic substances. Further research should focus on the causes of the difference between experimental and natural fluids in the contents of methane, light alkanes, oxygenated hydrocarbons, and water and on the stability of N-, S-, and Cl-containing fluid components.

KW - Deep cycle of carbon

KW - Fluid

KW - Hydrocarbons

KW - Inclusions in diamond

KW - Mantle

KW - Nitrogen

KW - OXYGEN FUGACITY

KW - P-T CONDITIONS

KW - hydrocarbons

KW - deep cycle of carbon and nitrogen

KW - LITHOSPHERIC MANTLE

KW - EARTHS MANTLE

KW - VOLATILE COMPONENTS

KW - inclusions in diamond

KW - HIGH-PRESSURES

KW - PERIDOTITE XENOLITHS

KW - YAKUTIAN KIMBERLITES

KW - mantle

KW - O-H FLUID

KW - fluid

KW - SULFIDE INCLUSIONS

UR - http://www.scopus.com/inward/record.url?scp=85089018185&partnerID=8YFLogxK

U2 - 10.15372/RGG2020103

DO - 10.15372/RGG2020103

M3 - Article

AN - SCOPUS:85089018185

VL - 61

SP - 663

EP - 674

JO - Russian Geology and Geophysics

JF - Russian Geology and Geophysics

SN - 1068-7971

IS - 5-6

ER -