Abstract
C- and N-bearing species in reduced fluids weree studied experimentally in C–O–H–N and muscovite–C–O–H–N systems and in natural carbonate-bearing samples at mantle P–T parameters. The experiments reproduced three types of reactions leading to formation of hydrocarbons (HCs) at 3.8–7.8 GPa and 800–1400 C and at hydrogen fugacity (fH2) buffered by the Fe–FeO (IW) + H2O or Mo–MoO2 (MMO) + H2O equilibria: (i) Thermal destruction of organic matter during its subduction into the mantle (with an example of docosane), (ii) hydrogenation of graphite upon interaction with H2‑enriched fluids, and (iii) hydrogenation of carbonates and products of their reduction in metamorphic clayey rocks. The obtained quenched fluids analyzed after the runs by gas chromatography-mass spectrometry (GC–MS) and electronic ionization mass-spectrometry (HR–MS) contain CH4 and C2H6 as main carbon species. The concentrations of C2-C4 alkanes in the fluids increase as the pressure and temperature increase from 3.8 to 7.8 GPa and from 800 to 1400 C, respectively. The fluid equilibrated with the muscovite–garnet–omphacite–kyanite–rutile ± coesite assemblage consists of 50–80 rel.% H2O and 15–40 rel.% alkanes (C1 > C2 > C3 > C4). Main N-bearing species are ammonia (NH3) in the C–O–H–N and muscovite–C–O–H–N systems or methanimine (CH3N) in the fluid derived from the samples of natural pelitic rocks. Nitrogen comes either from air or melamine (C3H6N6) in model systems or from NH4+ in the runs with natural samples. The formula CH3N in the quenched fluid of the C–O–H–N system is confirmed by HR–MS. The impossibility of CH3N incorporation into K-bearing silicates because of a big CH3NH+ cation may limit the solubility of N in silicates at low fO2 and hence may substantially influence the mantle cycle of nitrogen. Thus, subduction of slabs containing carbonates, organic matter, and N-bearing minerals into strongly reduced mantle may induce the formation of fluids enriched in H2O, light alkanes, NH3, and CH3N. The presence of these species must be critical for the deep cycles of carbon, nitrogen, and hydrogen.
Highlights
Hydrocarbons (HCs) and ammonia have been important agents in the Earth’s carbon and nitrogen cycles
This study focuses on C- and N-bearing species in reduced fluids synthesized at 3.8–7.8 GPa and 800–1400 ◦ C in the model systems of C–O–H–N and muscovite–C–O–H–N and in charges with natural samples of pelite (Maykop Fm. shale, Russia) and N-bearing mica schist (Polar Ural, Russia) which represented carbonate-bearing slab material subducted to mantle depths
This study has provided the first evidence that methanimine (CH3 N) may be an essential N-bearing specie in the reduced fluid obtained from natural pelite samples in the P–T conditions of 6.3 GPa and 1000 ◦ C corresponding to a hot slab subducted to
Summary
Hydrocarbons (HCs) and ammonia have been important agents in the Earth’s carbon and nitrogen cycles. Their stability in different tectonic settings in the course of geodynamic evolution has been. Thermodynamic calculations and experimental data indicate that organic species, such as light alkanes, acids, and salts, e.g. acetic acid and acetates, can be stable in the silicate mantle under the appropriate redox conditions [4,7,10,12,13,14,15,16,17,19,20,21,22]. Light alkanes were found out [22] to be stable in a wide range of redox conditions in the presence of a lherzolitic mineral assemblage at 5.5–7.8 GPa and 1150–1350 ◦ C. The discovery of hydrocarbons in diamonds from the mantle and in crustal alkaline rocks [5,17,23,24,25,26,27] supports this conclusion
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