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Abstract
The existence of methane in the Earth’s mantle does not cause any doubt, however, its possible chemical transformation under the mantle thermobaric conditions is not enough known. Investigation of methane at the upper mantle thermobaric conditions, using diamond anvil cells, demonstrated the possible formation of ethane, propane and n-butane from methane, however, theoretical calculations of methane behaviour at extreme temperature and pressure predicted also heavier hydrocarbons. We experimentally investigated the chemical transformations of methane at the upper mantle thermobaric conditions, corresponding to the depth of 70–80 km (850–1000 K, 2.5 GPa), using “Toroid”-type Large reactive volume device and gas chromatography. The experimental results demonstrated the formation of the complex hydrocarbon mixture up to C7 with linear, branched and cycled structures and benzene. Unsaturated hydrocarbons were detected on the trace level in the products mixture. The increasing of exposure time leaded to growth of heavier components in the product systems. The data obtained suggest possible existence of complex hydrocarbon mixtures at the upper mantle thermobaric conditions and provide a new insight on the possible pathways of the hydrocarbons synthesis from methane in the upper mantle.
Highlights
The existence of methane in the Earth’s mantle does not cause any doubt, its possible chemical transformation under the mantle thermobaric conditions is not enough known
We experimentally investigated the transformation of methane at the moderate thermobaric conditions, corresponding to the depth of 70–80 km (850–1000 K, 2.5 GPa), using a “Toroid”-type large reactive volume (LRV) device with analysis by gas chromatography
In the present paper the main attention was focused on the experimental data providing the methane transformation under extreme thermobaric conditions
Summary
The existence of methane in the Earth’s mantle does not cause any doubt, its possible chemical transformation under the mantle thermobaric conditions is not enough known. Investigation of methane at the upper mantle thermobaric conditions, using diamond anvil cells, demonstrated the possible formation of ethane, propane and n-butane from methane, theoretical calculations of methane behaviour at extreme temperature and pressure predicted heavier hydrocarbons. The investigation of methane chemical transformations demonstrated its decomposition into molecular hydrogen and pure carbon (in the form of soot, graphite and diamond) at severe thermobaric conditions – 10–50 GPa and 2000–3000 K14,15. The absence of more complicated hydrocarbons in the methane transformation products at extreme pressure and temperature can be possibly explained by the small amount of the sample in DAC (the most commonly used method for such experiments) and, as a result, trace amounts of heavier hydrocarbons, which are not indicated due to the limitation of Component/fraction, % Methane Ethane Propane i-Butane n-Butane C5 fraction C6 fraction C7 fraction Benzene
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