Abiotic Hydrocarbons Generation Simulated by Fischer-Tropsch Synthesis under Hydrothermal Conditions in Ultradeep Basins

FTT experiments with water as a hydrogen source and three types of possible carbon sources in the subsurface(diiron nonacarbonyl,siderite and formic acid,representing CO,CO_(2)and a simple organic acid,respectively)were carried out in this study.Our experimental results showed that n-alkanes with the highest carbon number of C_(33)were produced when CO was used as a carbon source(series A);a variety of polycyclic aromatic hydrocarbons(PAHs)were detected in series B with CO_(2)as a carbon source;gaseous hydrocarbons were also detected with formic acid added(series C).The different products in the three series showed that there were different hydrocarbon generation mechanisms and reaction processes with different carbon sources.The generation of long-chain n-alkanes in series A provided experimental support for the formation of abiogenic petroleum underground,which was of significance to early membranes on the Earth.PAHs in series B provide experimental support for the possibility of an abiotic source of reduced carbon on other planets.The carbon isotopes of gaseous hydrocarbons produced by CO exhibited a partial reversed order(δ^(13)C_(1)<δ^(13)C_(2)>δ^(13)C_(3)>δ^(13)C_(4)>δ^(13)C_(5)),while the gaseous hydrocarbons produced by CO_(2)and HCOOH showed a positive order(δ^(13)C_(1)<δ^(13)C_(2)<δ^(13)C_(3)<δ^(13)C_(4)<δ^(13)C_(5)).Based on these,the alkylene mechanism and the carbonyl insertion mechanism were used to reasonably explain these characteristics.

In-situ electrochemical study on the eff ects of Fe(Ⅲ)on kinetics of pyrite acidic pressure oxidation

Fe(Ⅲ)has been proved to be a more eff ective oxidant than dissolved oxygen at ambient temperature,however,the role of Fe(Ⅲ)in pyrite acidic pressure oxidation was rarely discussed so far.In this paper,in-situ electrochemical investigation was performed using a flow-through autoclave system in acidic pressure oxidation environment.The results illustrated that increasing Fe(Ⅲ)concentrations led to raising in redox potential of the solution,and decreased passivation of pyrite caused by deposition of elemental sulfur.Reduction of Fe(Ⅲ)at pyrite surface was a fast reaction with low activation energy,it was only slightly promoted by rising temperatures.While,the oxidation rate of pyrite at all investigated Fe(Ⅲ)concentrations increased obviously with rising temperatures,the anodic reaction was the rate-limiting step in the overall reaction.Activation energy of pyrite oxidation decreased from 47.74 to 28.79 kJ/mol when Fe(Ⅲ)concentration was increased from 0.05 to 0.50 g/L,showing that the reaction kinetics were limited by the rate of electrochemical reaction at low Fe(Ⅲ)concentrations,while,it gradually turned to be diffusion control with increasing Fe(Ⅲ)concentrations.

Reactions between olivine and CO2-rich seawater at 300℃:Implications for H2 generation and CO2 sequestration on the early Earth

To understand the influence of fluid CO2 on ultramafic rock-hosted seafloor hydrothermal systems on the early Earth,we monitored the reaction between San Carlos olivine and a CO2-rich NaCl fluid at 300 C and 500 bars.During the experiments,the total carbonic acid concentration（∑XO2） in the fluid decreased from approximately 65 to 9 mmol/kg.Carbonate minerals,magnesite,and subordinate amount of dolomite were formed via the water-rock interaction.The H2 concentration in the fluid reached approximately 39 mmol/kg within 2736 h,which is relatively lower than the concentration generated by the reaction between olivine and a CO2-free NaCl solution at the same temperature.As seen in previous hydrothermal experiments using komatiite,ferrous iron incorporation into Mg-bearing carbonate minerals likely limited iron oxidation in the fluids and the resulting H2 generation during the olivine alteration.Considering carbonate mineralogy over the temperature range of natural hydrothermal fields,H2 generation is likely suppressed at temperatures below approximately 300℃ due to the formation of the Mg-bearing carbonates.Nevertheless,H_2 concentration in fluid at 300℃ could be still high due to the temperature dependency of magnetite stability in ultramafic systems.Moreover,the Mg-bearing carbonates may play a key role in the ocean-atmosphere system on the early Earth.Recent studies suggest that the subduction of carbonated ultramafic rocks may transport surface CO2 species into the deep mantle.This process may have reduced the huge initial amount of CO2 on the surface of the early Earth.Our approximate calculations demonstrate that the subduction of the Mg-bearing carbonates formed in komatiite likely played a crucial role as one of the CO2 carriers from the surface to the deep mantle,even in hot subduction zones.