Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p71c0473f&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P71C-0473
Other
1040 Isotopic Composition/Chemistry, 8135 Hydrothermal Systems (8424)
Scientific paper
The occurrence of volatile hydrocarbon species in mid-ocean ridge vent fluids, suggests an inorganic (abiogenic) origin. Since isotopic composition is used as an effective tool for identifying the origin and mechanism of hydrocarbon formation, determining isotopic fractionation during inorganic hydrocarbon formation in hydrothermal experiments is of importance. An experiment involving carbon dioxide reduction at 400°C, 500bar was carried out in hydrothermal solution equipment. Magnetite (5g, powder), a common product of subseafloor metamorphism, was used as a catalyst. Formic acid was used as carbon precursor, as it instantly decomposes to CO2 and H2 at elevated temperatures and pressures. The concentration of formic acid in the starting solution (0.5M NaCl + 0.001M HCl) was 189.11 mmolal. To monitor reaction progresses in solution, separate fluid samples were collected over 1070 hours of reaction, and analyzed for chemical and isotopic species. Dissolved CO2, CO, CH4 and H2 were observed in aqueous fluid. No formate ion (HCOO-) was detected by ion chromatography. The total amount of carbon in solution, however, was always less than that in the fluid at the start of the experiment. At the end of the experiment, 35% of the initial carbon was lost. This suggests formation of other C-bearing species in solution and/or attached to magnetite surfaces. To better understand this, we performed isotopic measurements of dissolved carbon species. The carbon isotopic composition of starting formic acid, is -29.81‰ (V-PDB). The δ13C values of CO2 range from -29.39 to -29.56‰ , which are close to that of the starting formic acid. The δ13C values of CH4 and CO are lower than that of starting formic acid, ranging from -35.61 to -39.05‰ and -46.14 to -47.78‰ , respectively. During the experiment, the variations of δ13C values of CO2, CH4, and CO were relatively small. Data suggest that CO2 and CO achieved chemical equilibrium, while the amount of CH4 was much less than theoretical prediction. An additional experiment, using 13C-spiked formic acid as carbon source, showed that about 85% of CH4 was from a carbon phase other than the starting formic acid. The 13C/12C isotope fractionation between CO2 and CO, 1000lnα (CO2-CO), was approximate 17.9+/-0.7‰ , which is smaller than an equilibrium value at 400°C (23‰ ), suggesting that the entire system was not in isotopic equilibrium. Surface compositional analysis was conducted using X-ray Photoelectron Spectroscopy (XPS) on magnetite before and after reaction. The concentration of carbon increased by a factor of two following reaction. The carbon spectra from reacted and unreacted magnetite revealed similar functional groups with C-C, C-H, C-O, and O-C=O bonds, however, hydrocarbon compounds containing those functional groups may not be the same. The isotopic analysis showed that the carbon on the magnetite surface has δ13C values (-28.8+/-2.0‰ ) similar to that of formic acid. The carbon on the magnetite surface, only accounted for the half of lost the carbon, suggesting the existence of aqueous hydrocarbons. A complex mechanism, other than a simple kinetic chain reaction, is implied for magnetite-catalyzed hydrothermal carbon dioxide reduction processes. The detection of long chain, branched and/or aromatic hydrocarbons in aqueous products, is the key to understanding the overall reaction mechanism.
Fu Qiang
Horita Juske
Seyfried William E.
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