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Author
Date
2018-10Type
- Doctoral Thesis
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Abstract
Unlike in many other stable isotope systems, equilibrium isotope fractionation in
the carbon system is predicted by statistical mechanics’ theory to be in the order
of several permil at high temperatures corresponding to Earth’s deep and extreme
environments. Experimental studies confirming such theoretical predictions
are rare, and lacking for most mineral-mineral, mineral-fluid, and fluid-fluid pairs.
This thesis investigates carbon isotope exchange experiments between 300 – 1500
°C in the systems (i) CH4-CO2-CO, (ii) graphite-Na2CO3-CaCO3 (melt) and (iii)
CH4-CO2-CO-graphitic carbon. Experiments have been conducted with box- and
gas-mixing furnaces, externally-heated pressure vessels (“cold seals”), and piston
cylinder apparatuses. Sample characterization (before and after experiment) involved
gas-chromatography (GC), elemental analyzer (EA) and gas bench (GB)
systems connected to an isotope ratio mass spectrometer (IRMS).
Project (i) examined the carbon isotope exchange in a gas phase by using a variety
of organic starting materials that were, sealed under vacuum in quartz tubes,
thermally decomposed (300 -1200 °C) to create a CH4-CO2-CO gas mix plus an
elemental carbon residual. While the decomposition and gas generation is nearly
instantaneous, chemical and isotopic equilibration is protracted: vials with an additional
piece of Ni-foil catalyzed the equilibration reactions, whereas experiments
without Ni yield only minor chemical and isotopic reactions, if any. Measured gas
speciation of catalyzed gases correspond well to the expected range from thermodynamic
calculations. The experiments define carbon isotope partition functions for
the CO2/CH4, CO2/CO and CH4/CO pairs as (T in Kelvin)
10^3ln a (CO2/CH4) = 8.9(±0.6)*10^5 (1/T^2)^0.825(±0.005)
10^3ln a (CO2/CO) = 1.07(±0.05)*10^6 (1/T^2)^0.830(±0.003)
10^3ln a (CH4/CO) = 1.1(±0.2)*10^3 (1/T^2)^0.462(±0.001)
Project (ii) explored the carbon isotope exchange during graphite crystallization
in a Na2CO3-CaCO3 melt at 900-1500 °C, 1 GPa using a piston-cylinder device. Graphite
was grown anew from organic material during the melting of the carbonate mixture. Graphite growth proceeds by (1) decomposition of organic material into
globular amorphous carbon, (2) restructuring into nano-crystalline graphite, and
(3) recrystallization into hexagonal micron-sized graphite flakes. Each transition is
accompanied by carbon isotope exchange with the carbonate melt. As the experiments
did not yield bulk-equilibrated graphite at lower temperatures, the >1200
°C data was combined with empirical fractionation factors for carbonate-graphite
obtained from upper amphibolite and lower granulite facies carbonate-graphite pairs
reported by Kitchen & Valley (1995; J. metamorphic Geol., 13: 577-594) and Valley
& O’Neil (1981; Geochim. Cosmochim. Acta 45: 411-419) which results in the
general fractionation function (T in Kelvin)
∆13C(carbonate-graphite) = (3.37(4)*10^6)/T^2
This function is usable as a geothermometer for solid or liquid carbonate at
>600 °C. Similar to previous observations, lower-temperature experiments (<1100
°C) deviate from equilibrium. By comparing the experimental results to diffusion
and growth rates in graphite, it is shown that at <1100 °C diffusion rates are slower
than graphite growth and equilibrium surface isotope effects control the isotope fractionation
between graphite and carbonate-melt. The competition between diffusive
exchange and growth rates requires a more careful interpretation of isotope zoning
in graphite and diamond, especially since in high-temperature systems isotope
fractionation is often assumed to proceed at or near equilibrium.
Project (iii) investigates the influence of a COH-fluids redox state on the carbon
isotope composition of graphite/diamond precipitated from it. Time series experiments
were run to examine the carbon isotope exchange between carbonaceous
system-components during the progressive oxidization of an initially CH4-dominated
fluid. Stearic acid, thermally decomposed at 800 °C and 2 kbar, produced a reduced
COH-fluid together with an elemental carbon phase. Progressive hydrogen loss from
the capsule caused continuous methane dissociation accompanied by the precipitation
of elemental carbon. The precipitating carbon, which aggregates as globules, is
always 6.8 ±0.3 ‰ lighter than the methane, the opposite of what is expected from
equilibrium isotope fractionation. In dynamic environments, kinetic isotope fractionation
may hence control the carbon isotope composition of graphite or diamond.
The commonly observed 13C-enrichment trends in diamonds are then consistent with
deep reduced fluids oxidizing upon their rise.
Finally, with the experimentally-derived fractionation functions now available
it is possible to calculate carbon isotope fractionation factors for the CO2 – graphite/
diamond and CH4 – graphite/diamond pairs, which is necessary, as their isotopic
equilibration appears impossible in experiment. The fractionations are defined
as (T in Kelvin):
10^3ln a (CO2/graphite) = -(9*10^4/T^2)+(9580/T)-2.72
and
10^3ln a (graphite/CH4) = (8.9*10^5/T^1.65)-(9*10^4/T^2)-(9580/T)+2.72
With this extended set of equilibrium carbon isotope fractionation factors, tools
are provided that will be useful for applied geochemical and industrial problems,
and help developing more sophisticated models on the origin, evolution and fate of
Earth’s carbon reservoirs. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000314017Publication status
publishedExternal links
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Contributors
Examiner: Schmidt, Max W.
Examiner: Lilley, Marvin D.
Examiner: Bernasconi, Stefano M.
Examiner: Cody, George D.
Publisher
ETH ZurichSubject
Carbon isotopes; Experimental petrology; Graphite; Diamond; CO2; Carbon monoxide; Methane; CarbonatesOrganisational unit
03592 - Schmidt, Max / Schmidt, Max
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