Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (4) Citing Articles via Google Scholar Google Scholar Articles by Sharp, Z.D. Articles by Durakiewicz, T. Search for Related Content GeoRef GeoRef Citation

The effect of thermal decarbonation on stable isotope compositions of carbonates

¹ Department of Earth and Planetary Sciences, Northrop Hall, The University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
² Institute of Meteoritics, Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
³ Los Alamos National Laboratory, Condensed Matter & Thermal Physics Group, Mailstop K.764, Los Alamos, New Mexico 87545, U.S.A.
⁴ Mass Spectrometry Laboratory, Institute of Physics, Maria Curie-Sklodowska University 20-031 Lublin, Poland

Correspondence: ^* E-mail: zsharp{at}unm.edu

Oxygen and C isotope compositions of CO₂ gas released by thermal decomposition of siderite, calcite, and dolomite were measured using a new "real-time" continuous-flow technique to determine whether fractionation associated with simple thermal decarbonation could explain the large isotopic variations and mineralogy such as those found in the ALH84001meteorite.

Oxygen and C isotope fractionation between calcite or dolomite and evolved CO₂ gas during thermal decarbonation in a 3 bar He pressure environment is very small. The ¹³C and ¹⁸O values of evolved CO₂ gas are nearly identical to those of the carbonate, very different from the calculated equilibrium ¹⁸O _calcite-CO2 value of –4 to –5 at 800–900 °C or from previous experimental results of decarbonation in vacuum. The kinetic ¹⁸O_siderite-CO2 values are ~–2, whereas ¹³C_siderite-CO2 values increase logarithmically with time, from ~1 for the earliest stages of decarbonation to >5 in the final stages. Incomplete siderite decomposition produces both magnetite (¹⁸O = 3.5 SMOW) and minor graphite. CO and O₂ were detected during the decarbonation process. The data can be explained by simultaneous oxidation and reduction by the reaction:

where x and y are between 0 and 1. Siderite decomposition in the presence of H₂ gas produces wüstite and Fe metal in place of oxidized Fe minerals.

The experiments in this study are not a perfect analog for possible decarbonation conditions that might have occurred to the carbonates in ALH84001 Nevertheless, the large ¹³C and ¹⁸O variations observed in ALH84001(>10 for O) are significantly larger than those expected by thermal decarbonation, suggesting instead a low-temperature mechanism for their formation.

This article has been cited by other articles:

				R. Han, T. Shimamoto, T. Hirose, J.-H. Ree, and J.-i. Ando Ultralow Friction of Carbonate Faults Caused by Thermal Decomposition Science, May 11, 2007; 316(5826): 878 - 881. [Abstract] [Full Text] [PDF]

				D. S. Barker Origin of cementing calcite in "carbonatite" tuffs Geology, April 1, 2007; 35(4): 371 - 374. [Abstract] [Full Text] [PDF]