Exposure to ionizing radiation causes the mean combustion temperature of naturally occurring, solid, terrestrial organic matter, derived from the radiation-induced polymerization of methane, to increase

Court, R. W.

Gilmour, I.

Parnell, J.

Sephton, M. A.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe combustion characteristics and stable carbonisotopic compositions of irradiated organic matter:implications for terrestrial and extraterrestrial sampleanalysisConference or Workshop ItemHow to cite:Court, R. W.; Sephton, M. A.; Parnell, J. and Gilmour, I. (2005). The combustion characteristics and stablecarbon isotopic compositions of irradiated organic matter: implications for terrestrial and extraterrestrial sampleanalysis. In: 36th Lunar and Planetary Science Conference, 14-18 Mar 2005, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1845.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukTHE COMBUSTION CHARACTERISTICS AND STABLE CARBON ISOTOPIC COMPOSITIONS OF IRRADIATED ORGANIC MATTER: IMPLICATIONS FOR TERRESTRIAL AND EXTRATERRESTRIAL SAMPLE ANALYSIS.  R. W. Court1, M. A. Sephton1, J. Parnell2 and I. Gilmour1 11Planetary and Space Sciences Research Institute, Open University, Milton Keynes, Buckinghamshire, MK7 6AA, UK (r.w.court@open.ac.uk); 2Department of Geology & Petroleum Geology, University of Aberdeen, Aberdeen, AB24 3UE, UK.  Introduction:  Ionizing radiation is the dominant creator and modifier of organic matter in space. It is capable of inducing free-radical chemistry in interstel-lar ice grains and the interiors of comets and asteroids, in planetary atmospheres and on the surfaces below [1,2]. Therefore, all analyses of extraterrestrial organic matter need to be aware of the nature and extent of radiolytic alteration.  This is especially true for the technique of stepped combustion-isotope ratio-mass spectrometry (stepped combustion-IRMS). Incremental heating allows the separation of carbon-bearing phases, such as kerogen, graphite, carbonate and diamond, by virtue of their different combustion temperatures [3]. The gas evolved at each temperature step is then analyzed iso-topically. This technique, used extensively on terres-trial samples [4], was to have been employed by the Gas Analysis Package on the ill-fated Beagle 2 Mars Lander, and will be performed by the Ptolemy evolved gas analyzer on board the Rosetta spacecraft, upon arrival at Comet 67P/Churyumov-Gerasimenko [5,6].  The ability of stepped combustion-IRMS to sepa-rate carbon-bearing phases requires these to have sig-nificantly different combustion temperatures. Conse-quently, a full understanding of any process capable of causing variations in the combustion temperature of carbon-bearing phases is essential. Although thermal maturation is capable of significantly raising the com-bustion temperatures of organic matter and kerogen [5], this is not considered to be a significant problem on the cold and volcanically inactive surfaces of Mars and comets. Here, we show that ionizing radiation, abundant in extraterrestrial environments, can also cause significant increases in the temperature of com-bustion of organic matter, formed by the polymeriza-tion of simple hydrocarbons, and discuss the implica-tions of this for extraterrestrial sample analysis.  Methods: Several naturally-occurring bitumens have been subjected to stepped combustion-carbon isotope ratio-mass spectrometry, using the MS86 ma-chine at The Open University, UK. Three types of bi-tumen were analyzed: irradiated bitumen derived from the radiolytic polymerization of methane, and irradi-ated and nonirradiated mixtures of complex, biogenic hydrocarbons (Table 1). Irradiation was provided by uranium and thorium naturally present within the bitu-mens. These samples, and the effects of ionizing radia-tion on their organic chemistry and other properties, are described more fully elsewhere [7,8].  Results and Discussion: The results reveal the ef-fects of ionizing radiation on the carbon isotopic com-position and combustion characteristics of organic matter. There is a positive relationship between in-creasing radioelement concentration, and, hence, de-gree of irradiation, and the calculated mean combus-tion temperature, amongst the methane-derived bitu-mens. This is attributed to the progressive radiolytic formation of larger, more refractory polyaromatic hy-drocarbons [8]. However, no relationship is apparent between the mean combustion temperature and radio-element concentration in the bitumens derived from complex hydrocarbon mixtures. This is probably due to the influence of other processes, such as variations in the original composition of the complex hydrocar-bon mixtures, thermal maturation and oxidation. Implications for extraterrestrial sample analy-sis: This data indicates that solid organic matter, formed by the radiation-induced polymerization of methane, combusts around 500 °C, and that the com-bustion temperature increases with increasing radiation dose Since ionizing radiation is the dominant producer and modifier of extraterrestrial organic matter [1], ex-traterrestrial organic matter may possess similar com-bustion characteristics. Accurate predictions of the combustion characteris-tics of extraterrestrial organic matter would be very useful, especially for in-situ analyzes. It would allow the combustion profile of the experiment to be tailored in advance, enabling a greater number of smaller tem-perature steps to be used for the temperature range over which gas is predicted to be evolved from the material. This would provide greater analytical resolu-tion where it is most required. This is useful in terres-trial studies, but, for extraterrestrial analyses, where the number of temperatures steps or repeat analyses possible may be strictly limited, it could be extremely important.  This data could also prevent a potential problem. Terrestrial, biogenic organic matter normally combusts at 300-450 °C. This enables stepped combustion to separate this material from carbonates, which generally decrepitate around 500-700 ºC, enabling isotopic analysis of the individual components [3]. However, the methane-derived organic polymers analyzed here Lunar and Planetary Science XXXVI (2005) 1845.pdfcombust at temperatures that not only are significantly higher that those of biogenic terrestrial organic matter, but also increase with increasing radiation dose. Hence, it is possible that certain terrestrial or extrater-restrial material might contain organic matter and car-bonate with similar combustion/decrepitation tempera-tures. If combustion or decrepitation of two isotopi-cally distinct phases occurred in the same temperature step, then the separate isotopic compositions of these phases are not be resolved. Instead, a value represent-ing a mixture of the components would be returned. This is especially significant for in-situ extraterres-trial analyses [5,6]. However, accurate predictions of the combustion characteristics of the target material would enable the combustion profile to be tailored so as to minimize, if not eliminate, this problem. Conclusions: 1. Ionizing irradiation causes the mean combustion temperature of organic matter de-rived from the polymerization of methane to increase. This is attributed to the progressive radiolytic polym-erization of small hydrocarbons into larger, more re-fractory and combustion-resistant PAHs. 2. This effect is not observed in bitumens derived from complex-hydrocarbon mixtures. This may be due to variations in chemical composition inherited from the source material, or to the effects of oxidation – influences that are greatly reduced in the methane-derived bitumens. 3. The methane-derived bitumens combusted at higher temperatures than the complex-hydrocarbon mixtures. This presumably reflects variations in or-ganic chemistry. 4. Stepped combustion-IRMS results provide indi-cations of the likely combustion behavior of extrater-restrial organic matter, on planetary surfaces or com-ets, which has been exposed to significant amounts of ionizing radiation. The data reported here should aid better interpretation of results of, and design and im-plementation of instrumentation associated with, in-situ extraterrestrial organic analyses. References: [1] Kissel J. et al. (1997) In: Comets and the Origin and Evolution of Life, 69-109. [2] Mi-yakawa S. et al (2002) Proc. Natl. Acad. Sci. U. S. A., 99, 14628-14631. [3] Wright I. P. and Pillinger C. T. (1998) Planet. Space Sci., 46, 813-823. [4] Wright I. P. and Pillinger C. T. (1989) In: New frontiers in stable isotopic research; laser probes, ion probes and small-sample analysis, 9-34. [5] Wright I. P. and Pillinger C. T. (1998) Adv. Space Res., 21, 1537-1545. [6] Wright I. P. et al. (2003) Acta Astronautica, 52, 219-225. [7] Court R. W. et al. (submitted) Geochim. Cosmochim. Acta. [8] Court R. W. et al. (submitted) Geochim. Cosmochim. Acta. Acknowledgements: We are very grateful to J. Leventhal, P. Eakin and B. Kříbek for supplying the bitumen samples, and to A. Verchovsky for the assis-tance in operating the MS86 machine. This work was supported by PPARC.   Table 1. Details and analytical results of the bitumens.  Bitumen type Bitumen source and host rock age Total radioelement concentration (%) Calculated mean organic combustion temperature (ºC) Bulk organic δ13C values (‰) Boliden, Sweden; Palaeoproterozoic 7.7 458 -42.4 Cluff Lake, Canada; Mesoproterozoic 12.3 474 -46.4 Methane- derived bitumens  Příbram, Czech Republic; Carboniferous 24.2 480 -59.6      Dingwall, Scotland; Devonian 0 350 -31.5 Nonirradiated complex- hydrocarbon mixtures Nash Scar Quarry A, Powys, Wales; Silurian 0 458 -31.0      Cerro Huemul, Argentina; Tertiary 0.21 418 -27.9 Moonta, South Australia; Archaean 4.2 279 -21.8 Temple Mountain, Utah; Triassic 7.0 437 -26.7 Irradiated complex- hydrocarbon mixtures    Laxey, Isle of Man, UK; Ordovician 12.3 402 -13.6 Lunar and Planetary Science XXXVI (2005) 1845.pdf

The combustion characteristics and stable carbon isotopic compositions of irradiated organic matter: implications for terrestrial and extraterrestrial sample analysis

http://oro.open.ac.uk/8669/1/The_Combustion_Characteristics_and_Stable_Carbon_Isotopic_Compositions_of_Irradiated_Organic_Matter_Implications_for_Terrestrial_and_Extraterrestrial_Sample_Analysis.pdf

The combustion characteristics and stable carbon isotopic compositions of irradiated organic matter: implications for terrestrial and extraterrestrial sample analysis

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