MINERALOGICALMAGAZINE, VOLUME 62A

Geochemistry of the organic evenkite in septarian concretions in the Oxfordian marls of the French Alps

J. E. Spangenberg Laboratoire des Isotopes Stables, Institut de Min~ralogie et P6trographie, Universit6 de Lausanne, BFSH-2, 1015 Lausanne, Switzerland N. Meisser Mus~e de G~ologie, Institut de Min6ralogie et P&rographie, Universit6 de Lausanne, BFSH-2, 1015 Lausanne, Switzerland

Crystals of evenkite, paraffin-type hydrocarbon, from Tunguska River, Siberia by Skropyshev (1954). the septarian concretions in the Oxfordian (Upper This waxy hydrocarbon was described as colourless Jurassic) marls of the Dauphinois subalpine tectonic or yellowish, pseudohexagonal tabular crystal with domain, France (Martin, 1985) have been analysed mica like and hardness of 1. The crystal- by gas chromatography mass spectrometry (GC/MS) lographic data of the evenkite from the type locality and compound specific isotope analyses based on gas matches those of other straight-chain normal chromatography - combustion - isotope ratio mass paraffins (n-alkanes), which showed it to be a spectrometry (GC/C/IRMS). Evenkite was first monoclinic modification of normal tetracosane (n- discovered in geodes lined with and C24H50, Strunz and Contag, 1965). The molecular associated in a polymetallic vein cutting and isotopic data of evenkite from two localities in vesicular tuff in the Evenki district, Lower the French Alps, Serres (Hautes-Alpes) and

n-024 n-C25 -28.9 f-28.5

n-C23 I n-026 -28.6 -28.6

n-C22 rl- ~'27 -28.6 -28.7

n-C28 n-C21 -29.3 -28.5

n-C29 n-C2o 1-29.6 -2841 , I n'C3o n'C31 -- L t. L . JL /l-28.2-28.0

Time FI6. 1. Total ion chromatogram and carbon isotope composition of the n-alkanes from the evenkite (Serre, France).

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Condorcet (Dr6me) areas, give insight into the relative concentrations of the individual n-alkanes is composition of this and permit the C24.4H5o.8, with an expected chemical composition of distinction between the two possible origin of the 85.21% C and 14.78% H. These values are similar to source hydrocarbons: a) hydrocarbons produced in the measured carbon and hydrogen of the Condorcet situ during thermal evolution of organic material evenkite (83.4% C, 14.8% H), and are very close to trapped in the septarian concretions, or b) infiltration the composition of the Siberian evenkite (85.43% C, of extrinsic migrated hydrocarbons caused by the 14.99% H, Skropyshev, 1953). These results indicate passage of formation/metamorphic hot fluids through that evenkite is an assemblage of solid straight chain the Oxfordian rocks. During the Jurassic burial, the alkanes with 20 to 30 carbon atoms (see Fig.), and host sediments were heated up to 200~ the nodules not a crystalline phase of pure tetracosane as believed cracked and were cemented with epigenetic previously. The uniform isotopic composition of the (Barlier, 1974). Mechanical and geothermal stress bulk mineral (-28.7%0) and of the individual n- during the alpine uplift (15 Ma) enhanced the thermal alkanes (-28.0 to -29.6%0, mean - -28.6 ___ 0.4%0) maturation of these sediments and produced probably indicate that the hydrocarbons are indigenous, and new fracture of the nodules and partial recrystallisa- were produced by thermal cracking of the organic tion of the vug-filling minerals. The evenkite at matter (marine plants and higher organism) trapped Serres is associated with , celestite, dolomite in the septarian concretions during Jurassic burial and and quartz, and at Condorcet with quartz, chalcedony tectonic/low-temperature-metamorphism history of and calcite. Evenkite appears to be the latest stage the calcareous rocks in the Dauphinois subalpine epigenetic mineral, which forms tabular crystalline domain. aggregates of white to brownish waxy crystals with strong basal cleavage. The crystals from both References localities have the same X-ray powder diffraction and FT-IR (KBr pressed pellet) patterns and are Barlier J. (1974) Th~se 3~me. cycle. Univ. Paris Sud, 96 identical to those described for evenkite from the p. Siberian type locality (Skropyshev, 1953) and for Hasnaoui N., Dellacherie J., Schuffenecker L., Dirand, synthetic n-tricosane - n-tetracosane mixed crystals M. and Balesdent, D. (1988) J. Chimie Physique, 85, (Hasnaoui et al. 1988). The molecular analyses show that evenkite is an assemblage of n-alkanes waxes in 153-60. the range C2o to C3o, with no odd-even predominance Martin, R. (1985) Schweizer Strahler, 7, 13-41. and highest concentrations in the C23-C25 range. Skropyshev, A.V. (1954) Miner. Abstr., 12, 305. Trace amounts of branched alkanes elute between the Strunz, H. and Contag, B. (1965) N. Jb. Miner. Mon., 1, n-alkanes. The empirical formula calculated from the 19-25.

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