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Chemical and carbon isotopic evolution of hydrocarbons during prograde metamorphism from 100°C to 550°C: Case study in the Liassic black shale formation of Central Swiss Alps

AuthorsSchwab, Valérie; Spangenberg, Jorge E.; Grimalt, Joan O.
KeywordsHydrocarbon distributions
Prograde metamorphism
Central Swiss Alps
Issue DateApr-2005
CitationGeochimica et Cosmochimica Acta 69(7): 1825-1840 (2005)
AbstractHydrocarbon distributions and stable isotope ratios of carbonates (δ13Ccar, δ18Ocar), kerogen (δ13Cker), extractable organic matter (δ13CEOM) and individual hydrocarbons of Liassic black shale samples from a prograde metamorphic sequence in the Swiss Alps were used to identify the major organic reactions with increasing metamorphic grade. The studied samples range from the diagenetic zone (<100°C) to amphibolite facies (~ 550°C). The samples within the diagenetic zones (<100 and 150°C) are characterized by the dominance of C<20 n-alkanes, suggesting an origin related with marine and/or bacterial inputs. The metamorphic samples (200 to 550°C) have distributions significantly dominated by C12 and C13 n-alkanes, C14, C16 and C18 n-alkylcyclopentanes and to a lesser extend C15, C17 and C21 n-alkylcyclohexanes. The progressive 13C-enrichment (up to 3.9‰) with metamorphism of the C>17 n-alkanes suggests the occurrence of cracking reactions of high molecular weight compounds. The isotopically heavier (up to 5.6‰) C<17 n-alkanes in metamorphic samples are likely originated by thermal degradation of long-chain homologous with preferential release of isotopically light C1 and C2 radicals. The dominance of specific even C-number n-alkylcyclopentanes suggests an origin related to direct cyclization mechanism (without decarboxylation step) of algal or bacterial fatty acids occurring in reducing aqueous metamorphic fluid conditions. The regular increase of the concentrations of n-alkylcycloalkanes vs. C>13 n-alkanes with metamorphism suggests progressive thermal release of kerogen-linked fatty acid precursors and degradation of n-alkanes. Changes of the steroid and terpenoid distributions are clearly related to increasing metamorphic temperatures. The absence of 18α(H)-22,29,30-trisnorneohopane (Ts), the occurrence of 17β(H)-trisnorhopane, 17β(H), 21α(H)-hopanes in the C29 to C31 range and 5α(H),14α(H),17α(H)-20R C27, C29 steranes in the low diagenetic samples (<100°C) are characteristic of immature bitumens. The higher thermal stress within the upper diagenetic zone (150°C) is marked by the presence of Ts, the disappearance of 17β(H)-trisnorhopane and thermodynamic equilibrium of the 22S/(22S + 22R) homohopane ratios. The increase of the ααα-sterane 20S/(20S + 20R) and 20R ββ/(ββ + αα) ratios (from 0.0 to 0.55 and from 0.0 to 0.40, respectively) in the upper diagenetic zone indicates the occurrence of isomerization reactions already at <150°C. However, the isomerization at C-20 (R → S) reaches thermodynamic equilibrium values already at the upper diagenesis (~ 150°C) whereas the epimerisation at C-14 and C-17 (αα → ββ) arrives to constant values in the lower anchizone (~ 200°C). The ratios Ts vs. 17α(H)-22,29,30-trisnorneohopane [(Ts/(Ts + Tm)] and 18α(H)-30-norneohopane (C29Ts) vs. 17α(H),21β(H)-30-norhopane [C29Ts/(C29Ts + C29)] increase until the medium anchizone (200 to 250°C) from 0.0 to 0.96 and from 0.0 to 0.44, respectively. An opposite trend towards lower values is observed in the higher metamorphic samples.
The occurrence of specific hydrocarbons (e.g., n-alkylcyclopentanes, cadalene, hydrogenated aromatic compounds) in metamorphic samples points to kerogen degradation reactions most probably occurring in the presence of water and under reducing conditions. The changes of hydrocarbon distributions and carbon isotopic compositions of n-alkanes related to metamorphism suggest that the organic geochemistry may help to evaluate the lowest grades of prograde metamorphism.
Description16 pages, 8 figures.
Publisher version (URL)http://dx.doi.org/10.1016/j.gca.2004.09.011
ISSN0016-7037 (Print)
Appears in Collections:(IDAEA) Artículos
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