Hydrocarbon distributions and stable isotope ratios of carbonates
(delta(13)C(car), delta(18)O(car)), kerogen (delta(13)C(ker)),
extractable organic matter (delta(13)C(EOM)) 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 degrees C) to amphibolite facies (similar to 550
degrees C). The samples within the diagenetic zones (< 100 and 150
degrees 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 degrees C) have distributions
significantly dominated by C-12 and C-13 n-alkanes, C-14, C-16 and C-18
n-alkylcyclopentanes and to a lesser extend C-15, C-17 and C-21
n-alkylcyclohexanes. The progressive C-13-enrichment (up to 3.9 parts
per thousand) 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 parts per thousand) C-< 17 n-alkanes in
metamorphic samples are likely originated by thermal degradation of
long-chain homologous with preferential release of isotopically light
C-1 and C-2 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 18
alpha(H)-22,29,30-trisnorneohopane (Ts), the occurrence of 17
beta(H)-trisnorhopane, 17 beta(H), 21 alpha(H)-hopanes in the C-29 to
C-31 range and 5 alpha(H),14 alpha(H),17 alpha(H)-20R C-27, C-29
steranes in the low diagenetic samples (< 100 degrees C) are
characteristic of immature bitumens. The higher thermal stress within
the upper diagenetic zone (150 degrees C) is marked by the presence of
Ts, the disappearance of 17 beta(H)-trisnorhopane and thermodynamic
equilibrium of the 22S/(22S + 22R) homohopane ratios. The increase of
the alpha alpha alpha-sterane 20S/(20S + 20R) and 20R beta beta/(beta
beta + alpha alpha) 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 degrees C. However, the
isomerization at C-20 (R -> S) reaches thermodynamic equilibrium values
already at the upper diagenesis (similar to 150 degrees C) whereas the
epimerisation at C-14 and C-17 (alpha alpha ->beta beta) arrives to
constant values in the lower anchizone (similar to 200 degrees C). The
ratios Ts vs. 17 alpha(H)-22,29,30-trisnorneohopane [(Ts/(Ts + Tm)]
and 18 alpha(H)-30-norneohopane (C29Ts) vs. 17 alpha(H),21
beta(H)-30-norhopane [C29Ts/(C29Ts + C-29)] increase until the medium
anchizone (200 to 250 degrees C) from 0.0 to 0.96 and from 0.0 to 0.44,
respectively.
An opposite trend owards 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. Copyright (c) 2005 Elsevier
Ltd.