Geology Reference
In-Depth Information
Isotope signal
Mineralogy of Components
Bulk sediments
Preservation potential
components
Skeletal
Non-skeletal
HIGH
Pristine aragonite
Mollusks
Marine cements
Good chance of
Pristine
Brachiopods,
Marine cements,
Pelagic sediments,
preservation of
Low-Mg calcite fossils,
belemnites,
LowMg calcite ooids
particularly
carbon and oxygen
grains or cements
foraminifera,
coccolith oozes
isotope signals
bivalves
Phosphatic fossils
Conodonts,
fish teeth
MODERATE
Secondary calcites
Carbon isotope
(stabilized in relatively
Molluscs,
Marine cements,
Some micrites,
signals may be
closed systems
foraminifera,
ooids, peloids,
some shallow
preserved, oxygen
with low
corals,
intraclasts
water carbonates,
isotope signals are
water/rock ratio)
echinoderms,
some dolomites
commonly altered
calcareous algae
LOW
Secondary calcites
Carbon and oxygen
(stabilized or cemented in Limestones altered by near surface meteoric diagenesis
isotope signals very likely
relatively open systems
of intensive cementation or recrystallization during
to have been altered
with high water/rock ratio)
burial; many dolomites
Fig. 3.9.
Preservation potential of carbon and oxygen isotopes
in ancient carbonate sediments and skeletal and non-skeletal
constituents, and phosphatic fossils. Modified after Marshall (1992).
(AAS) is widely used in the analysis of limestones and
dolomites because of elements hidden in minor amounts
within the carbonate lattice. Inductively-coupled Plasma
Atomic Emission Spectrometry Analysis (ICP-AES)
and Neutron Activation Analysis (NAA) are used for
determining the contents of trace elements and rare earth
elements in carbonate rocks by whole-rock and selec-
tive analyses (Fairchild et al. (1988).
Minor elements in carbonate rocks and in fossils
are important paleoenvironmental indicators. The com-
bined use of microfacies data, trace elements and iso-
tope geochemistry in facies analyses continues to evolve
in current studies (see Chap. 13).
sisting of Low-Mg calcite are rather resistant to diage-
netic alteration, isotope analyses of brachiopods are
enjoying increasing popularity.
Stable isotope analyses of ancient carbonates pro-
vide proxies for paleoenvironmental constraints (e.g.
temperature and salinity variations of the sea water, ter-
rigenous input), unravel changes in carbon cycling of
ancient ocean-atmosphere systems and document glo-
bal fluctuations in the climate. Isotope abundance mea-
surements for geochemical research are determined on
a mass spectrometer.
Basics: Laboratory work
Techniques and microscopical methods
Beckett, D., Sellwood, B.W. (1991): A simple method for
producing high-quality porecasts of carbonate rocks. - Sed.
Geol.,
71
, 1-4
Brown, R.J. (1986): SEM examination of carbonate micro-
facies using acetate peels. - J. Sed. Petrol.,
56
, p. 538
Delgado, F. (1977): Primary textures in dolostones and re-
crystallized limestones: a technique for their microscopy
study. - J. Sed. Petrol.,
47
, 1339-1341
Folk, R.L. (1987): Detection of organic matter in thin-sec-
tions using a white card. - Sed. Geol.,
54
, 193-200
Friedman, G.M. (1959): Identification of carbonate minerals
by staining methods. - J. Sed. Petrol.,
29
, 87-97
Gardner, K.L. (1980): Impregnation technique used colored
epoxy to define porosity in petrographic thin sections. -
Canadian Journal of Earth Sciences,
17
, 1104-1107
Germann, K. (1965): Die Technik des Folienabzuges und ihre
Ergänzung durch Anfärbemethoden. - Neues Jahrbuch für
Geologie und Paläontologie Abhandlungen,
121
, 293-306
Gies, R.M. (1987): An improved method for viewing micro-
pore systems in rocks with the polarizing microscope. - Soc.
Petroleum Engineers Formation Evaluation,
1987
, 209-214
Stable Isotopes:
The isotope ratios of
18
O/
16
O and
13
C/
12
C are very often used to trace processes involved in
the production of carbonate grains and their subsequent
conversion into limestones. Isotope studies are per-
formed as whole-rock bulk analyses or analyses of
single components (e.g. isolated fossils, carbonate ce-
ments) or as microsamples of individual parts of fos-
sils (e.g. shells) or cement crystals. The preservation
potential of isotope signals depends on the preserva-
tion of the primary mineralogy and the degree of di-
agenetic alterations (see Fig. 3.9). Diagenesis can ob-
scure the original composition of constituents of car-
bonate rocks, and if undetected, can lead to erroneous
conclusions. All samples, therefore, must be tested prior
to isotope analysis with respect to potential diagenetic
overprinting, using petrographic and CL microscopy,
SEM criteria and chemical data. Because shells con-
