Geology Reference
In-Depth Information
• type and distribution of non-carbonate minerals,
• changes taking place during burial diagenesis and in-
cipient metamorphosis.
the chemical environment and/or in the growth
speed; early diagenetic and burial cements reflect-
ing burial depths; mineral transformations, such
as the alteration of calcite to dolomite (Richter 1984),
•
evaluating pore water chemistry, reflected by sector
zoning in calcite crystals (Bruckschen et al. 1992),
and interpretation of the diagenetic history of reser-
voir rocks,
3.2.3.3 Fluorescence, Cathodoluminescence
and Fluid Inclusion Microscopy
Fluorescence
is a form of luminescence and represents
the property of material to emit light when exited by
visible or ultraviolet light (Rost 1992). Fluorescence
microscopy is a standard technique in coal petrogra-
phy, paleobotany, and the organic petrography of sedi-
ments of mainly organic origin or with a high organic
content (e.g. petroleum source rocks). Fluorescence in
limestones is probably caused by organic matter
(Machel et al. 1991). Samples are studied as polished
slices and thick thin sections (Clausing 1991). Epiflu-
orescence in carbonates is used for
•
•
reconstructing diagenetic events which identify
stages of cementation and provide a 'cement strati-
graphy' that are used to establish cement-based
stratigraphic correlations (Meyers 1991),
•
recognizing fabrics (e.g. distinction of recrystallized
structures, recognition of microcracks),
•
studying the diagenesis of fossils.
Fluid inclusions in minerals
have been studied dur-
ing the last twenty years with respect to the genesis
and migration of hydrocarbons, mineral deposits, di-
agenesis of sedimentary rocks and problems of struc-
tural geology and petrology (Roedder 1984, 1990;
Lattanzi 1991). Fundamental for the interpretation of
fluid inclusion data is the assumption, that the fluid
inclusions still have the same composition and volume
as at the time of formation in a closed system. Alter-
ations may be caused by changes in geometry and vol-
ume or by a temporary or permanent opening of the
inclusions.
Carbonate crystals with fluid inclusions appear dark
in thin sections and polished sections. Cloudy inclu-
sions of aqueous liquid and vapor within carbonate crys-
tals provide information on the diagenetic environment
of cements (vadose zone, low-temperature phreatic
zone, high-temperature burial zone) and their thermic
history. The size of the inclusion ranges from < 10
m
to 30
m.
Fluid inclusions are caused by primary engulfment
of fluids within growing crystals or by secondary en-
trapment of fluids in microfractures that are subse-
quently healed. Samples should not have been subjected
to significant overheating beyond the temperature of
entrapment or the temperature of homogenization
(Barker and Goldstein 1990) and should have been
reequilibrated during recrystallization and deformation.
Calcite crystals should display distinct growth zones.
Genetic interpretations and classifications are based on
the composition, form, and position of the inclusions
as well as their relationship to crystal structures
(Roedder 1984; Goldstein 1993).
Investigations require a petrographical microscope
(with UV epifluorescence capabilities for differentiat-
ing aqueous and hydrocarbon inclusions) and a heat-
ing and freezing stage. Fluid inclusion studies are com-
recognizing hidden microfabrics (Dravis and Yure-
wicz 1985; Klotz 1989),
•
studying organic constituents of fine-laminated lime-
stones (e.g. cyanobacteria, algae, pollen and spores),
•
differentiating automicrites and allomicrites (Neu-
weiler 1995),
•
understanding biomineralization processes.
Cathodoluminescence (CL) microscopy
of carbon-
ate rocks, starting with the work of Sippel and Glover
(1965), is a significant tool in the petrography of ma-
rine and non-marine carbonates (Marshall 1988; Barker
and Kopp 1991; Barbin et al. 1991) and in the investi-
gation of primary or diagenetic microstructures of fos-
sils (Amieux 1987; Elorza et al. 2001). CL microscopy
stimulates luminescence on polished thin sections
(thickness < 30
m) and polished rock surfaces by elec-
tron bombardment. Luminescence depends on the ma-
terial characteristics of the excited solid. These charac-
teristics comprise the chemistry, crystal structure, lat-
tice defects and other factors (Hemming et al. 1989).
The main activator elements of carbonates are Mn
2+
and Pb
+2
. Activation of calcite and dolomite by Rare
Earth elements is also a common feature of Phanero-
zoic marine limestones and sinter calcites (Habermann
et al. 1996). The subjective determination of CL colors
should be supplemented by spectrometric analyses. CL
studies are often combined with petrographic micros-
copy (Gregg and Karakus 1991) and rare element mi-
croanalyses, and have become a significant part of
microfacies analyses. The major applications of CL
microscopy for carbonate rocks (see Pl. 5) are for
•
observing and interpreting of diagenetic phases (e.g.
zonar structures within crystals reflecting changes in
