Geology Reference
In-Depth Information
Plate 5 Cathodoluminescence (CL): A Possibility to See More
Cathodoluminescence microscopy of thin sections has become an important tool of microfacies analysis.It
assists in observing diagenetic phases recorded by different cement types (-> 1, 2, 8) and establishing a
'cement stratigraphy'as well as recognizing zonation patterns of calcite crystals and the mode of porosity
occlusion (-> 3).
CL reveals the original structures of fossils and their growth modes (-> 4) and facilitates the recognition
of diagenetic not or little altered shells which can be used for the determination of stable isotopes. Generally
CL accentuates diagenetic and sedimentary textures (-> 6, 7), thus allowing a more precise observation of
microfacies criteria. The combined use of CL and petrographic microscopy is strongly recommended.
1 and 2 Primary interparticle pore in transmitted light (-> 1) and under CL (-> 2). Note the different appearance of
cement successions. The succession consists of a light-brown luminescent radialfibrous cement (A) circumcrusting
a grain followed by non-luminescent dogtooth cement (B) and banded yellow to brown luminescent burial coarse
mosaic calcspar cement which fills the remaining pore space. Early Jurassic: Carbonate platform of Jbel Bou Dahar,
High Atlas, Morocco.
3
Zoned crystal growth. The luminescence pattern exhibits the distinct zonation of a large crystals filling the pore space
in a travertine. Quaternary: Tivoli near Rome, Italy.
4
Ulltastructure and growth modes of fossils: Shell of the inoceramid species
Antarcticeramus rabotensis
(Crame and
Luther).
Under cathodoluminescence a distinct yellow luminescent lamina is visible which interrupts the normal sized prisms
constructing the shell. A recovery to normal crystal sizes is rather slow as seen by the smaller prisms (here) left of
the yellow lamina. This interruption in shell growth is interpreted as the result of a cooling interval or even a freeze
shock which influenced the growth of prisms in the shell. Growth direction of the shell is from right to the left in this
picture. Late Cretaceous (Campanian): James Ross Island, Antarctica.
5
Grade of diagenetic alterations of shells. Punctate brachiopod. The brachiopod shell appears dark (unaltered) with
some luminescent parts (diagenetically altered). The punctae are filled by orange luminescent diagenetic calcite.
Sediment in the lower part shows bright orange luminescence due to Mn
2+
incorporation ('activator' ion) during the
diagenetic transformation of metastable aragonite and High-Mg calcite to the stable phase Low-Mg calcite and are
therefore relatively stable to diagenetic alteration. Unaltered shells usually show no luminescence. The shell belongs
to the genus
Schizophoria
. Devonian: Iowa, USA.
6
Diagenetic stages: Relictic ooids. The ooid layers were dissolved mainly during late burial diagenesis; the preserved
core consists of recrystallized shell fragments which show a weak red to orange luminescence. Open moldic porosity
appears dark. The outline of the ooids is traced by orange to red luminescent early diagenetic micritic envelopes and
fine crystalline cements.
Interparticle porosity between ooids is filled by coarse crystalline sparite showing bright orange luminescence. This
is interpreted as the result of late diagenetic neomorphism of an original early diagenetic marine-phreatic sparite. Late
Cretaceous: Sinai Peninsula, Egypt.
7
Dolomitization and compaction. Dolomite rhomboids. The rhomboids have a non-luminescent to weakly luminescent
core surrounded by a bright orange luminescent zone. The weak luminescence of the cores indicates iron-rich
diagenetic fluids and the incorporations of Fe
2+
in the dolomite lattice ('quench' ion) during crystal growth.
By contrast, the final bright orange luminescent zone indicates changing pore fluid chemistry with more Mn
2+
incorporation in the crystal lattice. Broken cores cemented by the outer luminescent zone indicate ongoing
compaction during dolomitization. The rhomboids are interpreted as baroque dolomite typical of late burial
diagenesis. Same locality as -> 6
8
Diagenetic filling of a fossil (brachiopod shell not visible in figure). Left part shows sediment (micrite) with red to
orange luminescence due to diagenetic transformation to Low-Mg calcite and the incorporation of Mn
2+
in the calcite
lattice. Remaining porosity was cemented by large calcite crystals (right) formed during late diagenesis. The distinct
zoning of the crystals with weak and bright orange luminescent zones indicates slow crystal growth, changing pore
water chemistry and/or changing redox conditions during crystal formation. The latest cements exhibits bright orange
to yellow luminescence and document the final stage of the diagenetic cements. Middle Devonian: Anti-Atlas,
Morocco.
-> 1, 2: Blomeier and Reijmer 1999; 3: Courtesy of R. Koch (Erlangen); 4: Elorza et al. 2001; 5-8: Courtesy of R. von
Geldern (Erlangen)
