Geology Reference
In-Depth Information
13.2 Geochemical Proxies
linked to the carbonate phase, depending on the simi-
larity of the crystal chemistry of Sr and Ca, and Mg
and Mn and the substitution of CaCO 3 by MgCO 3 in
calcite and aragonite. The major proportion of lime-
stone studies using trace elements as facies indicators
is concerned with these elements. Examples of the ap-
plication of strontium and manganese distribution to
the facies analyses of carbonates are listed in Box 13.2.
This section summarizes trace element analysis and the
use of stable isotopes in evaluating paleoenvironments
and the diagenetic development of carbonate rocks. The
text focuses on the potential of combined geochemical
and microfacies studies. See Brand and Veizer (1980)
and Morse and Mackenzie (1990) for basic informa-
tion on the geochemistry of sedimentary carbonates.
Strontium
Starting with the investigations by Veizer and Demo-
vic (1973), who recognized an apparently facies-con-
trolled bimodal distribution of Sr in Mesozoic micrites,
strontium is used as a proxy for paleoenvironmental
and diagenetic conditions of carbonate sediments. A
common approach aims at recognizing significant cor-
relations between specific strontium contents and car-
bonate rock types (expressed by microfacies or lithol-
ogy) and/or depositional settings (e.g. shallow-marine
versus basinal).
Sr values decrease from modern sediments to an-
cient limestones and to dolomites. Modern shallow-
marine aragonite is characterized by high Sr values (up
to 10 000 ppm). Calcite has lower amounts of stron-
tium. The incorporation of Sr in carbonates depends
on the primary mineralogy, water temperature, salinity,
vital effects, and average Ca/Sr content of seawater.
Most ancient limestones have only a few hundred
ppm Sr, mainly due to the loss of Sr during neomor-
phic processes, such as transformation of aragonite to
calcite, repeated dissolution-precipitation processes and
recrystallization causing a progressive diagenetic elimi-
nation of Sr over time. Diagenesis in an open system
influenced by freshwater results in decreasing, low and
homogeneous values.
13.2.1 Trace Elements
Trace elements in carbonate rocks occurring at con-
centrations between 10 -2 and 10 -3 wt.%, or less, are ei-
ther bound to the carbonate phase studied in RFA analy-
ses, or to the non-carbonate phase represented by acid-
resistent residues and measured by AAS and ICP
(Bausch 1994). For a time trace elements of limestones
were usually investigated in bulk samples, but are now
studied in samples differentiating micritic matrix, car-
bonate cements, fossils and other grains using various
microsampling techniques. Trace element studies con-
centrate on specific trace elements (e.g. strontium or
manganese) or investigate a large set of elements fol-
lowed by statistical analyses. Multivariate analyses are
a fundamental prerequisite in testing possible relations
between microfacies types and chemical data (e.g. Röhl
and Strasser 1995; Kühl et al. 1996)
In the 1970s many authors stated that rare element
variations in carbonate rocks were controlled by varia-
tions in lithology, skeletal composition and diagenesis
and that rare element patterns might reflect sedimen-
tary facies patterns (Wedepohl 1970; Veizer and Demovic
1974). A more skeptical view is expressed in modern
studies concluding that geochemical data can be used
in facies diagnoses only when the lithologic and diage-
netic development of the rock unit are well known
(Oesterreich 1992). This view results partly from the
difficulties relating geochemical data to microfacies
types based on minor paleontological or textural dif-
ferences. A critical combination of microfacies types
and trace elements is successful if microfacies types
are defined using the major compositional criteria (e.g.
Lintnerova and Hladikova 1992; Masaryk et al. 1993).
Manganese
The incorporation of Mn into marine carbonates de-
pends on the primary mineralogy, crystallographic con-
Box 13.2. Selected references dealing with combined
facies and trace element analyses of carbonate rocks.
Strontium: Al-Hashimi 1976; Auernheimer 1983; Bausch
1968; Cerny 1978; Cioni et al. 1973; H.W. Flügel and
Wedepohl 1967; Gittinger 1969; Imreh et al. 1965; Kranz
1976; Krejci-Graf 1965; Jorgenson 1981, 1986; Llavador
et al. 1983; Masaryk et al. 1993; Matheos et al. 1990;
Oesterreich 1992; Prasada and Naqvi 1977; Sakae et al.
1981; Santos Garcia 1992; Strohmenger and Dozet 1991;
Veizer and Demovic 1973, 1974; Veizer et al. 1971
Manganese: Bencini and Turi 1974; Corbin et al. 2000;
Oesterreich 1992; Pingitore 1978; Santos Garcia 1992;
Shanmugam and Benedict 1983; Shimmield and Price
1986; Turi et al. 1980
13.2.2 Strontium and Manganese - Favorite
Tools for Facies Studies
Many studies deal with common elements such as Sr,
Mn or Mg. A relatively new approach is the use of Rare
Earth Elements. Mg, Sr and and sometimes Mn are
 
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