102 Peter Stille and Graham Shields
As a result of several studies, which have used the above approaches, it has
become clear that scatter in Sr isotope data may derive from a number of sources:
1) Leaching technique. To avoid measuring more radiogenic Sr which derives
from clay minerals (authigenic or detrital) or Sr from detritus, it is necessary to
apply the gentlest leaching technique possible for that particular mineralogy. For
limestones, this is likely to involve an initial cleaning of easily leachable ions by
ammonium acetate or very dilute HCI and thereafter dissolution in acetic acid
buffered to a mild pH (about 4.5) by an acetate buffer. Dolomites and apatites may
require an HCI leach and this is one further reason why they yield such variable
results.
2) Rb correction. Rb which is leached from the carbonate or elsewhere will
have undergone radioactive decay to 87Sr since the time of formation. Provided
this STSr has not escaped from the system, it will tend to make the measured
87Sr/S6Sr ratio too high and this effect must be corrected for. The older the
samples, and the higher the initial Rb/Sr ratio, the greater is the effect! However,
such a correction involves several assumptions and it is best to avoid samples
requiring any extra correction of this nature.
3) Diagenetic exchange. This effect can be minimalized by the establishment
of diagenetic trends (Fig. 5.10) and by the careful selection of samples (Fig. 5.9).
These aspects will become clearer in the next section.
4) Errors in stratigraphic correlation. This effect can be highly significant
when comparing data from around the World (see Fig. 5.8).
5) Different standard values. All laboratories calibrate against an international
standard: NBS SRM 987. Sometimes batches of standard may give different
values due to contamination and machine effects. McArthur (1994) recognized
that some of the remaining inconsistency between Cenozoic Sr seawater curves
could be removed if normalization were carried out relative to a modern seawater
shell standard instead of to this artificial standard as a result of analytic artefacts.
5.1.3 Microstratigraphy at the Precambrian/Cambrian Boundary?
Improvements in our understanding of how the Sr isotopic system may react
during diagenetic alteration brings us right up to date. It is now possible to use the
Sr isotopic system as a high resolution tool in stratigraphy for much of the past
800 Ma. Its power lies in the assumption of isotopic homogeneity during times of
good ocean circulation and the possibility of combination with other
chemostratigraphic parameters such as the stable isotopes carbon and oxygen. One
recent study used this flexibility to make stratigraphic correlation of the
Precambrian / Cambrian boundary easier (Brasier et al. 1996). As this boundary
marks the introduction of animal shells in the geologic record, it was necessary to
measure bulk carbonate rock for all geochemical parameters.
This international ~oup of authors, representing part of the IGCP (International
Geological Correlation Programme) group working on the formal definition of the
boundary, measured C and O isotope ratios, Fe, Mn, Mg, Rb and Sr contents of
As a result of several studies, which have used the above approaches, it has
become clear that scatter in Sr isotope data may derive from a number of sources:
1) Leaching technique. To avoid measuring more radiogenic Sr which derives
from clay minerals (authigenic or detrital) or Sr from detritus, it is necessary to
apply the gentlest leaching technique possible for that particular mineralogy. For
limestones, this is likely to involve an initial cleaning of easily leachable ions by
ammonium acetate or very dilute HCI and thereafter dissolution in acetic acid
buffered to a mild pH (about 4.5) by an acetate buffer. Dolomites and apatites may
require an HCI leach and this is one further reason why they yield such variable
results.
2) Rb correction. Rb which is leached from the carbonate or elsewhere will
have undergone radioactive decay to 87Sr since the time of formation. Provided
this STSr has not escaped from the system, it will tend to make the measured
87Sr/S6Sr ratio too high and this effect must be corrected for. The older the
samples, and the higher the initial Rb/Sr ratio, the greater is the effect! However,
such a correction involves several assumptions and it is best to avoid samples
requiring any extra correction of this nature.
3) Diagenetic exchange. This effect can be minimalized by the establishment
of diagenetic trends (Fig. 5.10) and by the careful selection of samples (Fig. 5.9).
These aspects will become clearer in the next section.
4) Errors in stratigraphic correlation. This effect can be highly significant
when comparing data from around the World (see Fig. 5.8).
5) Different standard values. All laboratories calibrate against an international
standard: NBS SRM 987. Sometimes batches of standard may give different
values due to contamination and machine effects. McArthur (1994) recognized
that some of the remaining inconsistency between Cenozoic Sr seawater curves
could be removed if normalization were carried out relative to a modern seawater
shell standard instead of to this artificial standard as a result of analytic artefacts.
5.1.3 Microstratigraphy at the Precambrian/Cambrian Boundary?
Improvements in our understanding of how the Sr isotopic system may react
during diagenetic alteration brings us right up to date. It is now possible to use the
Sr isotopic system as a high resolution tool in stratigraphy for much of the past
800 Ma. Its power lies in the assumption of isotopic homogeneity during times of
good ocean circulation and the possibility of combination with other
chemostratigraphic parameters such as the stable isotopes carbon and oxygen. One
recent study used this flexibility to make stratigraphic correlation of the
Precambrian / Cambrian boundary easier (Brasier et al. 1996). As this boundary
marks the introduction of animal shells in the geologic record, it was necessary to
measure bulk carbonate rock for all geochemical parameters.
This international ~oup of authors, representing part of the IGCP (International
Geological Correlation Programme) group working on the formal definition of the
boundary, measured C and O isotope ratios, Fe, Mn, Mg, Rb and Sr contents of