review by b.c. schreiber why and how it is used in estimating stratigraphic position of carbonates...
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Review by B.C. Schreiber
Why and how it is usedin estimating stratigraphic position of
carbonates and evaporites
STRONTIUM ISOTOPES
ONE OF THE MOST USEFUL, CONSERVATIVE, AND STABLE ELEMENTS
FOR ISOTOPIC STUDY
IN SEDIMENTARY ROCKS(CARBONATES AND EVAPORITES)
IT HAS BEEN TREATED AS A GEOLOGICAL “TRACER”
BACKGROUND
During fractional crystallization, Sr tends to be come concentrated in the first minerals to crystallize, leaving Rb in the liquid phase.Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation.
Highest ratios occur in pegmatites. Typically, Rb/Sr increases in the order plagioclase, hornblende, K-feldspar, biotite, muscovite. Therefore, given sufficient time for significant production (ingrowth) of radiogenic 87Sr, measured 87Sr/86Sr values will be different in the minerals, increasing in the same order.
The Rb-Sr dating method has been used extensively in dating rocks. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and 87Sr/86Sr ratio.
The dates indicate the true age of the minerals only if the rocks have not been subsequently altered.
4 stable naturally occurring isotopes
84Sr (0.56%), 86Sr (9.86%), 81Sr (7.0%) and 88Sr (82.58%).
4 stable naturally occurring isotopes
84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%)
Present in many rock types.
Strontium is present as a ubiquitous minor element
in the crust of the Earth –
Typically found in concentrations of a few hundred parts per million
Strontium is present as a ubiquitous minor element
in the crust of the Earth –
Present in many rock types.
4 stable naturally occurring isotopes
84Sr (0.56%), 86Sr (9.86%), 81Sr (7.0%) and 88Sr (82.58%).
4 stable naturally occurring isotopes
84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%)
IN A MAGMA, IGNEOUS PROCESSESCONCENTRATE RUBIDUM IN THE RESIDUAL MELT
SO THE LATER FORMED MINERALS CONTAIN MORE AND MORE RUBIDIUM
SOURCES OF SrFrom radioactive decay of 87Rb
85Rb = 72.165%87Rb = 27.834%
Primordial 87Sr/86Sr ratio of 0.699, value derived from meteorites,
NOW HIGHER ON EARTHdue to the decay of 87Rb
Faure, 1991
GoldrichStabilitySeries
First toCrystallize
Last toCrystallize
SlowWeathering
FastWeathering
Bowen’sReaction
Series
First toCrystallize
Last toCrystallize
Last toCrystallize
WEATHERINGWEATHERING
Two sources of 87Sr in any material:
(1) formed primordial nucleo-synthesis along with 84Sr, 86Sr and 88Sr
(2) radioactive decay of 87Rb.
Sr IN GYPSUM REMAINS WITHIN SULPHATE EVEN WHEN IT DEHYDRATES
BUTWHEN IT REHYDRATES, MUCH Sr IS LOST
ALSO
WHEN ARAGONITE GOES TO CALCITEMUCH Sr IS LOST
BACKGROUND
During fractional crystallization, Sr tends to be come concentrated in the first minerals to crystallize, leaving Rb in the liquid phase.Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation.
Highest ratios occur in pegmatites. Typically, Rb/Sr increases in the order plagioclase, hornblende, K-feldspar, biotite, muscovite. Therefore, given sufficient time for significant production (ingrowth) of radiogenic 87Sr, measured 87Sr/86Sr values will be different in the minerals, increasing in the same order.
The Rb-Sr dating method has been used extensively in dating rocks. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and 87Sr/86Sr ratio.
The dates indicate the true age of the minerals only if the rocks have not been subsequently altered.
Secondly, differences in the relative mobilities of water at scales ranging from inter-grain pores tothe catchment scale may also profoundly affect 87Sr/86Sr (Bullen et al., 1996). For example, the chemical composition and the resultant 87Sr/86Sr in immobile waters at a plagioclase-hornblende grain boundary versus a quartz-mica boundary will be different from eachother.
Third, a difference in the relative "effective" surface areas of minerals in one portion o f the rock unit will also cause differences in chemistry and isotopic composition; "poisoning" of reactive surfaces by organiccoatings is an example of this kind of process.In a fundamental sense, because the waters in shallowsystems are not in chemical equilibrium with the rocks,it is unrealistic to expect that waters along flowpaths within even a constant-mineralogy unit should have a constant 87Sr/86Sr.
Instead, the waters moving along specific flowpaths slowly react with the rocks and gradually approach
The important concept for isotopic tracing is that Sr derived from any mineral through weathering reactions will have the same 87Sr/86Sr as the mineral itself.
Therefore, differences in 87Sr/86Sr among ground waters require either (a) differences in mineralogy along contrasting flowpaths, or (b) differences in the relative amounts of Sr weathered from the same suite of minerals.