Transcript 87 Sr

STRONTIUM ISOTOPES
Why and how it is used
in estimating stratigraphic position of
carbonates and evaporites
Review by B.C. Schreiber
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”
RESIDENCE TIME OF STRONTIUM
IN THE OCEANS
>2 M.Y.
MIXING TIME OF STRONTIUM
IN THE OCEANS
~103 YEARS
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.
44 stable
stable
naturally
naturally occurring
occurring isotopes
isotopes
84
84Sr
86Sr
87Sr
Sr (0.56%),
(0.56%), 86
Sr (9.86%),
(9.86%), 81
Sr (7.0%)
(7.0%) and
and
88
88Sr
Sr(82.58%).
(82.58%)
Strontium
is present as a ubiquitous minor element
in the crust of the Earth –
Strontium
is present as a ubiquitous minor element
in the crust of the Earth –
Present in many rock types.
Strontium
is present as a ubiquitous minor element
in the crust of the Earth –
Present in many rock types.
Typically found in concentrations of a
few hundred parts per million
87Sr
87Rb
half-life of 48,800,000 years
44 stable
stable
naturally
naturally occurring
occurring isotopes
isotopes
84
84Sr
86Sr
87Sr
Sr (0.56%),
(0.56%), 86
Sr (9.86%),
(9.86%), 81
Sr (7.0%)
(7.0%) and
and
88
88Sr
Sr(82.58%).
(82.58%)
4 stable
naturally occurring isotopes
84Sr
(0.56%), 86Sr (9.86%), 87Sr (7.0%) and
88Sr (82.58%)
Capo et al., 1998
SOURCES OF Sr
From radioactive decay of 87Rb
85Rb
= 72.165%
87Rb = 27.834%
IN A MAGMA, IGNEOUS PROCESSES
CONCENTRATE RUBIDUM IN THE RESIDUAL MELT
SO THE LATER FORMED MINERALS CONTAIN MORE AND
MORE RUBIDIUM
Primordial 87Sr/86Sr ratio of 0.699,
value derived from meteorites,
NOW HIGHER ON EARTH
due to the decay of 87Rb
Faure, 1991
WEATHERING
WEATHERING
First to
First
to
Crystallize
Crystallize
Fast
Weathering
Bowen’s
Reaction
Series
Goldrich
Stability
Series
Last
to
Last to
Last to
Crystallize
Crystallize
Crystallize
Slow
Weathering
CONTESSA QUARRY
marine carbonates
Modern seawater = 87Sr/86Sr = 0.7092
CONTROLLED BY THE EXTENT THE Sr2+ CAN
SUBSTITUTE FOR Ca2+ IN CALCIUM
BEARING MINERALS
Two sources of 87Sr in any material:
(1) formed primordial nucleo-synthesis
along with 84Sr, 86Sr and 88Sr
(2) radioactive decay of 87Rb.
The ratio 87Sr/86Sr is the
parameter
typically reported
in geologic investigations
Elderfield, 1986
PRESENT DAY SPREADING CENTERS
WHAT EFFECT DOES RAPID/SLOW SEAFLOOR
SPREADING HAVE
?
Based on results of Alt et al, 1986,
WHAT DOES RAPID SEA FLOOR SPREADING DO
TO THE OCEANS?
Elderfield, 1986
Visser, 1989
Sr IN GYPSUM REMAINS WITHIN SULPHATE EVEN WHEN IT
DEHYDRATES
BUT
WHEN IT REHYDRATES, MUCH Sr IS LOST
ALSO
WHEN ARAGONITE GOES TO CALCITE
MUCH Sr IS LOST
CONTESSA QUARRY
marine carbonates
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 to
the 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 organic
coatings is an example of this kind of process.
In a fundamental sense, because the waters in shallow
systems 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.