The alkali earth metal strontium has four stable, naturally occurring
isotopes: 84Sr (0.56%), 86Sr (9.86%), 81Sr
(7.0%) and 88Sr (82.58%). Only 87Sr is radiogenic;
it is produced by decay from the radioactive alkali metal 87Rb,
which has a half-life of 48,800,000 years. Thus, there are two sources
of 87Sr in any material: that formed during primordial nucleo-synthesis
along with 84Sr, 86Sr and 88Sr, as well
as that formed by radioactive decay of 87Rb. The ratio 87Sr/86Sr
is the parameter typically reported in geologic investigations; however,
some hydrogeochemists report values in "delta" units. Because
Sr has an atomic radius similar to that of Ca, it readily substitutes for
Ca in minerals. 87Sr/86Sr ratios in minerals and
rocks have values ranging from about 0.7 to greater than 4.0 (Faure, 1986).
The utility of the rubidium-strontium isotope system results from the
fact that different minerals in a given geologic setting can have distinctly
different 87Sr/86Sr as a consequence of different
ages, original Rb/Sr values and the initial 87Sr/86Sr.
For example, consider the case of a simple igneous rock such as a granite
that contains several major Sr-bearing minerals including plagioclase feldspar,
K-feldspar, hornblende, biotite, and muscovite. If these minerals crystallized
from the same silicic melt, each mineral had the same initial 87Sr/86Sr
as the parent melt. However, because Rb substitutes for K in minerals and
these minerals have different K/Ca ratios, the minerals will have had different
During fractional crystallization, Sr tends to be come concentrated
in plagioclase, 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 (10 or higher)
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 the 87Sr/86Sr ratio.
The dates indicate the true age of the minerals only if the rocks have
not been subsequently altered.
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. 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. This latter situation can arise
in several ways. First, differences in initial water chemistry within a
homogeneous rock unit will affect the relative weathering rates of the
minerals. For example, sections of the soil zone affected by evaporative
concentration of recharge waters or by differences in pCO2 can
be expected to have different 87Sr/86Sr. 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. Third,
a difference in the relative "effective" surface areas of minerals
in one portion of 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 chemical equilibrium over long time-periods.
For waters developed in multi-mineralic rocks or soils, 87Sr/86Sr
in any water parcel usually represents a mixture of Sr from several sources,
and thus the exact contributions from individual minerals are difficult
to determine with the Sr isotopic data alone. However, when considered
in conjunction with water chemistry, the Sr isotopes provide a powerful
tool for distinguishing among solute sources.
Thus far, there have been relatively few studies of applications of
Sr isotopes to catchment modeling; a good review is Graustein (1989). At
the Tesuque Watersheds in New Mexico (USA), extensive study of Sr systematics
have shown that 34% of the Sr in throughfall is botanically cycled, and
66% is derived from leaching of airborne dust from the foliage and represents
a net input of Sr to the ecosystem. Therefore, 20% of Sr in biomass is
derived from the bedrock and 80% from atmospherically-transported dust
Estimates for the release of Ca by weathering can be estimated using
strontium isotopes (Aberg et al., 1989). Miller et al. (1993) report that
87Sr/86Sr can be used to separate the total cations
exported from the catchment into the components derived from mineral weathering
reactions and cation-exchange reactions in the soil. At a forest in the
Adirondack Mountains, New York, they found that 70% of the cations in the
stream are derived from weathering and 30% from exchange reactions. The
87Sr/86Sr of various cation reservoirs was used to
determine the percent of Sr in each that was derived from atmospheric sources;
only about 8% of the Sr in stream water was atmospherically derived. Both
of the above two studies assumed a constant 87Sr/86Sr
for the weathering contribution, an assumption that is probably valid only
for monomineralic or very "young" bedrocks. Considerable variability
in the weathering contribution has been documented in a northern Wisconsin
watershed nested in multi-mineralic sands derived from Precambrian terrain
(Bullen et al., 1996).
Further information can be found in the section: Strontium
Isotopes in Geologic Processes in Clark and Fritz (1997), Environmental
Isotopes in Hydrology (CRC Press).
Other information can be found in the chapter: "Tracing
of Weathering Reactions and Water Flowpaths: A Multi-isotope Approach"
by Bullen and Kendall (1998).
Source of text: This review was assembled by Carol Kendall
and Dan Snyder, primarily from Kendall et al. (1995).
||Aberg, G., Jacks, G., and Hamilton, P.J., (1989). "Weathering rates
and 87Sr/86Sr ratios: an isotopic approach."
J. Hydrol., 109, 65-78.
||Bullen, T.D., and Kendall, C. (1998). "Tracing
of Weathering Reactions and Water Flowpaths: A Multi-isotope Approach".
In: C. Kendall and J.J. McDonnell (Eds.), Isotope
Tracers in Catchment Hydrology. Elsevier, Amsterdam, pp. 611-646.
||Bullen, T.D., Krabbenhoft, D.P., and Kendall, C., (1996), "Kinetic
and mineralogic controls on the evolution of ground-water chemistry and
87Sr/86Sr in a sandy silicate aquifer, northern Wisconsin",
Geochim. et Cosmochim. Acta, 60: 1807-1821.
||Faure, G. (1986). "The Rb-Sr method of dating", In: Principles
of Isotope Geology, Second Edition. John Wiley and Sons, New York,
||Graustein, W.C. (1989). "87Sr/86Sr ratios
meaure the sources and flow of strontium in terrestrial ecosystems."
In: P.W. Rundel, J.R. Ehleringer, K.A. Nagy (Eds.) Stable Isotopes in
Ecological Research. Springer-Verlag, New York. pp. 491- 511.
||Jäger, E. (1979). "The Rb-Sr method." In: E. Jäger
and J. C. Hunziker (Eds.), Lectures in Isotope Geology, Springer-Verlag,
New York. pp.13-26.
||Kendall, C., Sklash, M. G., Bullen, T. D. (1995). "Isotope Tracers
of Water and Solute Sources in Catchments", In: Solute Modelling
in Catchment Systems, John Wiley and Sons, New York, pp. 261-303.
||Miller, E.K., Blum, J.D., and Friedland, A.J., (1993). "Determination
of soil exchangeable-cation loss and weathering rates using Sr isotopes."
Nature, 362, 438-441.