Periodic Table--Strontium

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 Rb/Sr ratios.

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 (Graustein, 1989).

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).