Strontium Isotopes in Geologic Processes
Strontium is a divalent cation that readily substitutes for Ca2+ in carbonates, sulphates, feldspars and other rock-forming minerals. Like Ca2+, it participates in water-rock reactions, and is a minor component of most groundwaters. Strontium isotopes (87Sr/86Sr) have proven to be a useful indicator of water-rock interaction, and as a tracer for groundwater movement and the origin of salinity.
The strontium isotope ratio for carbonate rocks has been measured throughout the Phanerozoic (Fig. 9-11). This 87Sr/86Sr curve reflects the relative contributions of strontium to the ocean from continental weathering and from hydrothermal activity along mid-oceanic ridges (Veizer, 1989). The primordial 87Sr/86Sr ratio of 0.699, derived from meteorites, has been steadily increasing due to the decay of 87Rb. Modern seawater has 87Sr/86Sr = 0.709 (Faure, 1991), which is intermediate between 87Sr-depleted ocean basalts (~0.703) and 87Sr-enriched continental rocks (0.710 to 0.740).
Most of the Phanerozoic is characterized by a general decrease in 87Sr/86Sr (Fig. 9-11) due to increasing activity along mid-ocean spreading ridges. The late Cenozoic marine sediments experienced a dramatic increase in 87Sr due to climate cooling and increased rates of continental weathering by glaciation. This rapid and steady increase through the Pliocene and Pleistocene provides a high-resolution dating tool for sedimentary rocks. The strong variation in 87Sr/86Sr through Phanerozoic time and between rock types provides for strong contrasts between differing geological terrains.
The abundance of 87Sr, the daughter of 87Rb, is directly linked to the geochemistry of potassium, for which Rb+ will readily substitute. K-rich rocks will have high 87Rb and 87Sr contents, and this is reflected in the 87Sr/86Sr ratio of water with which they have equilibrated. Thus, groundwaters that have geochemically evolved in differing geological terrains will have contrasting strontium isotope ratios. For example, Yang et al. (1996) use this phenomenon to derive from 87Sr/86Sr ratios in the St. Lawrence river a pattern of contributions from various tributaries and landscapes.
Fig. 9-11 Strontium isotopes in sedimentary rocks throughout Phanerozoic time (modified from Veizer, 1989).
A similar approach can be taken if subsurface water-rock interaction influences the geochemistry of saline groundwaters in crystalline and basinal settings or even where normal groundwater has residence times long enough to see the influence of strontium leaching — which may go very fast. Bullen et al. (1996) report strontium isotope data from a very young groundwater system in crystalline terrain. They show that it is possible to recognize water that has been in contact with K- and Sr-poor feldspars and distinguish it from water flowing through K-rich terrain where biotite and K-feldspars dominate the strontium geochemistry. In this way, different groundwater bodies can be distinguished, mixing relationships examined, and allochthonous vs. autochthonous sources of salinity be identified.
An interesting use of 87Sr/86Sr data for a hydrogeological investigation was presented by Scholtis et al. (1996) who investigated the suitability of slightly metamorphosed Cretaceous marl for the containment of radioactive waste. Strontium isotope analyses show a clear differentiation of groundwaters with differing origins. More importantly, the fracture carbonates that precipitated from these waters have the same 87Sr/86Sr composition as the water. Therefore, it was possible to confirm that little or no cross-formational water movement took place, an important observation in a repository project.