Compositional evolution strontium isotopes

Although Sr/ Sr isotopic evolution for the mantle is only poorly constrained at present, this system has tremendous potential for revealing details of cmst-formation processes through time as well as for providing precise timescales on Earth formation. The test of this awaits determination of accurate initial strontium isotopic compositions from Precambrian samples. The potential for acquisition of this type of data is high, as advances in techniques including both in situ measurements (e.g., Christiensen et al., 1996) and low-level solution work (e.g., Mueller et al., 2000) now allow precise determination of isotopic compositions from extremely small amounts (<10 g) of strontium. This ability makes feasible analyses of rare, relict high-strontium, low-mbidium phases which may be preserved within ancient rocks and minerals and may accurately record mantle compositions. 

The duration of chemical exchange between seawater and basalt can also be determined by isotopic studies of hydrothermal minerals. A variety of techniques have been used, ranging from direct dating by K/Ar and Rb/Sr isochron techniques to comparisons of the initial strontium isotopic composition of alteration minerals with the isotopic evolution of seawater (Gallahan and Duncan, 1994 Richardson et al., 1980). 

The above considerations show that the strontium isotopic composition of seawater is controlled essentially by tectonic evolution, that is, by relative contributions from weathering processes on continents and from the intensity of submarine hydrothermal systems. Over geological time, however, the isotopic compositions of these two fluxes have evolved, because Sr is a decay product of Rb ... 

The starting point for the evolution of strontium isotopes is the Sr/ Sr ratio at the formation of the Earth. This is taken to be the isotopic composition of basaltic achondrite meteorites, thought to have a composition approximating to that of the solar nebula at the time of planetary formation. It is usually referred to as BAEl (Basaltic Achondrite Best Initial) and the measured value is 0.69897 0.000003. 

Faure G (1981) Strontium isotope composition of volcanic rocks Evidence for contamination of the Kirkpatrick Basalt, Antarctica. In O Connell RJ, Fyfe WS (eds) Evolution of the earth, Geodyntunics series, vol. 5. American Geophysical Union, Washington, DC, pp 75-81... 

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