Isotopic Studies of Carbonate Rocks

The Beacon Supergroup contains thin lenticular layers of carbonate minerals that are interbedded with virtually all of its formations. In addition, carbonate minerals occur as concretionary nodules, especially in siltstone and shale layers, and as calcite cleats in joints and on bedding planes in coal seams and in the overlying sandstones of Permian and Triassic age. 

The origin of such carbonate rocks is not clear because some may have formed at the time of deposition of the sediment while others originated during initial burial and subsequent diagenesis of the sediment. The difference is important because carbonates that are biogenic or formed as chemical precipitates at the water-sediment interface contain a record of the depo-sitional environment, whereas diagenetic carbonates reflect the properties of the porewater in the sediment after burial (Schmidt and Friedman 1974). 

The carbonate minerals that occur in the sedimentary rocks of the Beacon Supergroup consist primarily of calcite (CaCOj) which contains varying concentrations of strontium, magnesium, iron, and manganese as well as oxygen and carbon. The isotopic composition 

The isotopic composition of strontium (Section 3.4.2 and Appendix 3.7.3) in the crust of the Earth changes with time because of the decay of naturally-occurring radioactive Rb to stable, radiogenic Sr with a halflife of 48.8 X 10 years corresponding to a decay constant of 1.42 X 10 year- (Sections 3.4.2 and 3.7.3). Consequently, the Sr/ Sr ratios of rocks and minerals increase with time at rates that depend on their Rb/ Sr ratios which are calculated from their Rb/Sr concentration ratios (Eq. 3.9, Appendix 3.6.3). 

Faure and T.M. Mensing, The Transantarctic Mountains Rocks, Ice, Meteorites and Water, DOI 10.1007/978-90-481-9390-5 ll, Springer Science+Business Media B.V. 2010 

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