Significance of Strontium Isotope Ratios in Theories of Carbonatite Genesis

Abstract

RECENTLY, several investigators^1–3 have compared the ⁸⁷Sr/⁸⁶Sr ratios of carbonate rocks from carbonatite complexes with those of sedimentary limestones. Powell et al.¹ quote a mean value of 0.7035 for carbonatites while limestones are usually greater than 0.7090. All results are normalized to ⁸⁶Sr/⁸⁸Sr = 0.1194 and relative to a value of 0.7085 for normalized ⁸⁷Sr/⁸⁶Sr on the Eimer and Amend standard SrCO₃. Hamilton and Deans reported a mean 87Sr/86Sr ratio of 0.7060 for nine carbonatites. Powell et al. concluded that the difference in ⁸⁷Sr/⁸⁶Sr values demonstrates that carbonatites are not mobilized limestones. For this criterion to be effective, however, two requirements must be fulfilled: (a) there must be a negligible overlap of the strontium isotope ratios of carbonatites and limestones and (b) the ⁸⁷Sr/⁸⁶Sr ratios of limestones must not be altered by metamorphism. Powell et al.¹ showed that a xenolith of Trenton limestone (Ordovician) from the Mount Royal gabbro (Montreal) has essentially the same ⁸⁷Sr/⁸⁶Sr ratio as the Trenton limestone at some distance from the intrusion. From these measurements, they inferred that requirement (b) is generally satisfied. It seems to us, however, that the behaviour of a small xenolith that has merely been recrystallized but not otherwise reacted on by the enclosing magma cannot readily be extrapolated to cover the case of a large mass of limestone that might be melted and intimately incorporated in a magma with extensive reaction.