Generation and distortion of Rb/Sr whole-rock isochrons - effects of metamorphism and alteration
Hradetzky, Helmuth; Lippolt, Hans J.
European Journal of Mineralogy Volume 5 Number 6 (1993), p. 1175 - 1194
published: Dec 1, 1993
manuscript accepted: Jul 20, 1993
manuscript received: Dec 18, 1991
ArtNo. ESP147050506011, Price: 29.00 €
Abstract The Rb/Sr method is used for dating both rocks and minerals. Aliquots of a homogenized whole rock are called whole-rock samples. Two requirements have to be met for meaningful whole-rock isochrons: identical initial Sr ratios for all samples at the time of formation, and closed system conditions for the Rb/Sr ratio and Sr isotopic composition. Whole-rock isochrons especially those of metamorphic rocks may be disturbed. This study summarizes current knowledge on relevant problems. Magmatic processes generally produce initial isotopic homogeneity. However, heterogeneous isotopic composition of Sr is usually found in rocks derived from crustal-anatectic melts and hybrid magmas. Isotopic homogenization may also be the result of metamorphic overprinting. Greenschist-facies metamorphism, promoted by microtectonism in the presence of an abundant aqueous phase, may cause isotopic adjustments to take place over distances of hundreds of meters. On the other hand, under conditions of amphibolite facies metamorphism isotopic homogenization may be restricted to only a few millimeters or centimeters. Alteration of rocks may occur either by shallow (cold) or by intracrustal (hot) hydrous interaction. Sr-loss from rock specimens is observed in many places. Losses of mainly non-radiogenic Sr isotopes may result in the original age information being preserved. Losses of radiogenic 87Sr may locally be proportional to the Rb/Sr ratios, thus preserving linear arrays of the analytical data in an isochron diagram. Such apparent isochrons are lines with a decreased slope. However, even severely altered rock samples will yield the real age of an intrusion or a phase of metamorphism if the chemical attack took place immediately after their formation. For precise dating the boundary conditions of the Rb/Sr whole-rock dating method rarely appear to be fulfilled. Nevertheless, it seems noteworthy that all reported Rb/Sr ages from altered rocks are lower than ages determined on fresh samples from the same rock. Therefore altered rock samples may yield minimum Rb/Sr ages.