ABSTRACT

Conventional (multi-)mineral Rb-Sr isochron method requires several mineral species. Not all rock-forming minerals, however, provide the same chronological information. The closure temperature and the extent of weathering differ from mineral to mineral. On the other hand, the whole-rock isochron method has ambiguities on the initial Sr isotope equilibrium. In order to eliminate these complications and to date individual events related to each minerals in a given rock sample accurately, single mineral Rb-Sr isochron method was developed and applied to two different types of Cretaceous volcanic rocks in central Japan, Agigawa welded tuff sheet (Nohi Rhyolite Complex) and quartz porphyry (a dyke intruded into the Toki granite). Precise drilling was carried out on single phenocrystic grains of K-feldspar and chlorite (latter from quartz porphyry only) in order to obtain several aliquots from a single mineral grain. The internal difference of parent/daughter isotope ratios is expected fortuitously. Rb-Sr multi-mineral isochron method failed to yield a precise age from the Agigawa welded tuff sheet, probably because of partial alteration. A single grain K-feldspar age of 72.0 ± 4.1 Ma obtained from the same sample is likely to date annealing of the K-feldspar grain by the intrusion of the Toki granite. In the case of the quartz porphyry dyke, three K-feldspar grains gave the ages of 79.9 ± 6.8 Ma, 71 ± 14 Ma and 82 ± 18 Ma. Combined K-feldspar age for all separates from the three grains is 77.0 ± 4.6 Ma, probably corresponding to the intrusion of the dyke. On the other hand, a combined single mineral isochron age of chlorites (five grains) in the same quartz porphyry dyke is 54.5 ± 4.6 Ma. It seems to date the chlorite formation, which corresponds to the subsequent hydrothermal alteration. Single mineral isochron method proved its effectiveness in the case when Rb-Sr isotope system was partially disturbed between different mineral phases.