Migration of radiogenic strontium during metamorphism

Study of the concentration and isotopic composition of strontium and rubidium in hornblende diorite dikes and sills of Precambrian age from the Panamint Mountains of California showed that these rock systems were enriched in radiogenic Sr during a late Mesozoic metamorphism. Enrichments in radiogenic Sr were observed for total-rock samples which yielded apparent ages of up to 34,000 m.y., although there is no obvious petrographic evidence for such metasomatic changes. These results indicate that some caution is necessary in interpreting the Sr isotopic composition of rocks in terms of their original source or in assuming that ‘total rocks’ form closed systems. In general, the initial isotopic Sr composition should be determined, not assumed.