Rb-Sr Chronometry of Magma Differentiation

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A recently published isotopic study of the precaldera rhyolites of Glass Mountain, Long Valley, has shown that the timing of differentiation of rhyolitic magma can be determined from Rb-Sr dating and resolved from the age of eruption (as determined by K- Ar ages). Furthermore, the differences are difficult to explain unless the rhyolitic magmas persisted in the upper reaches of the chamber in stable, stratified zones for long periods, of the order of 105 to 106 years, with negligible mass transfer between the different layers. Such protracted residence periods are considerably in excess of the estimates implied by recently published thermal modelling of the longevity of high-level silicic magma chambers. This project is to extend this research to a comprehensive study of the mineral-glass Rb-Sr systematics and 40Ar-39Ar ages of the Glass mountain rhyolites and to develop similar research on other young silicic systems. The additional volcanic centers which have been selected are Coso (California), Mineral Mountains, Twin Peaks and Honeycomb Hills (Utah), La Primavera (Mexican Volcanic Belt) and Naivasha (Kenya). These particular volcanic centers were chosen because they contain pristine rhyolitic glasses ranging in composition from metaluminous to highly peralkaline with high Rb/Sr ratios, and have already been well characterized in terms of field relations, age, chemistry and petrography.