Time scales of large volume silicic magma systems: Sr isotopic systematics of phenocrysts and glass from the Bishop Tuff, Long Valley, California

Abstract

The initial Sr isotopic compositions of glass and mineral separates from the 0.74 Ma Bishop Tuff ashflow in eastern California were determined to investigate the time scales of magmatic processes in a large silicic system. It was found that there is substantial isotopic heterogeneity, both between eruptive units and between glass and phenocryst phases of individual units. The frist-erupted, lower temperature units generally have higher initial 87 Sr/ 86 Sr than later crupted, higher temperature units. Within each unit, feldspar phenocrysts have the lowest 87 Sr/ 86 Sr, associated glass has higher 87 Sr/ 86 Sr, and biotite phenocrysts have the highest 87 Sr/ 86 Sr. These isotopic differences were produced by processes in the magma chamber and not by post-eruptive alteration. Two samples were similar Fe−Ti oxide temperatures but from widely separated localities have nearly identifical Sr isotopic characteristics, indicating the existence of compositionally uniform layers of substantial volume within the chamber. Trace element data indicate that the feldspars crystallized from a liquid represented by the associated glass, and that the feldspar-glass pairs are not accidental. The rhyolitic liquids of the Bishoptuff magma chamber apparently experienced increasing 87 Sr/ 86 Sr at a rate too fast for feldspar phenocrysts to remain in isotopic equilibrium. The increasing 87 Sr/ 86 Sr is caused primarily by radioactive decay of 87 Rb in the high-Rb/Sr liquids and not primarily by assimilation of radiogenic wall-rock material. A self-consistent model can be constructed to account for all of the isotopic data except for those on biotite phenocrysts. The time scale for evolution of the system is bounded on the high side at about 500 ky by observations made on precaldera lavas, and on the low side at approximately 300 ky by the time necessary to establish homogeneous layers in an actively differentiating chamber. The deduced time scale is consistent with model Rb−Sr ages, which date the differentiation of low temperature liquids from higher temperature liquids, and is compatible with the observed isotopic disequilibrium between feldspars and glass because of the low diffusivity of Sr in fieldspars (<10 -16 cm 2 /s). The prolonged (about 500 ky) evolution of the Bishop Tuff system was facilitated by a large influx of basaltic material (about 10 -2 km 3 /y) to the base of the system, which compensated for diffusive heat loss from the top and allowed large volumes of magma to maintain low crystal contents for >3x10 5 years. The silicic-magma production rate within the Bishop Tuff magma chamber is estimated to be 10 -3 km 3 /y. The growth rate of alkali feldspar is estimated to be about 10 -14 cm/s based on the Sr isotopic difference between sanidine and glass of the lower Bishop Tuff. The biotite population is inferred to be partially (>50 ppm) xenocrystic, the xenocrysts being introduced to the chamber less than one year prior to eruption.