Rb[sbnd]Sr ages and Nd isotopic compositions of melt inclusions from the Bishop Tuff and the generation of silicic magma

Abstract

RbSr ages of melt inclusions in quartz have the potential to provide a unique and illuminating record of the differentiation and accumulation history of silicic magmas. Here we report the first Sr and Nd isotopic measurements of melt inclusion bearing quartz (MIBQ), extracted from the Bishop Tuff (BT), representing what is arguably the world's most controversial yet classic zoned silicic magmatic system. Early erupted BT airfall pumice, representing material from the top of the magma chamber, yields individual quartz crystals with Rb/Sr > 70 and variable apparent differentiation ages of 1,420 ± 80 ka, 2,100 ± 100 ka, 2,150 ± 150 ka and 2,500 ± 200 ka. A bulk quartz separate has an apparent age of 1,900 ± 300 ka, while a quartz separate from an individual airfall pumice has an apparent age of 1,330 ± 80 ka. These ages are similar to the RbSr differentiation ages (2,047-1,894 ka) and k-Ar eruption ages (2,100-1,300 ka) of the adjacent early pre-caldera rhyolites at Glass Mountain (GM), indicating that the upper portions of the BT magma chamber included components that episodically differentiated over the same time interval as early Glass Mountain volcanism. However, at this stage the magmatic systems were separate, as indicated by εNd of −1 for airfall melt inclusion, similar to the BT and late GM rhyolites, compared to εNd of −3 for early GM rhyolites. In contract to BT airfall, melt inclusions from intermediate and later erupted BT ignimbrite pumice, from deeper levels of the magma chamber, give younger apparent differentiation ages: 1,040 ± 140 ka (late erupted), 1,180 ± 80 ka (intermediate erupted clast 1), and 1,060 ± 110 ka, 1080 ± 110 ka (intermediate erupted clast 2, two quartz size fractions). These ages are indistinguishable from the RbSr differentiation age of the late pre-caldera GM rhyolites, 1,140 ± 80 ka. The εNd of intermediate and late erupted melt inclusions is −1, the same as BT airfall melt inclusions.These data are consistent with an early episodic development of the evolved upper portions of the BT magma chamber. However, we find no evidence that most of the differentiation took place at around 2 Ma, as recently suggested on the basis of 40Ar39Ar dating. Most of the Bishop Tuff magma, particularly the later erupted portions, differentiated in major events at around 1.0–1.2 Ma. Early differentiated melts repeatedly accumulated in the upper portions of the chamber concomitantly with differentiation, accumulation and eruption in the nearby Glass Mountain magma reservoir, which possibly represented a separate cupola to a larger complex system. By ∼ 1.2 Ma the Bishop Tuff magmatic system expanded to include the Glass Mountain region. We find no evidence for differentiation younger than ∼ 1.0 Ma in any samples from the Bishop Tuff (or Glass Mountain), confirming the view that over the > 300,000 years that elapsed before eruption of the Bishop Tuff, the stratified magmatic system remained largely stable.