OXYGEN ISOTOPE STUDY OF THE BISHOP TUFF MAGMA CHAMBER: THE ROLE OF CONVECTIVE HOMOGENIZATION

The 0.76 Ma Bishop Tuff (BT) provides a snapshot of a long-lived, batholith-scale magma chamber. Over 100 analyses of oxygen isotope ratios in individual quartz phenocrysts, and magnetite, zircon and glass within individual pumice clasts, were made of main depositional units of extracaldera BT, pre-caldera Glass Mountain, and post-caldera intracaldera rhyolites. We also studied quartz in samples varying in pre-eruptive depths from 3 to 8 km, based on volatiles in melt inclusions studies (W allace et al., 1999; Anderson et al., 2000). Quartz-magnetite oxygen isotope temperatures confirm the existence of temperature differences between hot (815+ALA-C) and cold (715+ALA-C) BT. Subtle and consistent variations of d¹⁸ O(Qz) from 8.19(hot) to 8.36 (cold) are entirely explained by temperature. Values of d¹⁸O(melt) were calculated using experimental quartz-rhyolite fractionations and show surprising <0.1+IDA-, lateral and vertical (4-5 km eruptive draw-down) unif ormity in the >650 km³pre-climactic BT magma chamber. Due to near-eutectic composition, d¹⁸O(melt)=d¹⁸O(magma). In contrast, d¹⁸O values of country rocks show wide variations in d¹⁸O (+-26 to -6+IDA-), and elevated d¹⁸O of BT (7.8+IDA-) requires assimilation of high-d¹⁸O rocks. Oxygen isotope homogeneity contrasts to BT zoning with respect to trace and major element composition, temperature, and crystal content. Older Glass Mountain (2.1-1.2 M a) domes exhibit larger variability of d¹⁸O of their quartz phenocrysts (+AH4-1+IDA-) among different domes. Younger Glass Mountain domes (1.2-0.8 Ma) and post-caldera lavas are similar to BT in values d¹⁸O(Qz) and homogeneity. We interpret t he large scale homogeneity of the BT magma body by coalescence of less homogeneous magma batches, and by convection which averaged isotopic differences. Later, the pre-climactic BT magma chamber became stratified with respect to composition and temperature. The BT magma body demonstrates that its plutonic equivalents (batholiths) are initially well-mixed and homogeneous (0.1+IDA-) with respect to the major element oxygen. 1 variations in batholiths which are often observed, are due to subsequent plutonic cooling and subsolidus hydrothermal alteration.