Boron isotopic variations in hydrous rhyolitic melts: a case study from Long Valley, California
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- Schmitt, A.K. & Simon, J.I. Contrib Mineral Petrol (2004) 146: 590. doi:10.1007/s00410-003-0514-6
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In this paper, we present boron isotope analyses of variably degassed rhyolitic glasses from Long Valley, California. The following results indicate that pre-eruptive boron isotopic signatures were preserved in degassed glasses: (1) averaged secondary ionization mass spectrometry (SIMS) measurements of H2O-rich (~3 wt%) melt inclusions from late erupted Bishop Tuff pumice are indistinguishable from positive thermal ionization mass spectrometry (PTIMS) analysis of vesiculated groundmass glass (δ11B=+5.0±0.9‰ and +5.4±5‰, respectively); (2) SIMS spot-analyses on H2O-poor obsidian (~0.15 wt% H2O) from younger Glass Mountain Dome YA (average δ11B=+5.2±1.0‰) overlap with compositionally similar late Bishop Tuff melt inclusions; and (3) four variably degassed obsidian samples from the 0.6 ka Mono Craters (H2O between 0.74 and 0.10 wt%) are homogeneous with regard to boron (average δ11B=+3.2±0.8‰, MSWD=0.4). Insignificant variations in δ11B between early and late Bishop Tuff melt inclusion glasses agree with published experimental data that predict minor 11B depletion in hydrous melts undergoing gas-saturated fractional crystallization. Melt inclusions from two crystal-rich post-caldera lavas (Deer Mountain and South Deadman Dome) are comparatively boron-rich (max. 90 ppm B) and have lower δ11B values (average δ11B=+2.2±0.8‰ and −0.4±1.0 ‰) that are in strong contrast to the boron isotopic composition of post-caldera crystal-poor rhyolites (27 ppm B; δ11B=+5.7±0.8‰). These variations in δ11B are too large to be caused by pre-eruptive degassing. Instead, we favor assimilation of 11B depleted low-temperature hydrothermally altered intrusive rocks subsequent to fresh rhyolite recharge.