Boron isotopic variations in hydrous rhyolitic melts: a case study from Long Valley, California

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 (similar to3 wt%) melt inclusions from late erupted Bishop Tuff pumice are indistinguishable from positive thermal ionization mass spectrometry (PTIMS) analysis of vesiculated groundmass glass (delta(11)B=+5.0+/-0.9parts per thousand and +5.4+/-5parts per thousand, respectively); (2) SIMS spot-analyses on H2O-poor obsidian (similar to0.15 wt% H2O) from younger Glass Mountain Dome YA (average delta(11)B=+5.2+/-1.0parts per thousand) 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 delta(11)B=+3.2+/-0.8parts per thousand, MSWD=0.4). Insignificant variations in delta(11)B between early and late Bishop Tuff melt inclusion glasses agree with published experimental data that predict minor B-11 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 delta(11)B values (average delta(11)B=+2.2+/-0.8parts per thousand and -0.4+/-1.0 parts per thousand) that are in strong contrast to the boron isotopic composition of post-caldera crystal-poor rhyolites (27 ppm B; delta(11)B=+5.7+/-0.8parts per thousand). These variations in delta(11)B are too large to be caused by pre-eruptive degassing. Instead, we favor assimilation of B-11 depleted low-temperature hydrothermally altered intrusive rocks subsequent to fresh rhyolite recharge.