|Title:||Boron isotopic variations in rhyolites from Long Valley, California: recycling of hydrothermally altered intrusive margins|
|Authors:||A.K. Schmitt, and J.I. Simon|
|Publication:||Contrib. Mineral. Petrol., v. 146, p. 590‐605.|
|Publish Date:||Jan 2004|
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.