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

• Published in: Contributions to mineralogy and petrology 2004-01, Vol.146 (5), p.590-605
• Main Authors: Schmitt, A. K., Simon, J. I.
• Format: Article
• Language: English
• Subjects: Boron; Calderas; Case studies; Chemical elements; Crystallization; Degassing; Ionization; Isotopes; Low temperature; Mass spectrometry; Mineralogy; Minerals; Petrology
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-003-0514-6

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 (delta11B=+5.0 plus or minus 0.9 permil and +5.4 plus or minus 5 permil, respectively); (2) SIMS spot-analyses on H2O-poor obsidian (~0.15 wt% H2O) from younger Glass Mountain Dome YA (average delta11B=+5.2 plus or minus 1.0 permil 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 delta11B=+3.2 plus or minus 0.8 permil, MSWD=0.4). Insignificant variations in delta11B 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 delta11B values (average delta11B=+2.2 plus or minus 0.8 permil and - 0.4 plus or minus 1.0 permil) that are in strong contrast to the boron isotopic composition of post-caldera crystal-poor rhyolites (27 ppm B; delta11B=+5.7 plus or minus 0.8 permil). These variations in delta11B 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. [PUBLICATION ABSTRACT]