New Perspectives on the Bishop Tuff from Zircon Textures, Ages and Trace Elements

Abstract

We present zircon textural, trace element and U-Pb age data obtained by secondary ion mass spectrometry (SIMS) (SHRIMP-RG: sensitive high resolution ion microprobe-reverse geometry) from 15 stratigraphically controlled Bishop Tuff samples and 2 Glass Mountain (GM) lava samples (domes OD and YA). Bishop zircon textures divide into four suites, (a) dominant sector-zoned grains, with (b) subordinate grains showing bright rims (lower U, Th, rare earth elements [REE]) in CL imaging, and sparse (c) GM-type grains (texturally similar to zircons from GM dome YA) and (d) Mesozoic xenocrysts from Sierran granitoid country rocks. All Bishop zircons from suites (a) -( c) combined have a weighted mean age of 777.9 ± 2.2 ka (95% confidence) and a tail back to ~845 ka. Our eruption age estimate using the weighted mean of 166 rim ages of 766.6 ± 3.1 ka (95% confidence) is identical within uncertainty to published estimates from isotope-dilution thermal ionization mass spectrometry (ID-TIMS) (767.1 ± 0.9 ka, 2σ) and 40 Ar/ 39 Ar (767.4 ± 2.2 ka, 2σ) techniques, the latter using the 28.172 Ma age for the Fish Canyon sanidine standard. We estimate also an eruption age for GM dome YA of 862 ± 23 ka (95% confidence), significantly older than the currently accepted 790 ± 20 ka K-Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean of 838.5 ± 8.8 ka (95% confidence), implying that the Bishop Tuff system was only active for ~80 kyr, and had effectively no temporal overlap with the GM system. Trace element variations in Bishop zircons are influenced strongly for many elements by sector zoning, producing up to 3x concentration differences between sides and tips within the same growth zones.Contrasting trends in molar (Sc+Y+REE 3+ )/P ratios between sides and tips indicate contrasting mechanisms of substitution in different sectors of the same crystal.Concentrations of Ti in tips are double those in the sides of crystals, hindering applicability of the Ti-in-zircon thermometer, in addition to variations inherent to the 0.15 -0.67 range in values proposed for aTiO 2 . The bright-rim portions of grains are inferred to have crystallized from the same magma as generated the bright rims seen under cathodoluminescence or back-scattered electron imaging on quartz and feldspar, respectively. This less evolved, slightly hotter magma

Timescales of mixing and mobilisation in the Bishop Tuff magma body: perspectives from diffusion chronometry

“…In all cases a ±30 °C (1 ) uncertainty on thermometry is assumed which is then propagated through to uncertainties on the diffusive timescales. The actual temperature controlling diffusion is, however, likely to have been slightly hotter than those modelled here from the cores of crystals due to the crystals being subsequently immersed in the 'bright-rim' forming melt (Hildreth 1979;Hildreth and Wilson 2007;Chamberlain et al 2014: see Table 3 for temperatures used).…”

“…Morgan and Blake 2006) from samples of the Bishop Tuff, eastern California. We also compare our data with alternative information on timescales of the processes involved from high-precision U-Pb chronometry and textural studies in co-erupted zircon (Chamberlain et al 2014). …”

“…However, more recent studies of zircon U-Pb ages and crystal size distribution (CSD) modelling indicate that this is unrealistic and infer timescales of between ~160 kyr to a few thousand years (e.g. Gualda et al 2012a;Pamukcu et al 2012;Reid and Schmitt 2012;Chamberlain et al 2014) for crystals residing in the melt-dominant magma body.…”

Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology

“…3c). The Red Hill and Raumid also showed apparent excess radiogenic Pb (the "High U effect"; Williams and Hergt, 2000, but this effect was not evident in the Bishop Tuff samples; Chamberlain et al, 2014;Ickert et al, 2015). There is not a strong correlation between degree of 274 anomaly and degree of excess apparent radiogenic 206 Pb (Fig.…”

“…However, the 0.8 Ma Bishop Tuff zircons, which do not seem to exhibit this high-uranium effect (Chamberlain et al, 2014;Ickert et al, 2015), have only one analytical spot with a 274 above 0.025, and none above (Fig. 10).…”