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

Abstract:We present two-feldspar thermometry and diffusion chronometry from sanidine, orthopyroxene and quartz from multiple samples of the Bishop Tuff, California, to constrain the temperature stratification within the pre-eruptive magma body and the timescales of magma mixing prior to its eruption. Two-feldspar thermometry yields estimates that agrees well with previous Fe-Ti oxide thermometry and gives a ~80 °C temperature difference between the earlier-and later-erupted regions of the magma chamber. Using this thermometry, we model diffusion of Ti in quartz, and Ba and Sr in sanidine as well as Fe-Mg interdiffusion in orthopyroxene to yield timescales for the formation of overgrowth rims on these phenocryst phases. Diffusion profiles of Ti in quartz and Fe-Mg in orthopyroxene diffusion both yield timescales of <150 years for the formation of overgrowth rims. In contrast, both Ba and Sr diffusion in sanidine yield nominal timescales 1-2 orders of magnitude longer than these two methods. The main cause for this discrepancy is inferred to be an incorrect assumption for the initial profile shape for Ba and Sr diffusion modelling (i.e., growth zoning). Utilising the divergent diffusion behaviour of Ba and Sr, we place constraints on the initial width of the interface and can refine our initial conditions considerably, bringing Ba and Sr data into alignment, and yielding timescales closer to 500 years, the majority of which are then within uncertainty of timescales modelled from Ti diffusion in quartz. Care must be thus taken when using Ba-in-sanidine geospeedometry in evolved magmatic systems where no other phases or elements are available for comparative diffusion profiling. Our diffusion modelling reveals piecemeal rejuvenation of the lower parts of the Bishop Tuff magma chamber at least 500 years prior to eruption. Timescales from our mineral profiling imply either that diffusion coefficients currently used are uncertain by 1-2 orders of magnitude, or that the minerals concerned did not experience a common history, despite being extracted from the same single pumice clasts. Introduction of the magma initiating crystallization of the contrasting rims on sanidine, quartz, orthopyroxene and zircon was prolonged, and may be a marker of other processes that initiated the Bishop Tuff eruption rather than the trigger itself. 2 AbstractWe present two-feldspar thermometry and diffusion chronometry from sanidine, orthopyroxene and quartz from multiple samples of the Bishop Tuff, California, to constrain the temperature stratification within the pre-eruptive magma body and the timescales of magma mixing prior to its eruption. Twofeldspar thermometry yields estimates that agree well with previous Fe-Ti oxide thermometry and gives a ~80 °C temperature difference between the earlier-and later-erupted regions of the magma chamber. Using this thermometry, we model diffusion of Ti in quartz, and Ba and Sr in sanidine as well as Fe-Mg interdiffusion in orthopyroxene to yield timescales for the formation of overgrowth rims on th...

biotite, orthopyroxene, clinopyroxene) and matrix glasses are presented from juvenile materials representing the full Bishop Tuff sequence from the earliest fall unit (F1) to the latest ignimbrite package (Ig2Nc). These data are combined with published information to investigate the nature and zonation of the pre-eruptive Bishop magma chamber. Our data confirm that this magma chamber was a single unitary body which was thermally and compositionally zoned. The zonation was largely established prior to the growth of crystals, and also prior to mixing in the lower parts of the chamber induced by late-stage intrusion of a magma of contrasting composition and slightly hotter temperature (the 'bright-rim' magma). Sparse mixed swirly and dacitic pumices show enrichments in Ba, Sr and Ti that identify these pumices as possible representatives of the 'bright-rim' magma. A model (revised from previously published work) for the pre-eruptive magma chamber comprises three main parts: (i) an upper, volumetrically dominant (~2/3), relatively unzoned region which was the source of the earlier, eastern-erupted ignimbrite units and their coeval fall units; (ii) a volumetrically minor transition zone which shows evidence for minor degrees of mixing and was the dominant source for the latest, eastern-erupted part of Ig1Eb (Sherwin subunit) and the earlier part of the northern-erupted ignimbrite (Ig 2Na); and (iii) a lower, volumetrically subordinate (~1/3) region which was affected by mixing with the 'bright-rim' invasive magma in the lead-up to the eruption, and fed later northern-erupted units. Ingress of the 'bright-rim' magma introduced orthopyroxene and bright-rimmed zircon crystals, induced partial resorption then overgrowth of rims enriched in Ti, Sr and Ba on sanidine and quartz, and development of zoning in clinopyroxene. Based on pumice proportions and associated crystal and glass chemistries through the eruptive sequence, we infer that the roof and floor of the magma chamber were stepped down to the north, such that the transition Sensitivity: Internal zone magma formed the floor of the southern part of the melt-dominant chamber and the roof of the northern part. Our data reinforce the previous concept of a single compositionally and thermally zoned Bishop magma chamber and additionally provides a temporally constrained role for pre-eruptive magma mixing and the introduction of melts and minerals with contrasting compositions to the resident Bishop magma.

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