The Bishop Tuff, California: New Insights into Magmatic Timescales and Processes from Micro-Analytical Approaches
Authors:   , Chamberlain Katy Jane 
Published: November 2021

<p>The Bishop Tuff is the product of one of the largest eruptions on Earth in the last 1 Myr. This thesis studies the Bishop Tuff in order to better understand the nature of the pre-eruptive magma body, with an emphasis on the processes that occurred within it and the timescales over which they operated. In situ geochemical analyses of crystals and glass from samples collected throughout the Bishop Tuff stratigraphic succession yields insights into the nature of zoning and mixing within this supervolcanic system. Timescales for zircon growth (inferred to represent longevity of the magma chamber) are investigated using U-Pb dating of zircons. Zircon textural and trace element data obtained by SIMS (SHRIMP-RG) are presented from 15 stratigraphically controlled Bishop Tuff samples and two older Glass Mountain (GM) lava samples. The resulting eruption age estimate derived from the weighted mean of 166 rim ages of 766.6±3.1 ka (95% confidence) is identical within uncertainty to published values from ID-TIMS and 40Ar/39Ar techniques. An eruption age is also derived for GM dome YA (the youngest GM dome) of 862±23 ka (95% confidence), significantly older than the widely used 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. Bishop zircon textures are divided into four suites whose proportions change systematically through the eruptive sequence. Trace element variations in Bishop zircons are influenced strongly by sector zoning for many elements, and thus restrict the value of trace element variations in discerning compositional stratification within the magma chamber. In later-erupted units, bright-rim overgrowths are common, and are inferred to have crystallized from the same „bright-rim‟ magma as generated the contrasting rims seen in CL or BSE imaging on quartz, feldspar and orthopyroxene. From zircon zonation patterns, this less-evolved, slightly hotter magma invaded deeper parts of the chamber represented in the late-erupted northern units possibly up to ~10 kyr prior to eruption. In order to better quantify the timescales of interaction with the „bright-rim‟ magma, two-feldspar thermometry data are presented on multiple Bishop Tuff samples to constrain temperature variations within the pre-eruptive magma body and yield values for diffusion modelling. Two-feldspar thermometry agrees well with published Fe–Ti-oxide thermometry and reveals a ~80 °C uniform thermal gradient between the upper and lower regions of the magma chamber. Using this thermometry, diffusion of Ti in quartz, Ba in sanidine, Sr in sanidine and Fe-Mg interdiffusion in orthopyroxene are modelled to estimate timescales for the formation of overgrowth rims on crystals. Ti in quartz and Fe-Mg in orthopyroxene diffusion both yield timescales of <150 years for the formation of overgrowth rims, although differing by about an order of magnitude in their timing. However, Ba and Sr diffusion modelling in sanidine yields disparate timescales 1-2 orders of magnitude longer than for Ti in quartz. 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. the profile is influenced by growth zoning). Using the comparison with Sr, constraints are placed on the initial width of the core-rim interface and the initial conditions can be refined, bringing Ba and Sr diffusion timescales into mutual alignment and closer to the values from Ti in quartz. This modelling shows that piecemeal rejuvenation of lower Bishop Tuff magma chamber occurred over a period of ~500 years leading up to eruption. In situ major and trace element analyses of sanidine, plagioclase, biotite, orthopyroxene, clinopyroxene, zircon and matrix glass from the Bishop Tuff and two GM lavas are presented to investigate the pre-eruptive stratification of the Bishop magma chamber and its chemical relationship to the GM system. Analyses of samples from the entire Bishop stratigraphy confirm that the magma chamber was thermally and compositionally zoned prior to growth of crystals and the intrusion of the „bright-rim‟ forming magma. Study of rare mixed swirly and dacitic pumice samples shows enrichments in Ba, Sr and Ti (the elements responsible for bright-rim overgrowths in phenocryst phases) and identifies these pumices as possible representatives of the „bright-rim‟ magma. This integrated study of phenocrysts and glass from the Bishop Tuff leads to development of a revised magma chamber model, in which there is a unitary chamber with a stepped or sloping roof. The chamber has an upper, volumetrically dominant (~2/3) part showing no evidence for convection and with unzoned crystals, and a lower part which had experienced mixing of crystals and interaction with the „bright-rim‟ magma. Intrusion of the „bright-rim‟ magma introduced orthopyroxene and dominantly bright zircon crystals, and caused overgrowth of bright rims enriched in Ti, Sr and Ba on sanidine and quartz phenocrysts. Chemical compositions of GM and Bishop Tuff materials show a shared consanguinity, implying common modes of magma generation, yet the generation of GM and Bishop eruptible magma bodies were physically and temporally separate events.</p>