AbstractsEarth & Environmental Science

The Bishop Tuff, California: New Insights into Magmatic Timescales and Processes from Micro-Analytical Approaches

by Katy Jane Chamberlain

Institution: Victoria University of Wellington
Year: 2014
Keywords: Bishop Tuff; Supereruption; Magma chamber processes
Record ID: 1300496
Full text PDF: http://hdl.handle.net/10063/3364


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…