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

Katy J. Chamberlain, Daniel J. Morgan, Colin J.N. Wilson

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83 Citations (Scopus)


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 evacuation. Two-feldspar 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 the thermometry results, 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 crystal phases. Diffusion profiles of Ti in quartz and Fe-Mg in orthopyroxene 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 exists). 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 crystallisation 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.

Original languageEnglish
Article number1034
Pages (from-to)1-24
Number of pages24
JournalContributions to Mineralogy and Petrology
Issue number1
Publication statusPublished - 1 Jul 2014

Bibliographical note

Funding Information:
Acknowledgements We thank Richard Walshaw (University of Leeds) and Joe Wooden (Stanford University) for assistance during CL imaging, and Aidan Allan and George Cooper for discussions. KJC was supported by an NZ International Doctoral Research Scholarship administered by Education New Zealand, a Jack Klein-man Award administered by the U.S. Geological Survey, and two Victoria University of Wellington Faculty Strategic Research Grants. CJNW thanks the Royal Society of New Zealand for support through Marsden Fund grant VUW0813 (which also facilitated the participation of DJM) and a James Cook Fellowship. Analyses at Leeds were part-funded by DJM through School of Earth and Environment start-up funds. We thank Kate Saunders and an anonymous reviewer for their helpful reviews and Jon Blundy for editorial handling.


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