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

Abstract

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...

Timescales of crustal magma reservoir processes: insights from U-series crystal ages

“…However, as with the data set presented by Reid et al (2011), Chamberlain et al (2014b do find some systematic relationships between trace elements that they infer to reflect melt evolution and differences between the 'normal Bishop' magma and a late addition of a less-evolved composition (the 'bright-rim' composition). In another paper looking at diffusion of trace elements in quartz, orthopyroxene and sanidine, Chamberlain et al (2014a) concluded that the interaction of the 'normal Bishop' magma with the 'bright-rim' composition occurred over at least 500 years prior to eruption, which they envision to be occurring in a melt-dominated body. Simon et al (2014) combined Nd, Hf and Pb isotopic data, trace-element data, Ar -Ar geochronology, and zircon geochronology for Long Valley rhyolites in a model of magma reservoir evolution over the pre-to post-caldera interval.…”

Rapid priming, accumulation, and recharge of magma driving recent eruptions at a hyperactive caldera volcano

“…Temperatures are averages from Barker et al (2015) using the orthopyroxene-liquid thermometer of Putirka (2008). Viscocity (log ) of melt (in Pa s ) calculated using the model of Giordano et al (2008) (Allan et al, 2013;Cooper, 2014;Chamberlain et al, 2014), and therefore the zoning observed in BSE images is inferred to be an accurate representation of the Fe-Mg content.…”

Months between rejuvenation and volcanic eruption at Yellowstone caldera, Wyoming

“…Other studies recognize the importance of alternate initial conditions for diffusion modeling (Costa et al, 2008), including deconvolution of the effects of simultaneous crystal growth and diffusion of Ba and Sr in sanidine (Chamberlain et al, 2014). By employing a third faster diffusing element (Mg), this study reveals that Ba and Sr profiles across the sanidine core-rim boundary reflect changes in melt composition rather than diffusive relaxation across a sharp boundary.…”

“…At appropriate degrees of undercooling, experimental crystallization rates (~10 -10 cm/s) predict ~15 yr for growth of the entire sanidine rim (~500 mm) and 1.5-3.5 mo for the growth of the 4-7 mm concentration gradients (see the Data Repository). Any time elapsed between sanidine dissolution in response to rejuvenation and the onset of renewed crystallization must be <~1200 yr due to preservation of 18 O/ 16 O disequilibrium in the resorbed cores of SCL zircons (Bindeman et al, 2008), but is likely much shorter because the entire population of SCL sanidine crystals would be consumed in only ~30-45 yr within heated rhyolite given their rate of dissolution (see the Data Repository). Diffusion during SCL lava emplacement is likely to have been insignificant because silicic lavas are typically emplaced as a series of quickly quenched lobes (e.g., Tuffen et al, 2013).…”