Evolution of Silicic Magma through Assimilation and Subsequent Recharge: Evidence from Sr Isotopes in Sanidine Phenocrysts, Taylor Creek Rhyolite, NM

Isotopic fingerprinting of individual mineral phases, complemented by crystal size data, provides a unique avenue for elucidating the details of evolutionary histories of crustal magma systems. Here we report the first measurements of Sr isotopic compositions of single crystals as a function of size and Sr isotopic profiles constructed through microdrill sampling of sanidine crystals from a high-silica rhyolite lava from the Taylor Creek Rhyolite, NM. Whole-rock 87Sr/86Sr increases monotonically with modal abundance of sanidine phenocrysts, suggesting Taylor Creek magma evolved through a coupled process of assimilation and crystallization. In contrast, sanidine phenocrysts do not show simple monotonic increases in 87Sr/86Sr as a function of crystal size and core-to-rim stratigraphy. Instead, 87Sr/86Sr ratios and Sr concentrations of individual sanidines increase with crystal size to a maximum at ∼4 mm and then decrease with further increase in size. Microsampling of two crystals greater than 4 mm in length showed core-to-rim increase then decrease in 87Sr/86Sr, whereas a single sanidine crystal less than 4 mm in length displayed a simple core-to-rim decrease in 87Sr/86Sr. Furthermore, in contrast to measured size distributions of crystals in volcanic rocks, which commonly decrease exponentially with increasing size, crystal size frequency histograms are bell shaped, with decreasing numbers of crystals in the smallest size class. All these results are consistent with a model involving continuous phenocryst nucleation and growth in a crustally contaminated magma into which a lower-87Sr/86Sr, lower-Sr magma was injected. In such a scenario, it is argued that curved crystal size distributions mirror variations in nucleation rate in response to changes in undercooling as the magma body evolved from assimilation- to recharge-dominated regimes.