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 cryst al size data, provides a unique avenue for elucidating the details of evolu tionary histories of crustal magma systems. Here we report the first measur ements of Sr isotopic compositions of single crystals a a function of size and Sr isotopic profiles constructed through microdrill sampling of sanidin e crystals from a high-silica rhyolite lava from the Taylor Creek Rhyolite, NM. Whole-rock Sr-87/Sr-86 increases monotonically with modal abundance of sanidine phenocrysts, suggesting Taylor Creek magma evolved through a coup led process of assimilation and crystallization. In contrast, sanidine phen ocrysts do not show simple monotonic increases zn Sr-87/Sr-86 as a function of crystal size and core-to-rim stratigraphy. Instead, Sr-87/Sr-86 ratios and Si concentrations of individual sanidines increase with crystal size to a maximum at similar to 4 mm and then decrease with further increase in si ze. Microsampling of two crystals greater than 4 mn in length showed core-t o-rim increase then decrease in Sr-87/Sr-86, whereas a single sanidine crys tal less than 4 mm in length displayed a simple core-to-rim decrease in Sr- 87/Sr-86. Furthermore, in contrast to measured size distributions of crysta ls in volcanic rocks, which commonly decrease exponentially with increasing size, crystal size frequency histograms are bell shaped with decreasing nu mbers of crystals in the smallest size class. All these results are consist ent with a model involving continuous phenocryst nucleation and growth in a crustally contaminated magma into which a lower-Sr-87/Sr-86, lower-Sr magm a war injected. In such a scenario, it is argued that curved crystal size d istributions mirror variations in nucleation rate in response to changes in undercooling as the magma body evolved from as-similation- to recharge-dom inated regimes.