THE CRYSTAL RECORD OF THE TUFF OF ELEVENMILE CANYON, NV

The ~25.1 Ma tuff of Elevenmile Canyon in western Nevada is one of the largest eruptions in the Great Basin with an estimated erupted volume of 2500-5000 km³. We target this voluminous, crystal-rich (25-60 vol.%), trachydacitic to rhyolitic tuff to explore the storage conditions, evolution, and longevity of large volumes of silicic magma in Earth’s crust. We obtained backscattered electron (BSE) images of alkali feldspar (kspar) and plagioclase grains on a scanning electron microscope (SEM) to qualitatively assess zoning textures and identify areas for targeted compositional analysis. Major element contents of feldspar core, interior, and rim zones were measured by electron microprobe (EPMA). Kspar grains fall into 3 textural categories: 1) dark, massive cores and bright rims ranging in thickness from <20 to >200µm; 2) oscillatory zoned with dark, resorbed cores; 3) complex, resorbed cores of kspar or dark, resorbed plagioclase cores or inclusions. Despite this textural complexity, the major element compositions of the kspar grains display a tight range from An_2-5Ab_31-37Or₆₆-₅₈, and show little to no core-to-rim variation. Preliminary Ba in kspar from EPMA ranges from 0.1-3.3 wt.% Ba, with decreasing Ba concentration from core to rim in 72% of grains, with no apparent correlation to zoning textures. Plagioclase also display several textural populations in BSE: 1) dark, resorbed or splotchy cores encased in a bright zone with a dark rim; 2) oscillatory zoned; 3) no zoning. Plagioclase compositions vary more widely than kspar, from An₇₂-₃₀Ab_27-61Or_1-9. They also show clear variations within and between spatial locations: rims vary from An_50-30, interiors from An_68-34, and cores from An_72-36. In the most common plagioclase population, bright regions around dark cores are enriched in An, reflecting a reversal in the expected fractional crystallization trend, after which An decreases. Feldspar compositions and textures are consistent with compositional mixing or recharge of the system. Trace elements will be used to further assess these trends. Ba and Sr in sanidine, Mg and Sr in plagioclase, and Ti in quartz diffusion chronometry will be leveraged to assess timescales of eruptible magmatic storage.