New Approaches for Identifying the P-T-X-t Histories and Eruption Triggers for Silicic Magmas; An Example Examining the Scaup Lake Rhyolite, Yellowstone Caldera, WY

The crystal cargoes from past eruptions provide petrologic records of the pressure, temperature and composition of a magma body preceding eruption. Recent advances in diffusion chronometry also now enable us to reconstruct the timing of magmatic events shortly before eruption. Here these techniques are combined to unlock detailed P-T-X-t histories of silicic magma bodies leading to eruption, using the 260 ka Scaup Lake rhyolite lava (SCL) from Yellowstone caldera as an example. The SCL contains 30% phenocrysts of reversely zoned quartz, clinopyroxene, orthopyroxene, plagioclase and sanidine. SCL sanidine and plagioclase reveal ubiquitous bright rims that are enriched in Ba, Sr, Ca and in some cases Mg and Ti relative to the grain interior. Major element transects across the full width of the sanidine rims reveal two pronounced changes in composition that can be equated to heating events (older +25°C, younger +100°C) using sanidine-liquid thermometry and compositional relationships predicted by Rhyolite-MELTS. Renewed precipitation of sanidine at higher temperatures could reflect magma ascent and concomitant exsolution of dissolved H2O, the addition of CO2 by new magma, and/or the addition of K-Na-enriched melt derived from melting sanidine-rich cumulates. The increase in magmaphile elements associated with the 25°C heating event indicate this episode of feldspar growth resulted from the injection of a hotter, less evolved magma 10-40 yrs prior to eruption based on diffusion chronometry (Till et al., Geology, 2015). Estimates using natural and experimental crystal growth rates suggest the second heating event of 100°C recorded in the outermost sanidine rims occurred within 1.5-2 yrs of eruption. This is consistent with a subset of the diffusion chronometry results that indicate rejuvenation-eruption timescales of <10 mo.s. Thermodynamic calculations suggest depressurization of 200-300 MPa could produce ≤25-30°C of heating, requiring additional processes to explain the late stage increase in temperature. MELTS modeling reveals the SCL was significantly above the point of H2O exsolution during late crystal growth. Thus it is unlikely second boiling triggered ascent and eruption. Instead it appears the necessary overpressure was provided by a second intrusion in the prior several years to months.