A GLIMPSE INTO THE THERMAL HISTORY OF THE FISH CANYON TUFF REVEALED BY QUARTZ PHENOCRYSTS

What are the storage conditions of large, silicic volcanos? Answering this question is essential for understanding how these systems form and erupt. The likelihood of eruption is largely controlled by the thermal conditions at which volcanic systems are stored in the crust. Recent studies [1, 2] suggest that large, silicic systems are stored near or below the solidus (650-700°C) and at high crystallinity (>50%) for 10s of 1000s of years prior to eruption. Evidence for their eruption are monotonous intermediates. These ignimbrites are homogeneous, crystal-rich and have a near-solidus mineral assemblage. One of the most notable is the ~5000 km³ Fish Canyon Tuff (FCT) in CO. Due to its pluton-like characteristics and resorption of anhydrous minerals, it is suggested to be a remobilized crystal mush [3]. Hence, FCT is optimal to study the storage conditions of silicic volcanos. We determined the pre-eruptive conditions of FCT using Ti-in-Quartz thermometry [4]. Our results suggest extremely homogeneous, near solidus crystallization temperatures of 737 to 672°C depending on Ti activity (0.5 or 0.8). Residence times at these temperatures were calculated by fitting diffusion profiles of Ti in quartz. Three different diffusion coefficients (D) by Cherniak et al. [5], Jollands et al. [6] and Audétat et al. [7] are compared. At 737°C residence times are significantly shorter for D of [5] (<700 a) than [6] (0.07-1 Ma) or [7] (0.15-4 Ma). In the latter case, residence times exceed the maximum storage time of ~0.44-0.7 Ma estimated from zircon ages [8] and eruption history in the vicinity of FCT. Residence times at 672°C calculated for [6] and [7] result in unrealistic times of >8 Ma. Hence, at 672°C, the D of [5] provides the best fit with independent estimates of magmatic residence time. Residence time above 737°C is <2% of the total storage time using D of [5] and imply cold storage. In contrast, D of [6] imply storage significantly above the solidus.

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