THE CRYSTALS LEFT BEHIND - QUANTIFYING TIME BREAKS WITHIN A SUPERERUPTION

Supereruptions are traditionally viewed as run-away events where, once triggered, they proceed unhindered. There are, however, examples where field evidence suggests pauses in eruption progression, such as the 2.08 Ma Huckleberry Ridge Tuff (HRT), the first and largest of the caldera-forming eruptions that comprise the Yellowstone Volcanic Field. The 2500 km³ eruption deposited three ignimbrite packages, Members A, B and C, with near-vent fall deposits beneath Members A and C. Intriguingly, multiple time breaks have been inferred between eruptive units: from days to weeks for the pre-Member A fall deposit and up to decades between Members B and C. Although important to identify and quantify time breaks during a supereruption, the procedure is challenging, as the time breaks are unresolvable with most dating techniques. Therefore, their identification and durations are largely based on qualitative field observations.

Here we consider the hypothesis that the HRT experienced a time break of decades between Members B and C through evaluation of quartz and sanidine crystals erupted at the onset of Member C, where partial decompression and reconfiguration of separate magma bodies could be recorded in the crystal rims. Bright Ti-rich rims occur on ~16% of 123 quartz crystals taken from five horizons, with Ti-in-quartz diffusion modeling yielding average boundary ages of 46¹ or 45,500 years² (diffusion coefficient dependent). Bright Ba-rich rims occur on ~24% of 82 sanidine crystals from the same layers. These rims yield ages of 1,050-4,650 years; however, these are likely overestimates based on the uncertainty of the original boundary profile³. Interestingly, the offset in these time estimates are remarkably similar to those present in other silicic systems (e.g., Lava Creek Tuff³, Bishop Tuff⁴), where rims have been attributed to late-stage recharge into the system, not eruption time breaks. We postulate that these rims and their resulting timescales might be ubiquitous across many erupted rhyolitic magmas, and thus their interpretation requiring further evaluation to deduce the if a particular process can be parsed out.

¹Cherniak et al. 2007 Chem. Geol. 236 65; ²Jollands et al. 2020 Geology 48 654; ³Shamloo & Till 2019 Contrib. to Mineral. Petrol. 174 32; ⁴ Chamberlain et al. 2014 Contrib. to Mineral. Petrol. 168 1034.