Repeated Perturbations, One Supereruption: Peach Spring Tuff, Arizona-California-Nevada, USA

Monday, 8 January 2018
Salon Maule (Hotel Quinamavida)
Calvin F Miller1, Michelle L Foley1, Guilherme A R Gualda1, Lily L Claiborne1 and Ayla S Pamukcu2, (1)Vanderbilt University, Earth and Environmental Sciences, Nashville, TN, United States, (2)Princeton University, Department of Geosciences, Princeton, NJ, United States
Abstract:
The Peach Spring Tuff (PST) is a ~103 km3 ignimbrite, the product of an 18.8 Ma supereruption (Glazner et al 1986; Ferguson et al 2013). Outflow is dominantly phenocryst-poor high-silica rhyolite (HSR), whereas intracaldera tuff and a sporadically exposed proximal unit that caps outflow is phenocryst-rich trachyte (TR). Elemental and isotopic compositions suggest that TR represents cumulate material complementary to extracted melt-rich HSR (Pamukcu et al 2013; Frazier 2013), which is consistent with the eruption sequence: HSR first (melt-rich upper portion), then TR (crystal-rich base). Glasses in TR fiamme have zircon saturation temperatures >100° C higher than HSR glasses and compositions indicating dissolution of cumulate phases (very high Zr/Hf, REE, Ba, Sr). High T is also evident in thick (10s of mm), Ti-rich, U-poor TR zircon rims (Pamukcu et al 2013).

Textural and geochemical evidence document multiple perturbations of the PST magma system on a range of timescales:

(1) The thick TR zircon rims, demonstrating relatively late high T, require >103 yrs for growth

(2) Melts coexisting with some zircon interiors in both TR and HSR suggest previous episodes of growth from high-T melts that dissolved cumulate crystals (modeled with Ti-calibrated zircon Kds, Claiborne et al in press)(>>103 yrs yrs before rims)

(3) Magnetite CSDs reveal a population that grew in ~10-1-101 yrs, probably during decompression that accompanied eruption (Pamukcu et al 2013)

(4) As many as three discrete, complexly mingled glasses are present in single TR fiamme (68, 72, 77 wt% SiO2 – the former two with compositions indicating cumulate dissolution; Foley 2017); glasses have sharp contacts (micron-scale diffusion profiles) that suggest interaction <1 yr before, and probably during, eruption.

Heating and resulting partial dissolution and remobilization of locked cumulate material, together with sparse but widespread mafic magmatic enclaves (Flansburg 2015), might suggest eruption triggering by mafic replenishment. However, the PST system yielded only a single supereuption, and there is no evidence for other eruptions of lesser size. Yet multiple intense thermal events occurred over many thousands of years. This indicates that, at least for the PST system, magma recharge alone was insufficient to trigger eruption.

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