Rhyolite genesis at the Picabo Volcanic Center of the Snake River Plain: Progressive recycling of hydrothermally altered rhyolites revealed by high resolution analysis of individual zircons

The Picabo eruptive center of the Snake River Plain (SRP) produced a series of normal and low δ¹⁸O rhyolites from 10.44 Ma to 6.62 Ma, providing the first evidence of progressive recycling of hydrothermally altered rhyolites during the formation of a caldera complex. In this study we present a characterization of ignimbrites and associated lavas based on U-Pb ages and δ¹⁸O compositions of individual zircon cores measured by ion microprobe, phenocryst δ¹⁸O values measured by laser fluorination, whole rock ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd compositions, and whole rock geochemistry. Our data define rhyolite genesis at the Picabo volcanic center through time and have implications for the transition between volcanic centers. Caldera complex evolution at Picabo began with eruption of the 10.44 ± 0.27 Ma Tuff of Arbon Valley (TAV), a chemically zoned unit with a normal δ¹⁸O_melt value (8.15‰), very high ⁸⁷Sr/⁸⁶Sr (up to 0.734430) and very low ɛ_Nd (-18). Eruptions continued with the ~9.1 Ma Two-and-a-Half-Mile Rhyolite (Kellogg et al., 1988), a unit significant in that it has an even lower ɛ_Nd than the TAV and a normal δ¹⁸O_melt value (8.10‰). This low ɛ_Nd of -23, of the Two-and-a-Half-Mile Rhyolite, reveals that greater than 40% of Archean crust was assimilated. These normal δ¹⁸O eruptions were followed by a series of lower δ¹⁸O eruptions distinguishable by Sr and Nd isotopes and whole rock chemistry. The 8.25 ± 0.26 Ma Rhyolite of West Pocatello has the lowest δ¹⁸O_melt value (3.34‰) of these eruptions, and based on nearly identical age, ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and whole rock chemistry, we correlate it to a 1,000 m thick intracaldera tuff (present in the INEL drillcore). Along with a distinct decrease in δ¹⁸O, from the TAV to the Rhyolite of West Pocatello, there is a corresponding increase in δ¹⁸O_zircon heterogeneity from the TAV (1‰ variation) to the low δ¹⁸O units with the greatest δ¹⁸O_zircon diversity (up to 5‰). Although morphological evidence for calderas is obscured, the following observations indicate that the Picabo center represents a series of nested calderas with a mechanism of formation similar to the eastern centers, Heise and Yellowstone: 1) 10's of meters thick 10.4-6.6 Ma ignimbrite outflow sheets with great spatial extent; 2) abundance of post TAV low δ¹⁸O units with diverse δ¹⁸O_zircon, indicative of remelting of previously erupted rhyolites; 3) a δ¹⁸O_melt signature distinguishable from contemporaneous eruptions in adjacent volcanic centers. The development of caldera clusters and the critical transitions between major volcanic centers provides deeper insight into magma genesis through "crustal cannibalism" and rhyolite recycling, processes involved in rhyolite genesis at Picabo, Heise, Yellowstone, and likely other caldera complexes in the SRP and worldwide.