Probing the source and timing of rejuvenation and hybridization in post-caldera rhyolite magmas at Yellowstone Caldera

We find petrographic, isotopic and geochemical evidence for rejuvenation and recycling of subvolcanic intrusions within low δ¹⁸O intracaldera rhyolite lavas erupted following the formation of Yellowstone Caldera. In order to resolve the timing and compositional end-members involved in rejuvenation and hybridization of Yellowstone's subvolcanic magma reservoir, we have analyzed the Pb isotopic composition of clinopyroxene and sanidine phenocrysts and performed U-Pb dating of zircons from the South Biscuit Basin (SBB) and Scaup Lake (SCL) rhyolite lava flows. Both SBB and SCL erupted ca. 260 ka based on indistinguishable ⁴⁰Ar/³⁹Ar ages [1,2] and represent a renewed episode of postcaldera volcanism after a hiatus of ~200 kyr. Zoned phenocrysts of quartz, clinopyroxene, orthopyroxene, plagioclase, and sanidine and accessory zircon and Fe-Ti oxides characterize both SBB and SCL. SCL and SBB clinopyroxene have identical compositions and core-to-rim zoning patterns in Fe, Mg, and trace elements and commonly exhibit exsolution lamellae in their cores, suggesting that subsolidus conditions were attained during the early evolution of these rhyolites. The exsolution-bearing cores are mantled by a zone of relatively high Mg/Fe and low HREE & Rb, which is in turn overgrown by a rim with slightly lower Mg/Fe, and higher HREE & Rb. This zoning pattern suggests rejuvenation of subsolidus rhyolite by the influx of at least two less evolved, hotter silicic magmas. Diffusion modeling of Fe-Mg concentration profiles in clinopyroxene suggests that these events occurred on the order of 10³ yrs prior to eruption. To identify the source of the rejuvenated rhyolite and delimit intra-grain variability, we analyzed the Pb-isotopic compositions of the SBB and SCL clinopyroxene zones via LA-MC-ICPMS. The different zones within SBB and SCL clinopyroxene yield indistinguishable Pb-isotope compositions with ²⁰⁸Pb/²⁰⁶Pb=2.165-2.185 and ²⁰⁷Pb/²⁰⁶Pb=0.882-0.890, which matches the Pb-isotope compositions of the associated sanidine. These Pb isotope compositions are similar to those determined for sanidine, glass, and bulk rock [1,3,4] for the younger (ca. 175-70 ka) rather than the older (ca. 480-520 ka) group of intracaldera rhyolites. High-resolution ion microprobe dating of the unpolished rims of SCL zircon yields two populations of ²³⁰Th-corrected ²³⁸U-²⁰⁶Pb crystallization ages of ca. 350 and ca. 240 ka, with approximately two-thirds of the grains yielding the younger age. The interiors of cross-sectioned zircon grains yield the same bimodal age population but with reversed proportions. The rims of SBB zircon yield crystallization ages of ca. 260 ka. Taken together, these results indicate that the SBB and SCL rhyolites share a common origin and evolution ca. 350-250 ka, distinct from of Yellowstone's older intracaldera rhyolites. The Pb isotope composition and zircon ages suggest that by ~350 ka the subvolcanic reservoir beneath Yellowstone was effectively hybridized due to intrusion of relatively juvenile silicic magma. Rejuvenation of this intrusive material eventually erupted to form the SBB and SCL lava flows. [1]=Watts, K. & Bindeman, I.N. (2012) CMP, v.164, p.45-67. [2]=Christiansen, R.L. et al. (2007) USGS OFR 2007-1071, 94 p. [3]=Vazquez, J.A. et al. (2009) JVGR, v.188, p.186-196. [4]=Pritchard & Larson (2012) CMP, v.164, p.205-228.