Generation of low ?18O silicic magmas, Bruneau-Jarbidge Volcanic Center, Yellowstone hotspot: evidence from zircons, including oxygen isotopes, U-Th-Pb dating, and melt inclusions

The Yellowstone hotspot is characterized by magmas with both normal and low 18O/16O ratios. The production of isotopically light magmas at the Heise volcanic center (6.6-4.5 Ma) and on the Yellowstone volcanic plateau (2.1 Ma to present) appears to be associated with cycles of multiple caldera-forming eruptions that lead to deep burial and remelting of hydrothermally altered rhyolites. At the Miocene Bruneau-Jarbidge eruptive center in the central Snake River Plain, all of the eruptive products are isotopically light, present no evident cyclical behavior, and increase in mafic content through progressive eruptions. This study describes the results of oxygen isotopic analyses and U-Th-Pb dating on individual crystals of zircon from Bruneau-Jarbidge eruptive center (BJEC) lavas erupted between 11 and 8 Ma. These mostly post-date the large-volume ignimbrites of the Cougar Point Tuff (CPT) erupted between 12.8-10.5 Ma. 109 zircons were analyzed with the Cameca IMS-1280 ion microprobe at the University of Wisconsin to determine ?18O of cores, rims, and interiors. U-Th-Pb ages for zircon spots previously analyzed for oxygen were determined on the Stanford SHRIMP together with trace element analyses including Ti. Zircons from all rhyolite lavas from the BJEC have low ?18O values. The ?18O analyses average 1.47 o/oo, with values ranging from -3.41 o/oo to 8.10 o/oo. The overall eruptive series of the lavas demonstrate that the ?18O of the zircons began with an average value of -0.47 o/oo in the first erupted lava, and ultimately rose to an average value of 1.81 o/oo with the eruption of Dorsey Creek rhyolite at the final stages of the BJEC. The rhyolite lava with the highest average ?18O in zircon was Sheep Creek rhyolite (3.3 o/oo), erupted near the end of the sequence, still nearly 1 o/oo below nonaltered values. Ti-in-zircon temperatures average 930 °C, with values as low as 817 °C and as high as 996 °C. This indicates the sustained high temperature of the magmatic system, even following the large-volume eruptions of the CPT.