Pervasive and Persistent Large-Volume, low delta 18O Silicic Magma Generation at the Yellowstone Hotspot, 12.7-10.5 Ma: Ion Microprobe Analyses of Zircon in the Cougar Point Tuff

High precision analyses of oxygen isotopes measured by ion microprobe in cores and rims of 111 zircon crystals from the voluminous Cougar Point Tuff (CPT) of the Bruneau-Jarbidge eruptive center in southern Idaho, USA, confirm the presence of the largest low δ18O silicic volcanic province yet discovered (c.f. Boroughs, 2005). These data provide an unprecedented, high resolution view into details of large-volume magma generation and source materials at a major eruptive center of the Yellowstone hotspot. The CPT consists of at least ten members of high-temperature (800-1000 °C) ignimbrites erupted from 12.7-10.5 Ma with a collective eruptive volume estimated at ≥ 7000 km3. Analyses of zircon were performed on the University of Wisconsin Cameca IMS-1280 using a spot diameter of 10 microns, with an average analytical precision for δ18O on the KIM5 standard of 0.3‰ (2 s.d.). Core and rim spot locations were carefully selected using cathodoluminescence and secondary electron imaging to evaluate zoning patterns, identify core locations and avoid inclusions. All but two zircon grains display rims that are depleted in 18O, and ~60 % of crystals display core-rim zonation of 0.5 to 7.4‰. The total range of δ18OVSMOW in cores of the 109 zircons with low δ18O rims is from -1.8 to 6.4 ‰; rims of these zircons range from -3.1 to 3.3 ‰. Within individual units, cores of zoned zircons are generally more variable than rims and vary over a minimum of 1.5 ‰ (Unit IX) and up to 8.2 ‰ (Unit VII). Rims vary 0.4 (Unit IX) to 5.2‰ (Unit XII). Our data indicate that the vast majority of zircons (96%) grew in low δ18O crustal melts, and that multiple discrete lowδ18O sources were involved in magma generation for most eruptive units. Whereas zircon cores display the most variable compositions, rims in several units converge to a more restricted range of δ18O values, although all units but IX show rim variability > 0.8 ‰. The presence of multiple discrete magma volumes at the time of eruption has been documented previously by polymodal assemblages of glass and pyroxene compositions in several units and is consistent with the zircon δ18O data. Temporal patterns of variation in δ18O over the sequence of eruptions are consistent with zircon core inheritance from unerupted residual magmas and/or remelting of solidified remains of earlier CPT magma reservoirs that were depleted in 18O by hydrothermal alteration. Oxygen isotopes in bulk quartz separates measured by laser fluorination range in δ18O from 1.3 to 4.5‰, and temporal variations in quartz track those of average δ18O in zircon rims in each unit, and thus are consistent with a comagmatic origin with zircon. From the fourth eruption in the sequence to the tenth, average δ18O in quartz (=3.4, sd=0.4) and zircon rims (=0.9, sd=0.5) does not change appreciably from one eruption to the next. Measured Delta (Qz-Zrc) values using δ18O in zircon rims range from 1.8-2.6, and most fractionations are consistent with predictions based on mineral thermometry. First order estimates of the averageδ18O of the total eruptive volume (i.e. 7000 km3) range from 2.4 to 3.4‰, making the Cougar Point Tuff the largest volume of low δ18O magma known