Oxygen Isotope Character of the Lake Owyhee Volcanic Field, Oregon

Oxygen isotope analyses of zircons from lavas and tuffs from the Lake Owyhee Volcanic Field (LOVF) of east central Oregon unequivocally demonstrate the presence of mid-Miocene low-δ¹⁸O magmas (δ¹⁸O_zrc<4.7 ‰). Despite the growing data set of low-δ¹⁸O melts within, and proximal to, the Snake River Plain (SRP) Large Igneous Province, debate persists regarding both the mechanisms for low-δ¹⁸O magma petrogenesis, and their relative influence in the SRP. The LOVF is associated with widespread silicic volcanism roughly concurrent with the eruption of the Steens-Columbia River Basalt Group between ~17-15Ma. Silicic activity in the LOVF is limited to 16-15Ma, when an estimated 1100km³ of weakly peralkaline to metaluminous rhyolitic lavas and ignimbrites erupted from a series of fissures and calderas. Geographically, the LOVF overlaps the Oregon-Idaho Graben (OIG), and straddles the ⁸⁷Sr/⁸⁶Sr= 0.704 line which, together with the 0.706 line to the east, delineate the regional transition from the North American Precambrian continental crust to the east to younger Phanerozoic accreted terranes to the west. Here we report high accuracy ion microprobe analyses of δ¹⁸O in zircons using a 10-15μm spot, with average spot-to-spot precision ±0.28‰ (2SD), to investigate intra-grain and intra-unit δ¹⁸O_zrc trends for LOVF rhyolites. Due to its high closure temperature, chemical and physical resistance, and slow oxygen diffusion rates, zircon offers a robust record of magmatic oxygen isotope ratios during crystallization and provides constraints on the petrogenesis of Snake River Plain (SRP) low-δ¹⁸O melts. Individual zircons from LOVF rhyolites show no evidence of core-rim δ¹⁸O zoning, and populations exhibit ≤0.42‰ (2SD) intra-unit variability. Unit averages range from 2.2 to 4.3‰, with the lowest values in caldera-forming ignimbrites, but all units show evidence of crystallization from low-δ¹⁸O melts. Quartz and feldspar analyses by laser fluorination (precision ±0.20‰, 2SD) indicate δ¹⁸O_qtz ranges between 5.7 and 6.8‰, and δ¹⁸O_Kfs between -5.4 and +6.7‰. Δ¹⁸O_(qtz-zrc) ranges from 2.1-2.5‰, while Δ¹⁸O_(Kfs-zrc) varies from 2.3 to -7.7‰. Quartz values are consistent with equilibration at magmatic temperatures, however, at least some feldspars have undergone subsolidus exchange and feldspar or whole rock δ¹⁸O values are not reliably igneous. The observed low-δ¹⁸O_zrc values document high temperature interaction of magmatic protoliths with meteoric waters. Current models to account for low-δ¹⁸O SRP rhyolites include remelting of hydrothermally altered cogenetic volcanic and subvolcanic rocks, or partial melting of deeper-seated low-δ¹⁸O source rocks altered either by SRP hot spot activity or during an earlier event. The SRP represents the greatest known concentration of low-δ¹⁸O magmas on Earth, arguing against pre-hotspot models. It's likely that plume-related doming and rifting created unusually deep and pervasive pathways for circulation and exchange between surface waters and magma source rocks.