Oxygen isotope evolution of the Lake Owyhee volcanic field, Oregon, and implications for the low-δ¹⁸O magmatism of the Snake River Plain–Yellowstone hotspot and other low-δ¹⁸O large igneous provinces

The Snake River Plain–Yellowstone (SRP-Y) hotspot track represents the largest known low-δ¹⁸O igneous province; however, debate persists regarding the timing and distribution of meteoric hydrothermal alteration and subsequent melting/assimilation relative to hotspot magmatism. To further constrain alteration relations for SRP-Y low-δ¹⁸O magmatism, we present in situ δ¹⁸O and U–Pb analyses of zircon, and laser fluorination δ¹⁸O analyses of phenocrysts, from the Lake Owyhee volcanic field (LOVF) of east-central Oregon. U–Pb data place LOVF magmatism between 16.3 and 15.4 Ma, and contain no evidence for xenocrystic zircon. LOVF δ¹⁸O(Zrc) values demonstrate (1) both low-δ¹⁸O and high-δ¹⁸O caldera-forming and pre-/post-caldera magmas, (2) relative increases in δ¹⁸O between low-δ¹⁸O caldera-forming and post-caldera units, and (3) low-δ¹⁸O magmatism associated with extension of the Oregon-Idaho Graben. The new data, along with new compilations of (1) in situ zircon δ¹⁸O data for the SRP-Y, and (2) regional δ¹⁸O(WR) and δ¹⁸O(magma) patterns, further constrain the thermal and structural associations for hydrothermal alteration in the SRP-Y. Models for low-δ¹⁸O magmatism must be compatible with (1) δ¹⁸O(magma) trends within individual SRP-Y eruptive centers, (2) along axis trends in δ¹⁸O(magma), and (3) the high concentration of low-δ¹⁸O magmas relative to the surrounding regions. When considered with the structural and thermal evolution of the SRP-Y, these constraints support low-δ¹⁸O magma genesis originating from syn-hotspot meteoric hydrothermal alteration, driven by hotspot-derived thermal fluxes superimposed on extensional tectonics. This model is not restricted to continental hotspot settings and may apply to several other low-δ¹⁸O igneous provinces with similar thermal and structural associations.