OXYGEN ISOTOPE VARIATIONS ALONG THE YELLOWSTONE HOTSPOT TRACE: CAN A SINGLE MODEL EXPLAIN IT ALL?

Products of Miocene rhyolitic volcanism in the central and southwestern portions of the Yellowstone hotspot trace (YHT) differ physically and chemically from younger rhyolites of the eastern Snake River Plain and Yellowstone regions. The older activity is characterized by hotter (> 850° C) and drier (< 2% H₂O) rhyolitic magmas and a higher volcanic flux. Volcanism also occurs over a diffuse region, and little evidence exists for discrete volcanic centers such as the calderas at Yellowstone and Heise. Rhyolites in the central and southwestern YHT are also strikingly similar over a large distance. Pyroclastic units throughout the 30,000 km² area share distinctive physical features, such as pervasive rheomorphism and low aspect ratios, and generally lack pumice. They also invariably have similar phenocryst assemblages (oligoclase + sanidine ± quartz + augite + pigeonite + ilmenite + magnetite ± fayalite), and in-situ analysis of individual mineral phases often returns very similar major and trace element compositions in units separated by hundreds of kilometers.

Feldspar δ¹⁸O_VSMOW values, and quartz when present, from ~50 units in the central and southwestern YHT range from -1.5 to 4.8‰. Magmatic oxygen isotope ratios are more “normal” in the McDermitt caldera (~16 Ma, 118°W, 5.9 to 7.7‰) at the southwestern end of the hotspot trace. δ¹⁸O values then appear to gradually decline northeastward to the central Snake River Plain (11 - 8 Ma, 115.5°W, -1.5 to 2‰), which has the lowest values observed along the YHT. δ¹⁸O values then steadily increase eastward to the Heise volcanic center and Yellowstone caldera system (7 - 0.1 Ma, 113°- 110°W, 0 to 8‰) where depleted and “normal” rhyolites coexist. We agree with previous workers who suggest that the change in chemical and physical expression of YHT rhyolites from the central and southwestern YHT to the eastern Snake River Plain at ~6 Ma and 113.5°W reflects a fundamental change in the petrogenetic controls on volcanism. Therefore, it is unlikely that a single unified mechanism will explain the anomalous oxygen isotope ratios along the entire length of the YHT.