Megacrystic, High-Pressure Pyroxenes From Lavas Co-eruptive with the Columbia River Basalt Group of Northeast OR, USA: Evidence of Deep Magmatic Storage

Primary author: Arron Steiner
Co-author(s): John Wolff; Joe Boro

Primary college/unit: Arts and Sciences
Campus: Pullman


The Basalts of Magpie Table (BMT) of northeastern Oregon, make up small volume lavas (4 cm) megacrysts of clinopyroxene (cpx) with spinel inclusions. Cpx thermobarometry indicates a two-stage path from the mantle to the surface with >8 wt% Al2O3 cpx grown from a liquid in the mantle at the Moho at ~35 km deep and ~1210-1240°C. The magma with megacrystic cpx then rose to a second magmatic storage chamber at ~ 15 km deep indicated by cpx rim compositions. MELTS models predict olivine to crystallize at ~ 5 kbars and ~ 1100 °C. Olivine diffusion modeling shows two different diffusion times: Type 1 olivines ~ 15 days and Type 2 olivines ~500 days. The differences between Type 1 and Type 2 may be related to a recharge event which triggered the eruption. Type 2 olivines may have been generated before the Type 1 olivines and sat in the magma chamber prior to a recharge which supplied the Type 1 olivines and triggered the eruption.