PRE-ERUPTIVE CONDITIONS AND MELTS MODELING OF MAGMATIC DENSITIES FOR IMNAHA AND POWDER RIVER OLIVINE BASALTS

Northeast Oregon hosts chemically diverse lavas including the Columbia River Basalt Group (CRBG, 16.6-6 Ma) and the Powder River Volcanic Field (PRVF, 13-1 Ma). The presence of an older Imnaha (CRBG) lava flow at the top of the Wallowa Mountains indicates that a significant (~2 km) amount of tectonic uplift occurred since emplacement. Mantle tomography indicates a dense anomaly beneath the region —attributed to foundering of the lower lithosphere, stimulated by injection of CRBG magmas. Wanapum CRBG lavas, devoid of olivine, were contemporaneous with PRVF olivine basalt lavas saturated in Ol + Pl+ Cpx. Normally-zoned olivines span a relatively narrow composition (Fo_80-85) and contain Cr-spinel. Spinels on margins of olivine and in the groundmass are titanomagnetite-rich and coexist with euhedral laths of ilmenite. Application of a geo-thermometer and geo-barometer to olivine and spinel compositions yields anhydrous pre-eruptive conditions, as well as temperatures (1190-1250°C) and oxygen fugacities (ΔQFM +0.1 to +0.4). Assuming the density of the upper continental crust as 2.67 g/ cm³, and calculating the corresponding pressures of crustal thinning (43 to 33 km), MELTS modeled magma densities to test whether the loss of the lower 10 km of the lithosphere facilitated the eruption of PRVF olivine basalts. The densities were calculated for the two CRBG basalts (Imnaha 336: 49.48% SiO₂, 11.33% FeO, Imnaha 758: 49.93% SiO₂, 12.54% FeO) and for PRVF olivine basalts (PF12-3: 50.15% SiO₂, 8.89% FeO, PF12-12: 48.86% SiO₂, 9.10% FeO, PF12-33: 51.12% SiO₂, 9.06% FeO) under anhydrous and hydrous conditions (4 wt.% H₂O) at various pressures (0.25, 1.5, 3, 9, and 11 kb) and empirically determined fO₂ using MELTS. No MELTS runs for the PRVF successfully reproduced observed Ol compositions at 9 kb and 11 kb but runs were successful at 0.25kb and 1.5kb at anhydrous conditions. Calculated densities of anhydrous magmas for 0.25-1.5kb range from 2.89-2.90 g/cm³ (Imnaha) and 2.73-2.76 g/cm³ (PRVF). Puzzlingly, these high-density magmas erupted without the presence of significant H₂O. A possible inference is that thinning of the lithosphere and disruption of the crust due to uplift of the Wallowa Mountains aided migration of the PRVF olivine basalts from their peridotitic mantle source.