The Role of Cumulate Plagioclase Entrainment in the Steens Basalt, SE Oregon: Insights from the Magma Chamber Simulator

The Steens Basalt, located in SE Oregon, is the oldest and most mafic member of the Columbia River Basalt Group (CRBG). Because the Steens Basalt contains large (up to 6 cm) and abundant (up to 40%) plagioclase crystals, it is classified as a Giant Plagioclase basalt (GPB). Understanding the origin of these giant plagioclase yields insight into magmatic processes like recharge, crustal assimilation, cumulate entrainment, and fractional crystallization. Because large and abundant plagioclase are found in other flood basalt provinces worldwide, these results could also enhance understanding of magma evolution and storage conditions for GPB worldwide. The Steens basalt is informally subdivided into upper Steens and lower Steens based on geochemical differences and variations in plagioclase textures and morphology. In situ plagioclase data, collected by Toth (2018) revealed that more of the lower Steens plagioclase are in Sr isotope equilibrium with their groundmass compared to upper Steens plagioclase, where fewer plagioclase are in Sr isotope equilibrium with their groundmass. This led to the hypothesis that entrainment of a plagioclase-bearing cumulate into a Steens-type magma chamber was a necessary process to produce the chemical and textural diversity seen in the upper and lower Steens plagioclase. This study employs mass and energy-balanced thermodynamic model called the Magma Chamber Simulator (MCS) to determine dominant magmatic processes affecting Steens magmas during storage and quantify magmatic processes that result in the crystallization of abundant plagioclase. Rhyolite-MELTS was also used to better understand the dominant magmatic processes affecting Steens-type magmas during transport. Thermal, mass, and compositional inputs were iteratively tested in order to replicate whole rock and mineral data, test the hypothesis of cumulate entrainment, and reproduce phase assemblages present in upper and Lower Steens. Lower Steens magmas were dominated by mid to upper crustal recharge, fractional crystallization, and cumulate entrainment. Plagioclase-bearing cumulate entrainment resulted in full to partial plagioclase resorption, followed by abundant new crystallization. Plagioclase formed in lower Steens magmas will therefore be in Sr isotope equilibrium with associated groundmass. Enthalpy transfer from lower Steens magma is hypothesized to have thermally primed the mid to upper crust. Upper Steens magmas were dominated by crustal assimilation and fractionation relative to recharge and also experienced cumulate entrainment. Entrainment resulted in partial resorption, followed by abundant crystallization, which produced plagioclase that are less likely to be in Sr isotope equilibrium with their groundmass. For both lower and upper Steens, an important process during transport of magmas was isothermal decompression, which resulted in full clinopyroxene resorption in lower Steens type magmas. Only models which tested isothermal decompression resulted in the correct differences in phenocrysts between lower and upper Steens. Results define time transgressive establishment of a mid to upper crustal magma system that shifts from a recharge to assimilation and crystallization dominated system where isothermal decompression occurred during ascent