Date of Degree Completion
Master of Science (MS)
Second Committee Member
Third Committee Member
Fourth Committee Member
Many of the lava flows comprising the Steens Basalt in SE Oregon, the oldest and most mafic formation of the Columbia River Basalt Group (CRBG), contain exceptionally large (up to 5 cm) and abundant (up to 40 modal %) plagioclase crystals. This thesis presents a model for giant plagioclase formation in a complex system that integrates petrographic and in situ plagioclase compositional analyses with whole-rock data and computational modeling to explain systematic textural and chemical differences between plagioclase of the lower and upper Steens Basalt sub-sections.
Lower Steens plagioclase are typically more chemically homogenous than upper Steens plagioclase and exhibit textures and morphologies consistent with nucleation of crystals in equilibrium with the erupted magma. In contrast, upper Steens plagioclase are more chemically zoned and frequently exhibit textural and isotopic disequilibrium consistent with the preservation of antecrystic material. Computational modeling provides further support to the interpretation that plagioclase in the Steens Basalt likely formed in response to the entrainment of a plagioclase-bearing cumulate, and the composition of the magma chamber into which the cumulate plagioclase crystals were entrained dictates the behavior of plagioclase upon entrainment: cumulate plagioclase entrained into a lower Steens-type magma chamber (higher MgO) were mostly to completely resorbed, inducing plagioclase saturation and new crystal growth. In an upper Steens-type magma chamber (lower MgO), plagioclase would be stable, so entrained cumulate plagioclase crystals were likely preserved as antecrysts and coarsened after entrainment. These interpretations provide valuable insight into the magmatic processes that occur in large-volume basaltic systems.
Toth, Conner H., "Giant Plagioclase in the Steens Basalt, SE Oregon: Cumulate Entrainment Revealed by Textural and In Situ Chemical Analysis" (2018). All Master's Theses. 961.
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