New Petrologic and Geochemical Insights into Differentiation Processes Required to Produce the Lower Steens Basalt, Columbia River Basalt Province, SE Oregon

The Steens Basalt (~16.7 Ma) of SE Oregon is the oldest member and contains the most mafic compositions of the Columbia River Flood Basalt (CRB) event. The lower Steens lavas are more mafic and incompatible element poor than upper Steens flows, which are less magnesian, mildly alkalic, richer in incompatible elements, and mainly basaltic andesites, much like the CRB proper. At Steens Mountain, the lower Steens are thickest and comprise 35-40 mainly compound flows with modest soil development between, suggesting a period of slower effusion rate transitioning to high effusion rates as represented by stratigraphically higher flows that make up the prominent cliffs of the range. Stronger soil development between the uppermost flows implies a waning effusion rate. Whole rock major element data define two distinct trends in the lower Steens; the first has increasing Al2O3 with decreasing MgO at relatively low FeO, whereas the second exhibits increasing Al2O3 with decreasing MgO until an inflection point (~8 wt.% MgO), at which both Al2O3 and MgO decrease. Plagioclase mode is not coupled with Al2O3. MELTS closed system fractionation models across a range of pressures cannot reproduce these trends, suggesting open-system processes were at work. Olivine is homogeneous in some flows (~Fo80-78), but heterogeneous in others (~Fo84-62). Fo range does not correlate with whole rock MgO; two different flows with >10 wt.% MgO have Fo85-70 and Fo84-82. Plagioclase composition is restricted within and between flows (~An75-60). All these data suggest lower Steens magmas underwent large amounts of crystal fractionation and recharge with modest assimilation. Some assimilation requires an aluminous contaminant other than simple plagioclase accumulation. During other periods, magma mixing must have been efficient. Magma Chamber Simulator modeling will provide quantitative estimates for the roles of fractionation, recharge and assimilation in the lower Steens magma system.