- Continental flood basalts represent short-lived but immense blasts of mafic magma to the continental crust. The youngest and smallest continental flood basalt worldwide, the Columbia River Basalt, initiated with the eruption of the most mafic member, the Steens Basalt (~16.9 Ma). The Steens Basalt is exposed in southeast Oregon, southwest Idaho and northern Nevada. The flows were fed by dikes near Steens Mountain, with eruptions spanning no more than a few 100,000 years. The Steens Basalt has been informally subdivided into lower and upper units based on compositional distinctions. The lower flows are most mafic, tholeiitic and incompatible trace element poor, whereas the upper flows are less magnesian, mildly alkalic, and generally more enriched in incompatible trace elements. These chemostratigraphic excursions signal variations in petrogenetic processes occurred in time over the life of the flood basalt event.
The work presented here is motivated by questions evoked from the observed compositional changes with stratigraphic position in the Steens Basalt. The driving questions are how a flood basalt system evolves in response to changing mass and energy flux from the mantle, how the balance of recharge, fractional crystallization and assimilation processes impart geochemical patterns to the lavas, and how crustal architecture is modified by the influx of magma during a continental flood basalt event. In order to fully characterize the relative contributions of recharge, assimilation, and fractional crystallization (RAFC) processes to generation of the Steens Basalt, a comprehensive petrologic and geochemical data set is presented, including detailed petrography, whole-rock and mineral chemistry, and isotopic data. All of these data are utilized to interpret geochemical patterns, constrain intensive parameters (i.e., temperature, pressure, initial H2O content, and oxygen fugacity), and model RAFC processes using the Magma Chamber Simulator (MCS) and other modeling tools, with the goal of understanding, describing, and quantifying the full range of open system processes that combined to generate the Steens Basalt.
The volcanological and chemostratigraphic characteristics of the Steens Basalt record a three-stage history: the lower A, lower B, and upper Steens Basalt stages. The lower A stage is represented by a newly discovered and minimally exposed section at Steens Mountain that is characterized by initially heterogeneous flows that become increasing homogeneous upsection. During this stage, cooling and crystal fractionation of the magmas outpaces recharge. The lower B stage contains the most primitive compositions, is volumetrically dominant, and represents waxing of the basaltic pulse. The increasingly magnesian compositions upsection during this stage signal an interval of recharge dominance and high eruptive frequency. In the waning upper Steens Basalt stage, recharge is subordinate to fractionation and assimilation becomes an important contributor. These temporal petrogenetic changes are analogous to chemostratigraphic excursions in the Imnaha and Grande Ronde Basalts of the Columbia River Basalt Group, other continental flood basalts such as the Deccan and Siberian Traps, and to phase and cryptic layering present in layered mafic intrusions. Compositional changes in time in these other systems record a change in RAFC processes and suggest large mafic systems share similar evolutionary histories.
Two component isotopic mixing models of 5% partial mantle melts suggest that a combination of depleted upper mantle, and a more enriched mantle endmember contribute roughly equal proportions to magma generation up through the waxing stage of the Steens Basalt. Additionally, these mixing models indicate the contribution by the depleted mantle component increases during the onset of waning magmatism and that crustal contamination by partial melts from mafic accreted terranes contributes to magma generation during the upper Steens Basalt stage. High Os concentrations in the lower B Steens Basalt indicate consumption of phase in which Os is compatible, either sulfides in the mantle source or previously fractionated from Steens Basalt magmas in the crust. Cryptic deep fractionation of abundant clinopyroxene is suggested by enrichment of δ18O in some flows.
MCS modeling aids in the interpretation of the processes that dominate during the lower B Steens Basalt stage and elucidate additional processes at play during the waxing stage of magmatic input. Two trends in the lower B Steens Basalt, one at high and one at low MgO, initially increase in magnesian character upsection and have distinct evolutionary histories. Best fit models for the high MgO trend suggest that these compositions are staged in the mid to upper crust and are generated by increasingly voluminous recharge events with minor amounts of intervening crystallization. Where a decrease in MgO occurs upsection in the high MgO trend, lavas are modified by a combination of fractionation, cumulate entrainment, and magma mixing. The sparsely phyric flows of the low MgO trend result from fractionation of a primitive high MgO parent at shallower crustal levels, whereas low MgO giant plagioclase basalts are formed by cumulate entrainment into fractionated melts. The shallower low MgO reservoirs are pulsed by magmas from the recharge dominated high MgO trend, such that both high and low MgO trend lavas increase in MgO over the interval during which recharge is dominant. A preliminary integrated mass balance calculation for the lower B Steens Basalt stage suggests possible intrusive to extrusive ratios of ~8:1 to 14:1 and cumulate addition to the crust of 5-28 km. This initial attempt at an upsection integrated mass balance suggests that significant gabbroic underplating occurs over the life of the flood basalt event, indicating thickening of the crust and perhaps requiring foundering and delamination of deep crustal roots into the overlying mantle.
This research provides a framework for comparative studies of chemostratigraphic changes in other continental flood basalts. It also provides a strategy for modeling compositional changes with stratigraphy in large mafic systems using the relatively new Magma Chamber Simulator, and the first attempt to quantify and integrate the mass balance of RAFC contributions through a section of a continental flood basalt.