Collaborative Research: Crustal Modulation of Flood Basalts--Integrating Field, Geochemical and Computational Data for the Steens Basalts

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Flood basalt eruptions are geologically short-lived, catastrophic volcanic events that pour enormous volumes of lava?tens to hundreds of thousands of times greater than the 1980 Mount St Helens eruption? onto Earth's surface. They are important geologic events that completely change the landscape and also have the potential to impact climate and ecosystems. The only flood basalt in the United States is the Columbia River Flood Basalt (CRB), which dominates the landscape in parts of Washington and Oregon. Most of the CRB lavas erupted mainly 16 -15 million years ago and are noted for being among the world?s largest individual lava flows. Among the oldest part of the CRB is the Steens Basalt (16.8-16 million years old), which overran 53,000 km2 of land surface in what is now Oregon. The Steens Basalts are compositionally more magnesian than the bulk of the CRB and so provide a unique record of the interaction between mantle-derived basalt and the crust (the outer rocky shell of Earth that extends ~40 km below the surface). At Steens Mountain, in southeastern Oregon, as many as 200 lava flows are spectacularly exposed in a nearly kilometer-thick stack that records how magma (molten rock) changed composition during the earliest stages of the CRB event. Fieldwork at Steens Mountain will yield a temporal (older to younger) sequence of lava samples that will be chemically analyzed using an array of tools. These data will be assessed with a range of computer models, including the latest generation of model called the Magma Chamber Simulator. Modeling results will provide estimates of the amount of new magma added to the crust from the mantle, the ultimate source of the basalt, and also how much crust is incorporated into magma. The sequential sampling allows documentation of how these parameters change with time as the magmatic pulse waxes and wanes. The methodology of combining field and chemical data with computer analysis can be broadly applied to other volcanic terrains, and the results of this study provide a quantitative basis for understanding other large volume magmatic events on Earth. We hypothesize that the lower Steens Basalt represents magmatism dominated by high rates of magma input during an earlier, energetically waxing stage of magmatism. In contrast, Upper Steens Basalt experienced more interaction with the crust in a thermally waning system, as the locus of CRB activity migrated north and the crustal magmatic system moved to shallower depths in the crust. These hypotheses will be tested by fieldwork, where a representative suite of lava samples will be collected; collection of whole-rock major and trace elements, O-, Sr-, Nd-, Pb-, Hf-, Os- and He-isotopes, petrography and mineral chemistry data; and thermobarometric calculations and other computational petrologic models. Results of this study will allow the petrologic evolution of Steens magmas to be placed in context with mass and thermal exchange that occurred between crust and mantle during the onset of the Columbia River flood basalt event. The broader impacts include (1) involvement of a diverse group of scientists including 2 PIs, 1 PhD student, 2 MS students and several undergraduates. At least 5 are female scientists (including one Latina); (2) development/refinement and presentation of modeling tutorials; (3) scientific outreach in the form of public lectures and field trips; (4) dissemination of results via publications, and conferences; (5) integration of results and rock samples into classes.