Document Type

Thesis

Date of Degree Completion

Summer 2016

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Dr. Wendy Bohrson

Second Committee Member

Dr. Anita Grunder

Third Committee Member

Dr. Chris Mattinson

Fourth Committee Member

Dr. Jeff Lee

Abstract

Flood basalts are enormous volcanic events with volumes of volcanic cover and intrusive equivalents that are affected by and significantly affect the crust. Steens Basalt represents 31,800 km3 of flood basalt lavas that erupted in eastern Oregon ~16.8 Ma in less than 300,000 years. Analytical data of flows from a 1 km vertical exposure at Steens Mtn. documents time-transgressive changes in composition of two geochemically distinct units: (1) lower Steens, MgO-rich lavas with lower incompatible trace element concentrations and 87Sr/86Sr, and (2) upper Steens, MgO-poor, with higher incompatible trace element concentrations and 87Sr/86Sr. Results from the Energy-Constrained Recharge Assimilation Fractional Crystallization (EC-RAFC) computational modeling tool make predictions about the relative roles that magmatic processes (recharge, assimilation, and fraction crystallization) have in time (upsection), thus yielding estimates of the mantle vs. crustal mass contribution to the magmatic system. While all three processes are shown to be critical at Steens, results suggest sub-equal mantle-derived (magma recharge) and crustal (assimilation) input into lower and upper Steens basalts indicating that the fraction of inputs do not significantly change during the flood basalt event. In contrast, the modeled masses of crystals fractionated are significantly higher in the lower Steens compared to upper Steens, suggesting crustal thermal priming and substantial new mass added to the crust. Sub-equal mass balance of mantle and crust upsection doesn’t explain the more evolved character of upper Steens flows, perhaps the residual melt in the system has evolved with time. Also, the little variation in signature of the assimilant suggests the magma reservoirs shoaled to assimilate fresh crust. In support of the EC-RAFC results, four hypothetical magma behaviors are identified (from assessment of petrographic and select whole rock major and trace element data) that involve combinations of all magmatic processes and crystal entrainment. These behaviors are prevalent in the genesis of both lower and upper Steens lavas. EC-RAFC provides the first estimates of crustal vs. mantle mass input, as well as the first flow by flow modeling of geochemical changes. Additional work is needed to refine our understanding of the interaction between mantle and crust as the magmatic system matures.

Available for download on Saturday, February 02, 2019

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