Document Type

Thesis

Date of Degree Completion

Fall 2016

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Dr. Wendy Bohrson

Second Committee Member

Dr. Chris Mattinson

Third Committee Member

Dr. Walter Szeliga

Fourth Committee Member

Dr. Anita Grunder

Abstract

Flood basalts are gigantic basaltic eruptions that modify the mass of the crust via intrusion of mantle-derived magma, and change its composition through interaction between magma and crust. The Steens Basalt, located in southeast Oregon, erupted approximately 16.8 million years ago, and is the oldest member of the Columbia River Basalt Group, the youngest and best-exposed flood basalt province on Earth. The Steens Basalt has an eruption volume of approximately 31,800 km3 and the duration of eruption is estimated to be between ~50,000 and 300,000 years.

Major- and trace element whole rock data from 111 stratigraphically controlled samples from two vertical transects of Steens Mountain (SE Oregon) combined with mineral compositional data from a subset of these samples yield a time-transgressive record of commotional changes during this flood basalt event. These data document up section evolution from the more mafic, MgO-rich and incompatible element poor lower Steens lavas to the more evolved MgO-poor and incompatible element enriched upper Steens lavas. This well-documented change in composition affords an excellent opportunity to quantify how mantle and crustal influences change during a long-lived, large volume mafic magmatic event.

Computational modeling of whole-rock and mineral compositional changes up-section in Steens Basalt using MELTS and the Magma Chamber Simulator indicates that recharge, magma-mixing, and crystallization dominate the lower Steens, whereas crustal assimilation and crystallization dominate the upper Steens. Thus, the early history of the Steens Basalt is characterized by higher recharge masses, which likely reflect greater mass input from the mantle. Thermal priming of the crust associated with the Lower Steens, magma storage system occurred through heat transfer from high masses of recharge magma coupled with crystallization. Consequently, wall-rock melting promoted assimilation in upper Steens magmas. While modeling also supports recharge in upper Steens, crustal interaction is more significant compared to lower Steens. Thus, the ratio of crustal to mantle input apparently increased with time. Modeling also suggests the crustal magma storage system was located at about 10 km, and the crust was dominantly intermediate composition. These results establish a framework for future quantitative studies of flood basalt magma systems.

Appendix A Whole Rock Chemistry and Precision.pdf (1620 kB)
Whole Rock Chemistry Data and Precision

Appendix B BSE Microprobe Images.pdf (769756 kB)
BSE Microprobe Images

Appendix C Mineral Chemistry Data and Precision .xlsx (1219 kB)
Mineral Chemistry Data and Precision

Appendix D Modeling Process Example.pdf (3035 kB)
Modeling Process Example

Appendix E excursions and estimated masses.pdf (202 kB)
Excursions and Estimated Masses

Available for download on Wednesday, December 22, 2021

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