Changing Mantle Sources and the Effects of Crustal Passage on the Steens Basalt, SE Oregon: Chemical and Isotopic Constraints

Authors

Nicole E. Moore, Oregon State University
A. L. Grunder, Oregon State University
Wendy A. Bohrson, Central Washington University
Richard W. Carlson, Carnegie Institution for Science
I. N. Bindeman, University of Oregon

Document Type

Article

Department or Administrative Unit

Geological Sciences

Publication Date

6-23-2020

Abstract

Continental flood basalts are more prone to compositional modification from passage through thicker and (or) more felsic crust in comparison to their oceanic counterparts. The Steens Basalt in southeast Oregon (~17 Ma) is among the oldest and most mafic members of the Columbia River Basalt Group and provides a record of the early stages of flood basalt volcanism. We evaluate the balance of mantle sources in time during the onset of Columbia River Basalt Group magmatism and assess the effect of crustal passage using stratigraphically controlled Sr, Nd, Pb, Hf, Os, and O isotopic compositions, as well as whole rock major and trace element data.

Mixing models indicate that depleted and enriched mantle sources identified by previous workers contribute in varying proportions during the life of the magmatic system, with the greatest contribution by depleted mantle when eruption rate and presumed intrusion rate increase. During waxing, enrichment of δ¹⁸O in some flows signals cryptic deep fractionation of abundant clinopyroxene followed by shallow fractionation of olivine ± clinopyroxene ± plagioclase. Os concentrations are among the highest worldwide at a given MgO (0.29–0.86 ppb at 6.0 to 10.9 wt.%). We argue that high Os results from scavenging of sulfides by recharging magmas passing through earlier crystallized magmas. Elevated ⁸⁷Sr/⁸⁶Sr in the latest stage supports modest assimilation of partial melts from mafic accreted terranes, facilitated by thermal priming of crust by persistent magmatism. This work provides a more detailed schematic view of the Steens Basalt magmatic system, from mantle origin through crustal staging.

Comments

This article was originally published in Geochemistry, Geophysics, Geosystems. The full-text article from the publisher can be found here.

Recommended Citation

Moore, N. E., Grunder, A. L., Bohrson, W. A., Carlson, R. W., & Bindeman, I. N. (2020). Changing Mantle Sources and the Effects of Crustal Passage on the Steens Basalt, SE Oregon: Chemical and Isotopic Constraints. Geochemistry, Geophysics, Geosystems, 21(8). https://doi.org/10.1029/2020gc008910

Journal

Geochemistry, Geophysics, Geosystems

Rights

© 2020. American Geophysical Union. All Rights Reserved.