Title

A Geochemical and Multi-Isotopic Approach to Determine Mantle Source and Petrogenesis of Late Cenozoic Basalts in the Western Snake River Plain, Idaho

Publication Date

5-2008

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Major Advisor

Craig White

Advisor

Mark Schmitz

Abstract

Geochemical and multi-isotopic analyses can aide in determining the genesis and evolution of continental basalts in order to understand magma diversification on a variety of spatial scales. In the western Snake River Plain (WSRP), three chemically and isotopically distinct suites of Plio-Pleistocene basalts have been characterized. The M2 suite of lavas (2.0 - 0.9 Ma) is tholeiitic, and has radiogenic Sr and Pb, coupled with unradiogenic Nd. The M3b suite (< 0.80 Ma) is mildly alkaline in composition and has unradiogenic Sr and radiogenic Nd decoupled from radiogenic Pb. The M3a suite is intermediate in age (ca. 0.90 Ma) and shows geochemical similarities to the M2 tholeiites, but isotopic signatures similar to the M3b lavas.

Assuming a common parental magma, assimilation, fractional crystallization, and partial melting models show that group M2 cannot be related to either group M3a or M3b due to the significant differences in isotopic ratios. However, many of the geochemical characteristics of the group M3a tholeiites can be modeled by variations in partial melting of an M3b parental lava, followed by 30 to 70% fractional crystallization. These results necessitate two distinct source reservoirs to account for the strong isotopic variability among the three groups of lavas.

Consistent with previous hypotheses, the M2 tholeiites are attributed to a subcontinental lithospheric mantle (SCLM) source and a dominantly asthenospheric source is proposed for the M3a and M3b lavas. The isotopic signatures exhibited by the M3a and M3b lavas indicate the presence of a HIMU-type mantle reservoir in order to produce the decoupled Pb isotopic ratios from the unradiogenic Sr and Nd isotopic ratios.

Comparing WSRP data to isotopic data previously obtained for the Boise River Group (BRG) at Smith Prairie, Idaho, the Jordan Valley Volcanic Field (JVVF) of the Owyhee Plateau, and the Columbia River Group (CRG) shows similar isotopic and temporal trends on a regional scale. Strong similarities between WSRP and BRG data suggest similar reservoirs have sourced these basalts. In the JVVF, the isotopic data is offset to slightly more unradiogenic Sr and radiogenic Nd, but the data do not show the decoupling of Pb from Sr and Nd in the youngest alkaline lavas. Additionally, the coupled Pb, Sr, and Nd of the JVVF data suggest an asthenospheric source with a DMM contribution. Differences in the isotopic ratios and source reservoirs of the JVVF and WSRP are attributed to differences in the mantle beneath the craton (WSRP) and accreted terranes of oceanic affinity (JVVF). However, like the WSRP, both the JVVF and BRG show a shift from an SCLM source to an asthenospheric source with younger volcanism, implying regional-scale processes occurring, regardless of the nature of the mantle beneath the region. Finally, when the WSRP isotopic data is compared to that of the CRG, the opposite temporal trend is displayed. The oldest CRG lavas have been attributed to an asthenospheric source, trending between HIMU and DMM, whereas the youngest lavas have been attributed to an SCLM source.

The regional model developed from the comparisons between the WSRP and other basaltic provinces of the Pacific Northwest involves the role of an asthenospheric mantle plume beneath this region of North America. As the plume was first emplaced, it generated the oldest depleted-mantle derived basalts of the CRG. With time, the plume melted the SCLM, giving way to the isotopic signatures characteristic of the youngest of the CRG. As the plume spread beneath the WSRP, SCLM derived magmas ascended, producing the oldest basalts in this area. With time, as volcanism migrated toward the interior of the WSRP, the lithospheric component was reduced, yielding the isotopically depleted signatures typical of the younger suites. Reduction of the lithospheric component with time is consistent among the WSRP, BRG, and JVVF, indicating regional tectonic controls on Late Cenozoic magmatism in the Pacific Northwest.

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