Pulsed Eruptions of the Sentinel Bluffs Member, Grande Ronde Basalt, Determined from Geochemical and Paleomagnetic Characterizations of Lava Flows

Abstract:

The identification of compositionally distinct lava flows is essential to determining the stratigraphy, number of lavas, and hence timing and frequency of eruptions in flood basalt provinces. Here, we address this issue using paired geochemical and paleomagnetic sampling of lavas comprising the Sentinel Bluffs Member (SB), the youngest member of the Grande Ronde Basalt (GRB). We develop the SB stratigraphy using geochemical characterizations of lavas based on ratios and mass-normalized abundances of immobile elements, together with an integrated stratigraphy of sections containing continuous exposures of multiple SB flows. Mass normalization is part of a mass analysis methodology that enables precise determinations of magmatic immobile element abundances of lavas variably enriched by alteration-generated mass loss (see Sawlan, M.G., 2013, Alteration, mass conservation, and magmatic compositions of lavas of the Columbia River flood basalt province: Insights from the Sentinel Bluffs Member, Grande Ronde Basalt: AGU Fall Meeting, V13F-2689). Within the SB chemical stratigraphy five chemical series, each of which includes three or four more precisely defined chemical groups, are recognized. Paleomagnetic directions, averaged by chemical series, show distinct differences between lavas of Series I (I=64.8°, D=351.2°, N=9, α₉₅=3.8°), Series II-IV (I=56.5°, D=4.8°, N=29, α₉₅=1.7°), and Series V (I=71.4°, D=326.0°, N=9, α₉₅=4.6°). These differences in paleomagnetic directions of chemical series indicate SB eruptions occurred in pulsed eruptive episodes during which multiple, closely related, and geochemically distinct magmas erupted. Directions for Series II-IV lavas, near the geomagnetic dipole direction, are indistinguishable from one another (Series II: I=53.4°, D=2.0°, N=5, α₉₅=4.4°; Series III: I=55.7°, D=5.6°, N=15, α₉₅=1.8°; Series IV: I=59.6°, D=5.2°, N=9, α₉₅=4.1°) and indicate that these lavas erupted either closely spaced in time or within a more extended period with little paleosecular variation. Overall, this approach of mass analysis geochemistry-based stratigraphy, paired with paleomagnetic directions, forms the foundation for quantitative identification of individual flood basalt eruptions.