Chemical variation related to the stratigraphy of the Columbia River basalt

Study of major element chemical analyses of Columbia River basalt leads to a grouping of most of the analyses into 11 chemical types which are distinguished with little overlap on a SiO₂-MgO variation diagram. Other diagnostic variation diagrams are total iron (‘FeO’)-MgO, K₂O-MgO, and TiO₂-MgO.

A four-unit informal stratigraphy has been adopted in order to define the relations between chemical composition and stratigraphic position. From oldest to youngest, the four stratigraphic units are (1) lower basalt of Bond (1963) and Picture Gorge basalt, (2) lower Yakima basalt, (3) middle Yakima basalt, and (4) upper Yakima basalt.

Most of the Picture Gorge and lower basalt flows are relatively rich in MgO (approximately 4.5 to 7.1 percent) and are distinguished by intermediate SiO₂ relative to MgO. Furthermore, the Picture Gorge basalt generally has low K₂O relative to MgO. The lower Yakima basalt consists almost entirely of flows with relatively low MgO content (approximately 3.0 to 5.5 percent) and with the highest SiO₂ relative to MgO of any flows of the Columbia River basalt. The middle Yakima basalt contains flows of three distinct chemical types, which together cover the same MgO range as the lower Yakima flows but which have considerably lower SiO₂ and higher ‘FeO’ and TiO₂ relative to MgO. Flows in the upper Yakima basalt are of diverse composition; two of the youngest flows are distinguished by having the lowest SiO₂ and highest ‘FeO’, TiO₂, and P₂O₅ relative to MgO of any analyzed Columbia River basalt.

Flows of one or more chemical types may form the dominant lithology in a stratigraphic unit, but single flows of the same chemical types may occur in any stratigraphic unit.

Some lava sampled in the eastern part of the plateau has more TiO₂ than does lava of otherwise similar composition sampled in the western part of the plateau. This is tentatively interpreted as reflecting a heterogeneous composition for the mantle beneath the Columbia Plateau.