Columbia River flood basalts from a centralized crustal magmatic system

The Columbia River Basalt Group in the northwestern United States, comprising about 230,000 cubic kilometres of rock, exhibits unusual patterns in lava distribution, geochemistry and its apparent relationship to regional tectonics. Consequently, there is little consensus on the origin of its magmas. Here, we examine the isotopic ratios of Sr, Nd, Pb and Os and traceelement abundances in Columbia River basalts. The results suggest that most of the lava was produced whenmagma derived from a mantle plume assimilated continental crust in a central magma chamber system located at the boundary between the North American craton and the accreted terranes of Idaho and Oregon. Other, related basalts are the product of mixing between the mantle plume and different types of regional upper mantle. Magma was then transported over a wide region by an extensive network of dykes, a process that has been identified in other flood basalt provinces as well. Interactions of the plume with surrounding upper mantle, and of mantle-derived magmas with regional crust, provide a relatively simple model to explain the more unusual features of the main-phase Columbia River Basalts. The Columbia River Basalt Group (CRBG) consists of ∼230,000 km of tholeiitic basalt, basaltic andesite and scarce andesite that covers much of Oregon, Washington and western Idaho (Fig. 1). Most of the volume of lava (Steens, Imnaha and Grande Ronde basalts) erupted between 16.7 and 16.0Myr ago from vents associated with the Steens and Chief Joseph dyke swarms in eastern Oregon and Washington (Fig. 1), with a subsidiary area of eruption in north-central Oregon (Picture Gorge basalts) associated with the Monument dyke swarm. During this main phase, vent sites migrated rapidly northward from the Steens Mountain area through eastern Oregon to southeastern Washington (Fig. 1), resulting in an age-progressive distribution of Steens (older), Imnaha and Grande Ronde (younger) flows. Initial activity at Steens Mountain produced Lower Steens lavas, followed by more evolved and widespread Upper Steens and contemporaneous Imnaha flows, in turn followed by the 150,000 km Grande Ronde Formation which accounts for >60% of the total volume of the CRBG, with some flows reaching the Pacific Ocean. The Picture Gorge basalts represent a relatively localized, short-lived episode contemporaneous with mid-Grande Ronde activity. Following the main phase of activity, CRBG eruption rates went into a lognormal decline with the later Wanapum and SaddleMountains formations accounting for<10% of the total CRBG volume. Numerous origins, source materials and contributing components have been proposed for CRBG flood lavas, including a mantle plume, convecting mantle±subduction-related components, subcontinental lithospheric mantle, and lithologies within the continental crust. Many workers agree that a mantle plume is ultimately responsible for the CRBG and the Snake River Plain–Yellowstone hotspot track to the east, although different models of plume–lithosphere interaction have been proposed. Most also accept that the post-main phase, isotopically anomalous Saddle Mountains basalts, which are akin to basalts of the Snake River Plain to the east, are derived wholly or partly through melting of ancient, enriched, subcontinental lithospheric mantle. In Sr–Nd–Pb isotopic space, main-phase CRBG lavas lie on trends that radiate from Imnaha basalt (Fig. 2). The ‘Imnaha component’ is therefore present in all main-phase CRBG lavas. This component has He/He of 11.4±0.7 times the atmospheric ratio (RA), which is significantly elevated compared with mid-oceanridge basalt (He/He = 8± 1 RA), and conforms closely to the ‘moderately high He/He’ ocean-island basalt type of Class and Goldstein in its Pb isotope and incompatible element abundance characteristics (see the Supplementary Information). Following others, we identify the ‘Imnaha component’ with the mantle plume responsible for CRBG volcanism and the Snake–Yellowstone hotspot trace. Steens and Picture Gorge basalts lie between the Imnaha component and depleted mantle, as represented by Pacific midocean-ridge basalt, and hence are readily interpreted as the products of mixtures between the mantle plume and depleted mantle (Fig. 2). Despite isotopic similarity, Steens and Picture Gorge basalts have distinct incompatible trace-element ratios. Primitive Lower Steens lavas have similar ratios of large-ion lithophile elements to high-field-strength elements, as do Imnaha basalts (Fig. 3). In contrast, Picture Gorge lavas have elevated ratios of (Cs, Rb, Ba, U, K, Pb)/(Nb, Ta) despite having the lowest Sr/Sr of any CRBG lavas. In fact, the most primitive Picture Gorge basalts strongly resemble primitive island-arc tholeiites. We attribute this to a proportion of a subduction-related component in the depleted mantle that forms part of the source region of the Picture Gorge basalts, consistent with some previous interpretations. A slight tendency for increasing Sr/Sr among more evolved Picture Gorge and Steens lavas may reflect contamination with young accreted crust; note this trend is quite distinct from that of the Grande Ronde lavas, which have Sr/Sr > 0.704 (Figs 2,3). The