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[1] New 40 Ar/ 39 Ar ages of mineral separates and whole rock samples from nine volcanic edifices in the northern Line Islands region, between latitudes 20°N and 6°N, are incompatible with single or multiple hot spot models. Instead, two major episodes of volcanism, each lasting $5 Ma and separated by $8 Ma, occurred synchronously over long distances, not just along the main chain but also at nonaligned edifices. Volcanism during the older episode (81-86 Ma) extended over a distance of at least 1200 km along the eastern part of the complex seamount chain. Volcanism during the younger episode (68-73 Ma) was concentrated in the western part of the chain and may have extended over a distance of >4000 km. Chemical analyses of 68 samples represent a compositionally diverse suite, including tholeiitic, transitional, and alkalic basalt, strongly alkalic basanite and nephelinite, and alkalic differentiates ranging from hawaiite to trachyte. The most diverse assemblage of rocks was recovered from a crosstrending seamount chain south of Johnston Atoll. Although compositions of rocks from the two volcanic episodes overlap, compositions from the younger episode generally are more alkalic and include a larger proportion of highly differentiated compositions. None of the samples from the older episode, but many from the younger one, contain hydrous mineral phases such as amphibole and biotite. Extensive coeval volcanism along major segments of the chain is compatible with decompressional melting of heterogeneous mantle due to diffuse lithospheric extension along pre-existing zones of weakness. Episodes of volcanism are probably related to broad upwarping of the Superswell region in the eastern South Pacific, where these lavas originated.

[1] Seafloor spreading causes abundant magmatism along active ocean spreading centers, but the cause of magmatism along fossil spreading centers is enigmatic. Samples collected from Davidson Seamount, a typical volcanic ridge along an abandoned spreading center in the eastern Pacific, consist of an alkalic basalt to trachyte lava series; transitional basalts were sampled from another part of the abandoned axis, 20 km from the seamount. All samples experienced complex fractional crystallization prior to eruption, but they all share a common, compositionally heterogeneous mantle source. The parental magmas of the transitional basalts were produced from this source at higher degree of melting than those of the alkalic lava series. The composition of Davidson lavas overlaps with those of ridges along other fossil spreading centers and isolated near-and off-ridge seamounts in the eastern Pacific. Together they define a compositional continuum ranging from tholeiitic, normal mid-ocean ridge basalt (MORB)-like to alkalic, ocean island basalt (OIB)-like, similar to lavas that form linear island chains and ridges. We propose that this entire compositional spectrum of intraplate lavas that do not form linear volcanic chains in the eastern Pacific results from variations in the degree of partial melting of a common, compositionally heterogeneous mantle source. This source consists of more easily melted, geochemically enriched components of varying sizes and amounts embedded in a depleted lherzolitic matrix. Large degree of partial melting produces normal MORB-like melts represented by some near-ridge seamount lavas, whereas small degree of melting produces OIB-like fossil spreading center lavas. The small degree of partial melting beneath recently abandoned spreading centers results from either buoyancy-driven decompression melting of the hot lithospheric and asthenospheric mantle material beneath active spreading centers or rapid motion, with respect to the underlying asthenosphere, of abandoned spreading axes that are thickening over a fertile mantle. Mid-Tertiary volcanic rocks in coastal California, which are compositionally akin to intraplate lavas, are interpreted to be small degree partial melts of the same compositionally heterogeneous sub-Pacific mantle that has upwelled through windows in a subducted slab.

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