The Ontong Java Plateau

The Alaska-size Ontong Java Plateau (OJP) in the southwest Pacific is the largest of the world's large igneous provinces and formed entirely in an oceanic environment. Limited sampling of the upper levels of basaltic basement reveals strongly bimodal ages of ≈122 Ma and ≈90 Ma. Geochemical signatures indicate two isotopically distinct, ocean-island-like mantle-source types for the 122 Ma basalts and that the 90 Ma source was almost identical to one of the 122 Ma sources, strongly suggesting that a single mantle plume caused both eruptive events. In the 125-90 Ma period, the OJP appears to have been located near the Pacific Plate Euler poles and thus moved little relative to a postulated hotspot at about 42°S, 159°W the early phase of emplacement probably also occurred near a spreading center, but substantial volumes were emplaced off-axis. The eastern lobe of the plateau appears to have been rifted shortly after 90 Ma. Incompatible and major element data are consistent with 20-30% polybaric partial melting of a peridotite source, beginning in the garnet stability field and continuing in the spinel field. Most existing basaltic basement samples appear to have experienced 30-45% of crystal fractionation; the resulting cumulates would be wehrlitic to pyroxenitic in composition, with an average density of ≈3.25 g cm^-3. We conclude that these cumulates form much of the plateau's high velocity (≈7.6 km s^-1) basal crustal layer. The relatively high density of this layer appears to have prevented emergence of much of the OJP above sea level despite a total crustal thickness exceeding 30 km. Although the deepest levels of the crust could be eclogitized (further increasing density), post-emplacement subsidence of the plateau has probably been tempered by the presence of a roughly 85-km-thick melt-depleted mantle root with a relatively low density (≤3.25 g cm^-3) for mantle material. Estimates of partial melting deduced from the apparent thickness of the mantle root imply that the OJP formed by 17-31% partial melting, in excellent agreement with geochemical modeling.