One of the principal objectives of ODP Leg 192 was to test the plume-head hypothesis for the formation of giant ocean plateaus, and many of the results presented in this volume are consistent with such an origin. The discovery of high-MgO Kroenke-type basalt allows us to calculate the composition of the primary magma and, hence, deduce the nature of the mantle source and the degree of melting. Isotopic (Tejada et al., 2004) and chemical (Fitton and Godard, 2004; Chazey and Neal, 2004) data are consistent with a mildly depleted peridotite mantle source, and phase-equilibria (Herzberg, 2004) and trace element (Fitton and Godard, 2004; Chazey and Neal, 2004) modeling independently constrain the degree of melting of this peridotite source to ~30%. Melting to this extent can only be achieved by decompression of hot (potential temperature > 1500°C) peridotite beneath thin lithosphere. To achieve an average of 30% melting requires that the mantle is actively and rapidly fed into the melt zone; a start-up mantle plume provides the most obvious mechanism. A plume head impinging on thin lithosphere theoretically should have caused uplift of a sizeable part of the plateau above sea level, as in Iceland, and, indeed, at least part of the eastern salient was emergent (Thordarson, 2004). However, the abundance of essentially nonvesicular submarine lava and the absence of any basalt showing signs of subaerial weathering show that all the other sampled portions of the OJP were emplaced below sea level (e.g., Neal et al., 1997; Mahoney, Fitton, Wallace, et al., 2001). Volatile concentrations in quenched pillow-rim glasses suggest eruption depths ranging from 1100 m at Site 1183 to 2570 m at Site 1187 (Roberge et al., 2004).
We have not yet been able to resolve the paradox of apparent high mantle potential temperature coupled with predominantly submarine emplacement. Widespread melting of the mantle following the impact of an asteroid provides a possible means of avoiding uplift (e.g., Ingle and Coffin, 2004), but the resulting magma would be generated entirely within the upper mantle and should normally be expected to have the chemical and isotopic characteristics of Pacific mid-ocean-ridge basalt. OJP basalt is isotopically (Tejada et al., 2004) and chemically (Fitton and Godard, 2004) distinct from Pacific mid-ocean-ridge basalt. Furthermore, no mass extinction occurred at the time of OJP formation, even though the required asteroid would have had a diameter significantly greater than that thought to have been responsible for the extinctions at the Cretaceous/Tertiary boundary (Ingle and Coffin, 2004). A more detailed case against an impact origin for the OJP is set out by Tejada et al. (2004). An eclogitic source does not provide an alternative to the plume hypothesis because the high-Mg parental magma would require almost total melting, and, consequently, a very high potential temperature would still be needed to provide the latent heat of fusion. We can also rule out a hydrous mantle source because the magmas have very low H2O contents (Roberge et al., 2004).
This synthesis reviews the considerable progress that has been made in our understanding of the origin and evolution of the Ontong Java Plateau following its successful drilling during ODP Leg 192. We now have a much clearer view of the range and distribution of basalt types on the plateau, and we have identified a potential parental magma composition represented by Kroenke-type basalt. The age and duration of magmatism is still uncertain because we have still only scratched the surface of the 30- to 35-km-thick OJP crust. However, it now seems plausible that almost the entire plateau formed in a single, widespread magmatic event at ~120 Ma. The identification of a thick succession of volcaniclastic rocks at Site 1184 shows that at least part of the plateau was erupted in a subaerial environment. We conclude that the start-up plume hypothesis appears to fit more of the observations than do any of the alternative hypotheses, but the lack of uplift of the magnitude predicted by the plume hypothesis and the lack of an obvious hotspot track remain to be explained.