The Malaita Volcanic Group (Petterson, 2004) was divided by Tejada et al. (2002) into two chemically and isotopically distinct stratigraphic units: the Kwaimbaita Formation (>2.7 km thick) and the overlying Singgalo Formation (~750 m maximum exposed thickness). Basalt of the Kwaimbaita Formation was found to be compositionally similar to the basalt forming Units C–G at ODP Site 807 on the northern flanks of the OJP (Fig. F1), whereas the Singgalo Formation is similar to the overlying Unit A at Site 807. Thus, Kwaimbaita-type and Singgalo-type basalt flows with the same stratigraphic relationship are found at two sites 1500 km apart on the plateau (Tejada et al., 2002). A third basalt type, with higher MgO and lower concentrations of incompatible elements than any previously reported from the OJP, was recognized during ODP Leg 192 at Sites 1185 and 1187 on the eastern edge of the plateau (Mahoney, Fitton, Wallace, et al., 2001). We propose the term Kroenke-type basalt because it was discovered on the flanks of the submarine Kroenke Canyon at Site 1185 (Fig. F1).
Tejada et al. (2004) use radiogenic isotope (Sr, Nd, Pb, and Hf) ratios to show that Kwaimbaita-type basalt is found at all but one of the OJP drill sites and therefore represents the dominant OJP magma type (Fig. F3). Singgalo-type basalt, on the other hand, appears to be volumetrically minor. Significantly, Kroenke-type basalt is isotopically identical to Kwaimbaita-type basalt (Tejada et al., 2004) and may therefore represent the parental magma for the bulk of the OJP. Age-corrected radiogenic isotope ratios in Kroenke- and Kwaimbaita-type basalts show a remarkably small range (Fig. F3). Tejada et al. (2004) model the initial Sr, Nd, Pb, and Hf isotope ratios in these two basalt types as representing originally primitive mantle that experienced a minor fractionation event (e.g., the extraction of a small amount of partial melt) at ~3 Ga or earlier.
The remarkable homogeneity of OJP basalts is also seen in their major and trace element compositions (Fitton and Godard, 2004) (Fig. F4). Fitton and Godard (2004) use geochemical data to model the mantle source composition and, hence, to estimate the degree of partial melting involved in the formation of the OJP. Incompatible element abundances in the primary OJP magma can be modeled by ~30% melting of a peridotitic primitive mantle source from which ~1% by mass of average continental crust had previously been extracted. The postulated depletion is consistent with the isotopic modeling of Tejada et al. (2004). To produce a 30% melt requires decompression of very hot (potential temperature > 1500°C) mantle beneath thin lithosphere. Thin lithosphere is consistent with the suggestion by Kroenke et al. (2004) that the OJP may have formed close to a recently abandoned spreading center. Alternatively, lithospheric thinning could have resulted from thermal erosion caused by the upwelling of hot plume material.
An independent estimate of the degree of melting is provided by Herzberg (2004), who uses a forward- and inverse-modeling approach based on peridotite phase equilibria. He obtains values of 27% and 30% for fractional and equilibrium melting, respectively. Further support for large-degree melting is provided by the platinum group element (PGE) concentrations determined by Chazey and Neal (2004). The PGEs are highly compatible in mantle phases and sulfides, so their abundance is sensitive to degree of melting and sulfur saturation. Concentrations of PGEs in the OJP basalts are rather high and consistent with ~30% melting of a peridotite source from which sulfide phases had been exhausted during the melting process. Some basalt samples have PGE abundances that are too high to be accounted for by a standard model peridotite source; an additional PGE source appears to be needed. Chazey and Neal (2004) speculate that a small amount of material from the Earth's core may have been involved in the generation of OJP magmas.
Derivation of the dominant, evolved, Kwaimbaita magma type through fractional crystallization of the primitive Kroenke-type magma is consistent with the isotopic (Tejada et al., 2004) and geochemical (Fitton and Godard, 2004) evidence and with melting experiments carried out by Sano and Yamashita (2004). Sano and Yamashita's (2004) results show that the variations in phenocryst assemblage and whole-rock basalt major element compositions can be modeled adequately by fractional crystallization in shallow (<6 km) magma reservoirs.
Glass from the rims of basaltic pillows recovered from most drill sites on the OJP preserves a record of the volatile content of the magmas at the time of eruption. Roberge et al. (2004) show that water contents in the glasses are uniformly low (Fig. F5) and imply water contents in the mantle source that are comparable with those in the source of mid-ocean-ridge basalt. This is an important observation because it shows that the large degrees of melting estimated for the OJP magmas cannot have been caused by the presence of water but require high temperatures. The sulfur contents of OJP glasses confirm Chazey and Neal's (2004) inference of sulfur undersaturation in the magmas. The water depth of lava emplacement controls the CO2 content of the glasses, and data obtained by Roberge et al. (2004) imply depths ranging from ~1000 m on the crest of the OJP to ~2500 m on its eastern edge (Fig. F5). The amount of CO2 released during formation of the OJP is difficult to determine without reliable information on primary magmatic CO2 contents and precise knowledge of the duration of volcanism, but Roberge et al. (2004) estimate a maximum value that is ~10 times the flux from the global mid-ocean-ridge system. Erba and Tremolada (2004) estimate that the 90% reduction in nannofossil paleofluxes that they link to emplacement of the OJP requires a three- to six-fold increase in volcanogenic CO2.
The submarine emplacement of most of the OJP resulted in low-temperature alteration of the basalts through contact with seawater. The alteration ranges from slight to complete, and unaltered olivine and glass were found in some of the basaltic lava flows sampled in the drill cores. A detailed study of the alteration processes is reported by Banerjee et al. (2004) and Banerjee and Honnorez (this volume), who show that alteration started soon after emplacement and is indistinguishable from that affecting mid-ocean-ridge basalt. There is no evidence for high-temperature alteration in any of the basalt recovered from the OJP. The initial and most pervasive stage of alteration resulted in the replacement of olivine and interstitial glass by celadonite and smectite. Later interaction between basalt and cold, oxidizing seawater caused local replacement of primary phases and mesostasis by smectite and iron oxyhydroxides. The relationship between the state of alteration and physical properties of basaltic basement rocks recovered during Leg 192 is described by Zhao et al. (this volume). Glass shards in tuffs at Site 1184 show clear textural evidence of microbial alteration (Banerjee and Muehlenbachs, 2003).