Origin and evolution of magmas on the Ontong Java Plateau

The Early Cretaceous Ontong Java Plateau (OJP) represents by far the largest igneous event on Earth in the last 200 Ma and yet, despite its size, the OJP’s basaltic crust appears to be remarkably homogeneous in composition. The most abundant rock type is a uniform low-K tholeiite, represented by the Kwaimbaita Formation on Malaita and found at all but one of the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drill sites on the plateau and in the adjacent basins. This is capped by a thin and geographically restricted veneer of a slightly more incompatible-element-rich tholeiite (the Singgalo Formation on Malaita and the upper flow unit at ODP Site 807), distinguished from Kwaimbaita-type basalt by small but significant differences in Sr-, Nd- and Pb-isotope ratios. A third magma type is represented by high-Mg (Kroenke-type) basalt found in thick (> 100 m) successions of lava flows at two drill sites (ODP Sites 1185 and 1187) 146 km apart on the eastern flank of the plateau. The high-Mg basalt is isotopically indistinguishable from Kwaimbaita-type basalt and may therefore represent the parental magma for the bulk of the OJP. Low-pressure fractional crystallization of olivine followed by olivine+augite+plagioclase can explain the compositional range from high-Mg Kroenke-type to Kwaimbaita-type basalt. The Singgalo-type basalt probably represents slightly smaller-degree, late-stage melting of an isotopically distinct component in the mantle source. Primary magma compositions, calculated by incremental addition of equilibrium olivine to aphyric Kroenke-type basalt glass, contain between 15.6% (in equilibrium with Fo90) and 20.4% (Fo92) MgO. Incompatible-element abundances in the primary OJP magma can be modelled by around 30% melting of a peridotitic primitive-mantle source from which about 1% by mass of average continental crust had previously been extracted. This large degree of melting implies decompression of very hot (potential temperature >1500°C) mantle beneath very thin lithosphere. The initiation of an exceptionally large and hot plume head close to a mid-ocean ridge provides the best explanation for the size, homogeneity and composition of the OJP, but is difficult to reconcile with the submarine eruption of virtually all of the basalt so far sampled.

Accretionary-lapilli-bearing pyroclastic rocks at ODP Leg 192 Site 1184: a record of subaerial phreatomagmatic eruptions on the Ontong Java Plateau

Detailed analysis of lithologies and lithofacies associations within the 337.7 m thick basement volcaniclastic succession, recovered during Ocean Drilling Program (ODP) Leg 192 at Site 1184 on the Ontong Java Plateau, shows that in bulk it is made up of pyroclastic deposits of phreatomagmatic origin. The succession is essentially made up of two lithologies: lapilli tuff (59% of the total recovered core length) and tuff (34%), consisting almost entirely of juvenile clasts (>97%) and containing significant amounts of matrix-supported accretionary and/or armoured lapilli clasts. The evidence indicates that the succession was formed by at least six (and perhaps as many as 10) major phreatomagmatic eruptions that were subaerial and associated with the main phase of volcanism on the Ontong Java Plateau.

Phreatomagmatic eruptions on the Ontong Java Plateau: an Aptian 40Ar/39Ar age for volcaniclastic rocks at ODP Site 1184

The discovery of a thick (337.7 m drilled) succession of volcaniclastic sediments at Ocean Drilling Program (ODP) Site 1184, on the eastern salient of the Ontong Java Plateau (OJP), shows that at least part of the plateau formed at or near sea level. All the other parts of the OJP that have been drilled or are exposed on land are composed of basaltic lava flows erupted in a deep-marine environment. The composition of the volcaniclastic rocks at Site 1184 is essentially indistinguishable from that of basaltic lava flows recovered from the other drill sites on the OJP, suggesting that the eastern salient formed during the same magmatic event (at approximately 120 Ma) that formed the main part of the plateau. A steep (−54°) magnetic inclination preserved in the volcaniclastic succession is consistent with an Early Cretaceous age, but rare nannofossils recovered from the volcaniclastic rocks suggest a much younger Eocene age. In order to resolve this paradox, we attempted to date the succession by the 40Ar/39Ar technique, even though the rocks are highly altered. Two samples of feldspathic material separated from basaltic clasts give minimum age estimates of c. 74 Ma. The basaltic clasts have compositions similar to that of their host rocks and are therefore probably cognate. A weighted average of the results of total fusion experiments on four or five small euhedral plagioclase crystals separated from the matrix of the volcaniclastic rocks at each of four levels in the lower part of the succession gave a value of 123.5 ± 1.8 Ma, and this probably represents a reasonable estimate of the age of eruption. Although not very precise, the 40Ar/39Ar results are the best that presently can be obtained from such altered rocks. They rule out an Eocene age for the volcaniclastic succession at Site 1184, and we suggest that the Eocene nannofossils were introduced through contamination, probably along fractures.

Modelled palaeolatitudes for the Louisville hot spot and the Ontong Java Plateau

Formation of the Ontong Java Plateau (OJP), a large igneous province in the western Pacific, has been attributed to a rising plume head in the initial stage of the Louisville hot spot, approximately 120–125 Ma ago. However, the Neal et al. plate reconstruction suggests that the plateau formed approximately 9° north of the current location of this hot spot at 51°S. The magnetization of the plateau’s basement records a palaeolatitude of approximately 25°S which further increases the discrepancy with the plume-head model. Modelling the motion of the Louisville hot spot for the last 120 Ma yields a possible southward motion of up to about 6°. True polar wander (TPW) models also shift the predicted palaeolatitudes of the plateau farther north. Taking into account both hot-spot motion and TPW, formation of the OJP by the Louisville not spot remains a possibility.

Phreatomagmatic eruptions on the Ontong Java Plateau: chemical and isotopic relationship to Ontong Java Plateau basalts

The compositions of glass clasts in volcaniclastic rocks recovered from drilling at Site 1184 on the eastern salient of the Ontong Java Plateau (OJP) are investigated using microbeam analytical methods for major, minor and trace elements. These data are compared with whole-rock elemental and isotopic data for bulk tuff samples, and with data from basalts on the high plateau of the OJP. Three subunits of Hole 1184A contain blocky glass clasts, thought to represent the juvenile magmatic component of the phreatomagmatic eruptions that generated the volcaniclastic rocks. The glass clasts have unaltered centres, and are all basaltic low-K tholeiites, with flat chondrite-normalized rare earth element (REE) patterns. Their elemental compositions are very similar to the Kwaimbaita-type and Kroenke-type basalts sampled on the high plateau. Each subunit has a distinct glass composition and there is no intermixing of glass compositions between subunits, indicating that each subunit is the result of one eruptive phase, and that the volcaniclastic sequence has not experienced reworking. The relative heterogeneity preserved at Site 1184 contrasts with the uniformity of compositions recovered from individual sites on the high plateau, and suggests that the eastern salient of the OJP had a different type of magma plumbing system. Our data support the hypothesis that the voluminous subaerially erupted volcaniclastic rocks at Site 1184 belong to the same magmatic event as the construction of the main Ontong Java Plateau. Thus, the OJP would have been responsible for volatile fluxes into the atmosphere in addition to chemical fluxes into the oceans, and these factors may have influenced the contemporaneous oceanic anoxic event.