ALBERT

Description: Pb, Nd, and Sr isotopic results for lavas of the Cretaceous Ontong Java and Manihiki oceanic plateaus fall well within the modern-day oceanic island or hot pot field. The data provide no evidence of old continental basements but indicate a major involvement of 'Kerguelen-type' or 'EM-I'-like mantle in the sources of both plateaus, which appear to have probably been formed, at least in part, by hotspots. However, the presently active hotspots that Pacific plate reconstructions suggest might have been possible plateau sources lack Kerguelen-type isotopic compositions. Either these hotspots did not participate in the formation of the two plateaus, or if they did, Kerguelen-type material must have been volumetrically much more important early in their existence. Two hypotheses for the origins of these plateaus which involve hotspot sources are consistent with the sparse available geochemical, geochronological and geophysical data. The first holds that the plateaus formed cataclysmically in association with surfacing plume heads; the second posits a relatively steady but robust hotspot at or near a ridge crest and requires a much longer period of formation. A near-ridge origin appears to be indicated by evidence that most of the Pacific plateaus were built largely on relatively young ocean crust. However, we suggest that a near-ridge origin is also compatible with the plume head concept in that plume heads appear very likely to become associated with spreading axes through their influence on rift propagation, which should be substantially greater than for ordinary hotspots. In either case, the lack of hotspot tracks (seamount chains) attached to the two plateaus would be a consequence of ridge migration or rift propagation in a near-ridge setting.

Description: Basement rocks from the Ontong Java Plateau are tholeiitic basalts that appear to record very high degrees of partial melting, much like those found today in the vicinity of Iceland. They display a limited range of incompatible element and isotopic variation, but small differences are apparent between sampled sites and between upper and lower groups of flows at Ocean Drilling Program Site 807.40Ar-39Ar ages of lavas from Site 807 and Deep Sea Drilling Project Site 289 are indistinguishable about an early Aptian mean of 122 Ma (as are preliminary data for the island of Malaita at the southern edge of the plateau), indicating that plateau-building eruptions ended more or less simultaneously at widely separated locations. Pb-Nd-Sr isotopes for lavas from Sites 289, 803, and 807, as well as southern Malaita, reflect a hotspot-like source with epsilon-Nd(T) = +4.0 to +6.3, (87Sr/86Sr)T = 0.70423-0.70339, and 206Pb/204Pb = 18.245-18.709 and possessing consistently greater 208Pb/204Pb for a given 206Pb/204Pb than Pacific MORB. The combination of hotspot-like mantle source, very high degrees of melting, and lack of a discernible age progression is best explained if the bulk of the plateau was constructed rapidly above a surfacing plume head, possibly that of the Louisville hotspot. Basalt and feldspar separates indicate a substantially younger age of ~90 Ma for basement at Site 803; in addition, volcaniclastic layers of mid-Cenomanian through Coniacian age occur at DSDP Site 288, and beds of late Aptian-Albian age are found at Site 289. Therefore, at least some volcanism continued on the plateau for 30 m.y. or more. The basalts at Site 803 are chemically and isotopically very similar to those at the ~122 Ma sites, suggesting that hot plume-type mantle was present beneath the plateau for an extended period or at two different times. Surviving seamounts of the Louisville Ridge formed between 70 and 0 Ma have much higher 206Pb/204Pb than any of the plateau basalts. Thus, assuming the Louisville hotspot was the source of the plateau lavas, a change in the hotspot's isotopic composition may have occurred between roughly 70 and 90 Ma; such a change may have accompanied the plume-head to plume-tail transition. Similar shifts from early, lower 206Pb/204Pb to subsequently higher 206Pb/204Pb values are found in several other oceanic plateau-hotspot and continental flood basalt-hotspot systems, and could reflect either a reduction in the supply of low 206Pb/204Pb mantle or an inability of some off-ridge plume-tails to melt refractory low 206Pb/204Pb material.