SCIENTIFIC OBJECTIVES1. Age and duration of emplacement. Plume-related hypotheses for emplacement of large igneous provinces include rapid-eruption, age-progressive, and episodic growth models. A. Rapid- emplacement models are of two main types. The plume-head or "plume impact" model (e.g., Richards et al., 1991; Saunders et al., 1992) predicts that large oceanic plateaus are formed by widespread basaltic flood eruptions as the inflated head of a new mantle plume approaches the base of the lithosphere. An alternative model devised specifically for continental and continental-margin flood basalt provinces (White and McKenzie, 1989), sometimes called the "plume-incubation" model (e.g. Saunders et al., 1992), considers flood volcanism to result from cataclysmic pressure- release melting when a rift propagates above the enlarged top of a more or less steady-state mantle plume that has accumulated gradually (perhaps over several tens of millions of years) beneath thick, slow-moving continental lithosphere. Both types of model predict that the great bulk of magmatism occurs in only a few (probably <5) million years. B. In contrast, an age progressive, Icelandic style of construction, in which plateaus are formed over much longer intervals (tens of millions of years), remains a distinct possibility for many plateaus (e.g., Mahoney and Spencer, 1991; Coffin and Gahagan, 1995; Ito and Clift, 1998). C. Alternatively, plateau growth may occur in two or more discrete pulses of activity, dependent on mantle plume dynamics or the interplay between episodes of lithospheric extension and mantle melting (Bercovici and Mahoney, 1994; Larson and Kincaid, 1996; Neal et al., 2000).
As the world's largest oceanic plateau, the OJP is an important test case. Its great crustal volume implies partial melting of at least 150-400 x 106 km3 of mantle, which virtually necessitates involvement of the lower mantle if the bulk of the plateau was formed in the 122-Ma event (e.g., Coffin and Eldholm, 1994). Melting on such a scale is unknown today, and this consideration has helped fuel suggestions of fundamental differences in Cretaceous and Cenozoic mantle convection. On the other hand, if the plateau accreted more slowly over several tens of millions of years (like the much smaller Icelandic Plateau) or in two or more discrete pulses, then this partial melting requirement is eased considerably, but simple plume-head models must either be modified significantly or do not apply. The few existing basement sites demonstrate that 122-Ma and 90-Ma lavas are both present in far separated locations, but the importance of the 90-Ma episode is unclear. In many places, 90-Ma lavas may form a relatively thin carapace over a thick 122-Ma pile. Biostratigraphic dates of basal sediments and intraflow sedimentary beds, and 40Ar-39Ar dating of lavas recovered in the four widely distributed sites to be cored during Leg 192 will provide a much more detailed picture of the plateau's constructional history than is currently available. If suitable lava samples are recovered, some 40Ar-39Ar dating also may be carried out on feldspar separates.
2. Range and diversity of magmatism. Laboratory modeling suggests that starting-plume heads should be strongly zoned because of entrainment of large amounts of ambient, nonplume mantle during their rise to the base of the lithosphere (e.g., Campbell, 1998). Thus, even if a plume's source region (usually assumed to be at the base of, or deep within, the mantle) is compositionally homogeneous, significant isotopic and trace element heterogeneity is nevertheless predicted in magmas erupted from different parts of the plume head or at different times. Major element compositions are predicted to vary as well, with lavas erupted above the hottest (axial) parts of the plume head having picritic (e.g., Campbell, 1998) or possibly even komatiitic (Storey et al., 1991) affinities, whereas more ordinary basalts are predicted above cooler, more distal regions. In this regard, the two most remarkable features of the existing OJP basement samples are (1) the limited overall range of chemical and isotopic variation in the 122-Ma lavas, and (2) the isotopic and chemical similarity of the 90-Ma lavas to the 122-Ma ones. The isotopic and incompatible element results could indicate that the world's largest plateau had a much more homogeneous source (both relative to the scale of melting and in time) than predicted by plume-head models, whereas the combined major and trace element data imply a dominant role for large magma chambers.
Data for basement lavas from the four sites to be drilled on Leg 192 will place more accurate bounds on the compositional and thermal characteristics of the plateau's mantle source and on the nature, extent, and mechanisms of magmatic evolution. Isotopic work will include Nd, Sr, Pb, and Hf isotope-ratio and parent-daughter measurements of whole rocks and, where feasible, of clinopyroxene or plagioclase separates. Os and O isotopes also may be measured in suitable samples. Elemental data, complemented by petrographic studies, will include major element measurements on whole rocks and minerals, and a comprehensive suite of whole-rock trace element analyses.
3. Eruptive environment and style. Plume-head models predict as much as 1-3 km of dynamic uplift associated with the arrival of a large starting-plume head at the base of the lithosphere (e.g. Hill, 1991; Neal et al., 1997). The associated constructional volcanism also creates a much thicker crust than normal oceanic crust. The combination of these effects is predicted to elevate parts of a plateau's surface to shallow water depths and/or cause portions to emerge above sea level. Indeed, significant portions of several plateaus are known to have been initially shallow or subaerial (e.g., Richards et al., 1991; Coffin, Frey, Wallace, et al., 2000). However, although most of the OJP stands 2-3 km above the surrounding seafloor today, basement lavas from all locations studied thus far all came to rest at fairly great depths, probably beneath the calcite compensation depth in some cases (Neal et al., 1997; Ito and Clift, 1998). The reasons for this behavior and whether it is typical of the plateau as a whole are unknown, but critical for understanding how plateaus are constructed, and for testing the plume-head model in particular. Moreover, whether or not parts of the OJP were shallow has important ramifications for how its emplacement affected large scale climatic, oceanographic, and biospheric conditions. If significant amounts of magma were erupted at shallow depths or subaerially, the flux of climate-modifying volatile species (SO2, Cl, F, CO2) to the atmosphere would have been much greater than if the bulk of plateau volcanism occurred at greater depths.
All but one of the existing basement sites are located at the margins of the plateau. The part of the OJP most likely to have been originally at shallow levels is the broad domal region of the high plateau that today lies at water depths shallower than 2000 m (Fig. 1; see Site OJ-3), which is also where the crust of the plateau is thickest. This domal region is likely to have been the principal locus of eruptive activity during the ~122-Ma phase of plateau construction (Tejada et al., 1996; Neal et al., 1997). The other area most likely to have been at shallow depths is the crest of the eastern salient (Fig. 1; see Site OJ-6), which may correspond to the main locus of 90-Ma eruptive activity, as noted above. Sampling these summit regions during Leg 192, as well as the other two planned sites, will establish whether they were originally shallow or subaerial, or were emplaced in a deep-water environment. Paleodepths will be estimated from microfossils and physical-chemical characteristics of near-basement and intraflow sediments (Ito and Clift, 1998) and from measurements of volatile abundances in volcanic glass (Michael, 1999).
The physical volcanology of large-scale submarine lava flows is poorly known, as are the nature and scale of hydrothermal fluid fluxes associated with plateau magmatism. Knowledge of the physical volcanology of lavas is important for understanding eruption mechanisms and how the volcanic pile accumulated, whereas data on hydrothermal activity are critical for understanding the oceanographic and climatic effects of plateau formation. Flows comprising continental flood basalt provinces are typically 10-30 m thick and, in some cases, have been traced for distances of several hundred kilometers (e.g., Hooper, 1997). Areas distant from eruptive sources tend to be composed of simple flows, whereas compound flows are more indicative of relative proximity to eruptive vents. Previous sampling of OJP basement has revealed that flow thickness varies from <1 to 60 m, although most flows are in the 4-12 m range (e.g., Neal et al., 1997). The lavas are predominantly simple, consistent with the locations of most existing basement sites at the margins of the plateau and presumably far from their eruptive vents. Very little interlava ash has been found. With regard to hydrothermal activity, existing OJP basement sites show almost no evidence of anything but low- temperature seawater-mediated alteration in either the lavas or overlying sediments (e.g., Babbs, 1997). This lack of hydrothermal alteration may again indicate that these are distal portions of lava flows erupted far from eruptive vent systems, whereas major hydrothermal systems would be expected to be centered around major eruptive locii. During Leg 192, sites at the crest of the high plateau and eastern salient, in particular, may show a fuller range of physical features and evidence of significant hydrothermal activity.
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