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Scientific objectives

Primary objectives

1. Determine the paleolatitude change (if any) over time for the Louisville hotspot

High-quality paleolatitude data are required to establish the Louisville hotspot’s potential motion between 80 and 50 Ma relative to the Earth’s spin axis and to compare this to the 15° shift in paleolatitude observed for the Hawaiian-Emperor Seamount Trail during the same time period. Together with the measurement of high-resolution 40Ar/39Ar age dates for the cored lava flows, these paleolatitude data will help us distinguish between the possibilities that these primary Pacific hotspots moved coherently before 50 Ma or, as predicted by geodynamic mantle flow models, that they show significant interhotspot motion, with the Louisville hotspot showing less or no discernible latitudinal motion and a considerable longitudinal shift toward the east. Comparison of these results with predictions from geodynamic mantle flow and plate circuit models will allow us to critically test, calibrate, and improve these models. These comparisons are of fundamental importance for understanding the nature of hotspots, the convection of the Earth’s mantle, and true polar wander.

2. Determine the volcanic history of individual seamounts and the age progression along the Louisville Seamount Trail through 40Ar/39Ar age dating

Because volcanic activity for a single hotspot volcano can span up to 10 m.y. when including the possibility of posterosional volcanism, establishing an accurate framework of 40Ar/39Ar ages is essential to successfully determine the paleolatitude change over time and map out the magmatic evolution within single seamounts and along the Louisville Seamount Trail. Shield-building and postshield lavas typically form over short periods of 1–2 m.y. for Hawaiian-type volcanoes and can be readily distinguished from overlying posterosional sequences (if present) because of state-of-the-art 40Ar/39Ar age determinations. The high precision of incremental heating 40Ar/39Ar age dating will allow us to establish age histories within each drill core that can be used to establish the cessation of volcanism at the end of the shield-building phase and to determine the starting time (and minimal duration) of the postshield capping and posterosional stages.

3. Determine the magmatic evolution of the Louisville Seamounts and their mantle source through major and trace element and isotope geochemistry

Existing data from dredged lava suggest that the mantle source of the Louisville hotspot has been remarkably homogeneous for as long as 80 m.y. In addition, dredged lava is predominantly alkalic and likely represents a mostly alkalic shield-building stage, which contrasts sharply with the predominant tholeiitic shield-building stage of volcanoes and seamounts in the Hawaiian-Emperor Seamount Trail. Therefore, geochemical and isotopic data for basaltic lava from the five seamounts cored during Expedition 330 will provide key insights into the magmatic evolution and melting processes that produced and constructed the Louisville volcanoes while they progressed from shield to postshield (and possibly posterosional) volcanic stages. In turn, these data will help us to characterize the Louisville Seamount Trail as a product of one of only three primary hotspots in the Pacific and to test the apparently long-lived homogeneous geochemical character of its mantle source. Detailed analyses of melt inclusions, volcanic glass samples, and high-Mg olivine pheno- and xenocrysts will provide further constraints on the asserted homogeneity of the Louisville mantle plume source and the compositional evolution of this source between 80 and 50 Ma. Together, these geochemical and isotopic studies will allow us to document any fundamental differences between primary Hawaiian and Louisville hotspot volcanism.

Secondary objectives

1. Determine whether the Ontong Java Plateau formed from the plume head of the Louisville mantle plume around 120 Ma

One hypothesis states that the Ontong Java Plateau formed from massive volcanism around 120 Ma, when the plume head of the Louisville mantle upwelling initially reached the base of the Pacific lithosphere and started extensive partial melting (e.g., Richards and Griffiths, 1989; Mahoney and Spencer, 1991). If the Louisville Seamount Trail corresponds to the plume tail stage of the Louisville mantle plume and the Ontong Java Plateau to the plume head, the new paleolatitude estimates, 40Ar/39Ar ages, and geochemical data obtained from the recovered drill cores will help us to decide whether the oldest seamounts in the Louisville Seamount Trail were formed close to the 18°–28°S paleolatitude determined from ODP Leg 192 basalt for the Ontong Java Plateau (Riisager et al., 2003) and whether they are genetically linked or not. The outcome of this hypothesis test will have significant implications for the origin of the Ontong Java Plateau and large igneous provinces in general.

2. Determine the potential temperature and degree and depth of partial melting at which Louisville seamount magmas were generated

Characterizing Louisville as one of the primary hotspots in the Pacific requires estimation of the minimum potential temperature of its mantle plume source, the degree of partial melting in this source, and the depth of the melting zone beneath the oceanic lithosphere in order to distinguish this model from alternate models, such as intraplate volcanism originating in the upper mantle from more “fertile” (i.e., more refractory) materials (e.g., Foulger and Anderson, 2005). Evidence for temperatures higher than the mean 1350° ± 50°C temperature of an upper-mantle MORB source (Courtier et al., 2007; Putirka, 2008) will be important to prove the deep thermal origin of the Louisville mantle plume. Evidence for changes in the degree and depth of partial melting, on the other hand, will be important to document the changing plume-lithosphere interactions along the Louisville Seamount Trail.

3. Provide paleoceanographic and paleoclimate data at 40°–50°S paleolatitudes in the southern ocean from cored Louisville pelagic sediments

Thin packages of pelagic sediments cap the flat-topped Louisville Seamounts. These sediments contain abundant calcareous microfossils (e.g., foraminifers and nannofossils) because they were deposited in shallow waters and above the carbonate compensation depth. Limestone sequences including a well-recovered algae reef and macrofossils also were recovered on top of the volcanic basement or intercalated between lava flows. The fossil records can be compared with the 40Ar/39Ar radiometric age dates measured on basement samples. The timing of reef formation, and eventually the drowning of such carbonate banks, is of considerable interest because it provides evidence from the southeast Pacific for the expansion of tropical climates during past warm periods (Adams, 1967, 1983; Premoli Silva et al., 1995; Huber et al., 1995; Wilson et al., 1998; Jenkyns and Wilson, 1999). These sediments may provide a unique data set, adding to the very sparse paleoclimate record in the South Pacific at such high southern-latitude sites (Corfield and Cartlidge, 1992; Corfield and Norris, 1996; Barrera and Savin, 1999; Norris et al., 2001).