Scientific objectives

Primary objectives

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

High-quality paleolatitude data for the Louisville seamounts are required to establish its potential hotspot motion between 80 and 50 Ma relative to Earth's spin axis and to compare this movement to the 15° shift in paleolatitude that has been observed for the Hawaiian–Emperor seamounts 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 to determine whether primary Pacific hotspots moved coherently before 50 Ma or, alternatively, show significant interhotspot motion, with the Louisville hotspot possibly 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. The outcome of these comparisons is fundamentally important to understanding the nature of hotspots, the convection of Earth's mantle, and true polar wander.

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

Because volcanic activity for a single hotspot volcano can span as much as 10 m.y. when including the possibility of posterosional volcanism, it is essential to establish an accurate framework of 40Ar/39Ar ages to successfully determine paleolatitude change over time and map the magmatic evolution within single seamounts and along the Louisville Seamount Trail. Shield-building and postshield lavas typically form within 1–2 m.y. for Hawaiian-type volcanoes and can be readily distinguished from possible overlying posterosional sequences because of the high precision in 40Ar/39Ar age dates on the order of 0.2–0.5 Ma. For this reason, incremental heating 40Ar/39Ar age dating will allow us to establish age histories within each drilled core that can be used to establish the cessation of volcanism at the end of the shield-building phase and determine the starting time (and minimal duration) of the postshield capping and posterosional stages (if recovered). Drilling of the Louisville seamounts will likely recover mostly alkali basalts that contain high abundances of potassium, making the 40Ar/39Ar technique especially suitable for providing high-precision age dates necessary to test and calibrate geodynamic and geochemical models.

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

Existing data from dredged lavas suggest that the mantle source of the Louisville hotspot has been remarkably homogeneous for as long as 80 m.y. In addition, the recovered basalt samples are predominantly alkalic and likely represent a mostly alkalic shield-building stage, which sharply contrasts with the massive tholeiitic shield-building stage of volcanoes and seamounts in the Hawaiian–Emperor Seamount Trail. Therefore, geochemical and isotopic data for lavas from the proposed drill sites will provide key insights into the magmatic evolution and melting processes that produced and constructed the Louisville volcanoes during their progression from shield to postshield (and perhaps 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 test the apparently long-lived homogeneous geochemical character of its mantle source. Detailed analyses of melt inclusions, volcanic glass samples, primitive basalts, and high-Mg olivine pheno- and xenocrysts (if recovered) 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 map the fundamental differences between primary Hawaiian and Louisville hotspot volcanism.

Secondary objectives

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

One hypothesis states that the OJP formed from massive volcanism at ~120 Ma, coincident with when an oversized plume head preceding the deep Louisville mantle upwelling reached the base of the Pacific lithosphere and began extensive decompression melting (e.g., Richards and Griffiths, 1989; Mahoney and Spencer, 1991). Expedition 330 will generate essential data for the Louisville seamounts that will have significant implications for the origin of the OJP and LIPs in general. If the Louisville Seamount Trail corresponds to the plume tail stage of the Louisville mantle plume itself and the OJP corresponds to the plume head, then new paleolatitude estimates, 40Ar/39Ar ages, and geochemical data will help us to determine whether the oldest Louisville seamounts were formed close to the 18°–28°S paleolatitude determined from Leg 192 basalts for the OJP (Riisager et al., 2003) and whether they are genetically linked or not.

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

Characterizing Louisville as one of the primary hotspots in the Pacific requires the 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 (e.g., Foulger and Anderson, 2005), such as intraplate volcanism originating in the upper mantle from more "fertile" (i.e., more refractory) source materials. Evidence for temperatures higher than the mean 1350° ± 50°C temperature of an upper mantle MORB source (Putirka, 2008; Courtier et al., 2007) will be important for proving 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 for documenting the changing plume–lithosphere interactions along the Louisville Seamount Trail.

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

The flat-topped Louisville seamounts are capped by thin packages of pelagic sediments <10–40 m thick. These sediments possibly contain abundant calcareous fossils (e.g., foraminifers and nannofossils) because they were deposited in shallow waters and above the carbonate compensation depth (CCD). If so, this will provide good stratigraphic age control in these sediments. Similar sediments may also be recovered intercalated between lava flows deep in the volcanic basement, providing a fossil record that can be directly compared with the 40Ar/39Ar radiometric age dates for the basement samples. In addition, nummulitic limestones have been dredged from a few guyots in the Louisville Seamount Trail, indicating the possible presence of Eocene shallow-water reefs in the high- to mid-latitude Pacific (Lonsdale, 1988). 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). Although the rotary core barrel (RCB) system that we will use to drill through the hard igneous rocks is not suitable for undisturbed and continuous recovery of soft pelagic sediment, we will attempt to maximize the recovery of these thin pelagic caps by applying a "gravity-push" technique or by reducing RCB bit rotation and pump rate for this interval. Although not a primary objective during Expedition 330, these sediments may provide a unique data set that adds to the very sparse paleoclimate record in the South Pacific at high southern latitudes (Corfield and Cartlidge, 1992; Corfield and Norris, 1996; Barrera and Savin, 1999; Norris et al., 2001).