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doi:10.2204/iodp.pr.330.2011

Coring and drilling strategy

The drilling strategy used during Expedition 330 was the same as the Leg 197 drilling strategy, which provided compelling evidence for the motion of the Hawaiian mantle plume between 80 and 50 Ma (Tarduno, Duncan, Scholl, et al., 2002). For that reason we targeted four Louisville seamounts that have ages similar to the Detroit, Suiko, Nintoku, and Koko Seamounts in the Emperor Seamount Trail. Our principal drilling goal was to drill 350 m (or deeper) into the igneous basement of these Louisville seamounts in order to core and recover as many individual lava flows or cooling units as possible (see below). By analyzing these flows using modern paleomagnetic, ⁴⁰Ar/³⁹Ar geochronological, and geochemical techniques, we will be able to directly compare paleolatitude estimates and geochemical signatures between seamounts in the two longest-lived hotspot systems in the Pacific.

Paleosecular variation

Secular variation is defined by small deviations from the geomagnetic axial dipole (GAD) model caused by variations of the nondipole components in the Earth's magnetic field, occurring on timescales ranging from 10² to 10³ y (Butler, 1992; Tauxe, 2010). Because a paleomagnetic direction preserved in an in situ lava flow by its thermal remanent magnetization (TRM) is a geologically instantaneous recording of the fluctuating geomagnetic field, robust paleolatitude estimates based on the GAD model require us to average out these paleosecular variations by sampling a sufficiently large number of lava flows. This can be achieved by sampling multiple lava flows spanning a longer geological time interval, on the order of 10⁴–10⁵ y (Butler, 1992; Tauxe, 2010).

Planned penetration depths into the seamount volcanic basements were therefore primarily dictated by the need for accurate paleolatitude data. The expected errors on paleolatitude estimates at a ~50° southern latitude, appropriate for the Louisville hotspot, can be illustrated via Monte Carlo simulations of field directions as drawn from two widely used paleosecular variation (PSV) models—Model G of McElhinny and McFadden (1997) and TK03 of Tauxe and Kent (2004)—that are based on a global database of directions from lava flows of 0–5 Ma in age, assuming that the magnitude and timescale of paleosecular variations did not change with time. These simulations suggest that >42 independent flow units must be recovered to achieve a nominal 2σ error of 4° in the paleolatitude estimates (Fig. F9A), and only 25–30 units must be recovered if we aim more conservatively for 5° errors. These results also can be compared to a compilation of DSDP/ODP drilling statistics from other expeditions to seamount trails and large igneous provinces (Fig. F9B) that on average recovered >20 flow groups for ~200–300 m of coring into volcanic basement. Measured uncertainties (Fig. F9C) in these DSDP/ODP drill cores are generally compatible with the Monte Carlo simulations,but these data scatter significantly between 3° and 7° uncertainties for paleolatitude estimates based on 5 or more flow units. Because the mantle flow models and global plate circuit reconstructions predict small paleolatitude shifts for the Louisville hotspot, we required a basement penetration of at least 350 m to achieve a paleolatitude uncertainty better than 5°.

Hydrothermal and seawater alteration

Most of the dredged samples from the Louisville Seamount Trail were highly to completely altered by hydrothermal and seawater alteration. Even though drilling during Expedition 330 provided us with much fresher basaltic material, alteration remains problematic and thus requires special analytical attention in order to maximize the amount and quality of data on rock ages and their original (erupted) compositions. Holocrystalline groundmass samples that have been carefully hand-picked and acid-leached to remove alteration have been shown to provide ages concordant with ⁴⁰Ar/³⁹Ar ages of co-magmatic minerals and can be interpreted as eruption ages (Koppers et al., 2000, 2004). Recent data on basalts from the AMAT02RR site survey emphasize the suitability of this technique for the Louisville Seamount Trail (Fig. F4B) (Koppers et al., submitted).

Many studies also established that altered rocks can be effectively used for determining a hotspot's geochemical characteristics, particularly data for elements (lanthanides, Nb, Ta, Zr, Ti, Fe, and Al) and isotopic systems (Sm-Nd and Lu-Hf) that are resistant to seawater alteration. In addition, useful data can be obtained for the more sensitive Sr and Pb isotopic systems by applying mineral separation or acid-leaching to remove secondary minerals (Cheng et al., 1987; Mahoney et al., 1998; Koppers et al., 2003; Regelous et al., 2003). Additionally, magmatic compositions can be inferred from the microanalysis of major and trace elements, as well as isotopic ratios, in unaltered portions of various phenocrystic phases and melt inclusions. In any case, several ODP/IODP workshops have recommended a drilling depth of at least 100 m to retrieve relatively fresh samples suitable for age dating and geochemical studies.

Site selection and coring plan

Our principal science objectives were to acquire accurate paleolatitudes, geochemistry and isotope compositions, and ⁴⁰Ar/³⁹Ar age dates for at least four Louisville seamounts having formation ages similar to seamounts in the Emperor Seamount Trail drilled during Leg 197. In addition, we selected drill sites where seismic lines cross midway between centers of the targeted flat-topped seamounts (i.e., guyots) and their margins, a compromise designed to avoid eruptive centers (which may harbor fewer lava units) and marginal sites with potentially thicker volcaniclastic units and higher potential for tectonic disruption. The Louisville seamounts targeted for drilling are relatively small edifices and typically have thinner sedimentary covers and fewer subsidiary peaks and pinnacles that may represent late-stage (posterosional) lavas.

Four primary sites (Fig. F1; Table T1) were selected on four flat-topped seamounts at the older end of the Louisville Seamount Trail. On the basis of new ⁴⁰Ar/³⁹Ar ages from the AMAT02RR site survey, we estimate the ages of these dormant volcanic structures to be 75–77 Ma (prospectus Site LOUI-1C on Canopus Guyot at 26.5°S), 58.5 Ma (prospectus Site LOUI-2B on Achernar Guyot at 33.7°S), 54.0 Ma (prospectus Site LOUI-3B at 36.9°S), and 50.1 Ma (prospectus Site LOUI-4B on Hadar Guyot at 168.6°W). In addition, eight alternate sites (prospectus Sites LOUI-1B, LOUI-6A, LOUI-6B, LOUI-7A, LOUI-7B, LOUI-8A, LOUI-8B, and LOUI-9A) were selected (Table T2). Except for the 36.9°S guyot, which has a second set of crossing lines close to primary prospectus Site LOUI-8A, most alternate sites were located on three different but closely neighboring seamounts (also surveyed during AMAT02RR) in order to stay as near as possible to our original age-selection criterion and our overall drilling strategy. The eighth alternate site (Site LOUI-6B on Rigil Guyot at 28.6°S) was added during Expedition 330.

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