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Coring and drilling strategy

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

Paleosecular variation

Although the time-averaged magnetic field is well approximated by a geocentric axial dipole, secular variations in the magnetic field occurring on timescales as long as ~105 y (e.g., Constable and Johnson, 2005) result in significant deviations from this simple dipolar structure. Because a paleomagnetic direction preserved in a lava flow as thermal remanent magnetization is a geologically instantaneous recording of the fluctuating geomagnetic field, robust paleolatitude estimates based on the geocentric axial dipole model require us to average out these paleosecular variations by sampling a sufficiently large number of individual 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 volcanic basement were therefore primarily dictated by the need for a sufficient number of lava flows to provide 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 models—Model G of McElhinny and McFadden (1997) and TK03 of Tauxe and Kent (2004)—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 uncertainty of 4° in the paleolatitude estimates (Fig. F9A), and 25–30 units must be recovered if we aim more conservatively for 5° uncertainty. These results also can be compared to a compilation of Deep Sea Drilling Project (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 the 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, a basement penetration of at least 350 m was required 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 basalt from the AMAT02RR site survey emphasize the suitability of this technique for the Louisville Seamount Trail (Fig. F4B) (Koppers et al., 2011).

Many studies also established that altered rocks can be effectively used for determining geochemical source characteristics, particularly data for elements (lanthanides, Nb, Ta, Zr, Ti, Fe, and Al) and isotopic systems (Sm-Nd and Lu-Hf) that are largely 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 (laser) microanalysis of major and trace elements, as well as isotopic ratios, in unaltered portions of various phenocrystic phases and melt inclusions.

Site selection and coring plan

Our principal science objectives were to acquire accurate paleolatitudes, geochemistry and isotope compositions, and 40Ar/39Ar age dates for seamounts in the Louisville Seamount Trail having formation ages similar to those drilled in the Emperor Seamount Trail during Leg 197. In addition, drill sites were selected where seismic lines cross midway between the centers of the targeted guyots and their margins, a compromise designed to avoid eruptive centers (which may have fewer lava units) and marginal sites with potentially thicker volcaniclastic units and higher potential for tectonic disruption. The five 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) lava.

In preparation for Expedition 330, 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 and SO167 site surveys, the ages of these dormant volcanic structures were estimated 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 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 26.5°S and 36.9°S Guyots, which both have a second set of crossing lines close to primary prospectus Sites LOUI-1C and LOUI-3B, most alternate sites are located on 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.