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doi:10.2204/iodp.proc.330.106.2012 Site U13751Expedition 330 Scientists2Background and objectivesSite U1375 (prospectus Site LOUI-2B) on Achernar Guyot was the fourth site completed during Integrated Ocean Drilling Program (IODP) Expedition 330. Achernar Guyot (Fig. F1), the third of five seamounts drilled in the Louisville Seamount Trail, has an estimated age of ~59–63 Ma. Knowledge of this feature will fill an important gap in the age-versus-distance relationship of the Louisville Seamount Trail and will also provide information pivotal to reconstructing past plate motion and the motion of the Louisville hotspot. Compared to Rigil and Canopus Guyots to the northwest, this seamount is half the size (29 km long and 27 km wide) and is part of a trail of seven small guyots and seamounts that starts with Burton Guyot at the northern end (Fig. F1). Site U1375 was targeted in the middle of this small edifice (Fig. F2), away from the guyot’s shelf edges and any thick packages of dipping volcaniclastics on its flanks, the latter of which were preferentially targeted at Sites U1372, U1373, and U1374. Achernar Guyot shows no evidence of tilting, and Site U1375 was placed on its summit plain at 1258 m water depth. Side-scan sonar reflectivity and 3.5 kHz subbottom profiling data indicate that Site U1375 is covered with <15 m of pelagic sediment, and seismic reflection profiles suggest that this central part of Achernar Guyot is typified by <44 m of volcaniclastics overlying igneous basement. The original drilling plan was to recover soft sediment using a gravity-push approach with little or no rotation of the rotary core barrel assembly, followed by standard coring into the volcaniclastic materials and 350 m into igneous basement. A short downhole logging series was planned, including the standard triple combination and Formation MicroScanner-sonic tool strings and the third-party Göttingen Borehole Magnetometer tool. Shortly after Hole U1375A was spudded, drilling became problematic because of instabilities in the uppermost seamount sediment cover, likely from the presence of unconsolidated cobbles. After Hole U1375A had to be abandoned, Hole U1375B was spudded ~350 m to the northwest, but severe hole instabilities were also encountered there. In order to avoid further delay at yet another location on Achernar Guyot, Hole U1375B was abandoned and a new site was established on Burton Guyot about 91 nmi to the northwest along the Louisville Seamount Trail. Despite the drilling difficulties, three cores were recovered, including ~1.5 m of carbonate-cemented volcanic breccia in Hole U1375A and 57 cm of micrograbbro (dolerite) in Hole U1375B. ObjectivesDrilling during Ocean Drilling Program (ODP) Leg 197 provided compelling evidence for the motion of mantle plumes by documenting a large ~15° shift in paleolatitude for the Hawaiian hotspot (Tarduno et al., 2003; Duncan et al., 2006). This evidence led to testing two geodynamic end-member models during Expedition 330, namely that the Louisville and Hawaiian hotspots moved coherently over geological time (Wessel and Kroenke, 1997; Courtillot et al., 2003) or, quite the opposite, that these hotspots show considerable interhotspot motion, as predicted by mantle flow models (Steinberger, 2002; Steinberger et al., 2004; Koppers et al., 2004; Steinberger and Antretter, 2006; Steinberger and Calderwood, 2006). The most important objective of Expedition 330, therefore, was to core deep into the igneous basement of four seamounts in the Louisville Seamount Trail in order to sample a large number of in situ lava flows ranging in age between 80 and 50 Ma. With a sufficiently large number of these independent cooling units, high-quality estimates of their paleolatitude can be determined, and any recorded paleolatitude shift (or lack thereof) can be compared with seamounts in the Hawaiian-Emperor Seamount Trail. For this reason, Expedition 330 mimicked the drilling strategy of Leg 197 by drilling seamounts equivalent in age to Detroit (76–81 Ma), Suiko (61 Ma), Nintoku (56 Ma), and Koko (49 Ma) Seamounts in the Emperor Seamount Trail, with Achernar Guyot being equivalent to Suiko Seamount. Accurate paleomagnetic inclination data are required for the drilled seamounts in order to establish a record of past Louisville hotspot motion, and, together with high-resolution 40Ar/39Ar age dating of the cored lava flows, these data will help us constrain the paleolatitudes of the Louisville hotspot between 80 and 50 Ma. These comparisons are of fundamental importance in determining whether these two primary hotspots have moved coherently or not and in understanding the nature of hotspots and convection in the Earth’s mantle. Expedition 330 also aimed to provide important insights into the magmatic evolution and melting processes that produced and constructed Louisville volcanoes as they progressed from shield to postshield, and perhaps posterosional, volcanic stages. 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. (Cheng et al., 1987; Hawkins et al., 1987; Vanderkluysen et al., 2007; Beier et al., 2011). In addition, all dredged basalt is predominantly alkalic and possibly represents a mostly alkalic shield-building stage, in contrast to the tholeiitic shield-building stage of volcanoes in the Hawaiian-Emperor Seamount Trail (Hawkins et al., 1987; Vanderkluysen et al., 2007; Beier et al., 2011). Therefore, the successions of lava flows cored during Expedition 330 will help us characterize the Louisville Seamount Trail as the product of a primary hotspot and test the long-lived homogeneous geochemical character of its mantle source. Analyses of melt inclusions, volcanic glass samples, high-Mg olivine, and clinopyroxene phenocrysts will provide further constraints on the asserted homogeneity of the Louisville plume source, its compositional evolution between 80 and 50 Ma, its potential mantle plume temperatures, and its magma genesis, volatile outgassing, and differentiation. Incremental heating 40Ar/39Ar age dating will allow us to establish age histories within each drill core, delineating any transitions from the shield-building phase to the postshield capping and posterosional stages. Finally, basalt and sediment cored at Site U1375 were planned to be used for a range of secondary objectives such as searching for active microbial life in the old seamount basement and determining whether fossil traces of these microbes were left behind in volcanic glass or rock biofilms. We also planned to determine 3He/4He and 186Os/187Os signatures of the Louisville mantle plume to evaluate its potential deep-mantle origin, to use oxygen and strontium isotope measurements on carbonates and zeolites in order to assess the magnitude of carbonate vein formation in aging seamounts and its role as a global CO2 sink, to age date celadonite alteration minerals for estimating the total duration of low-temperature alteration following seamount emplacement, and to determine the hydrogeological and seismological character of the seamount basement. |
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