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doi:10.2204/iodp.proc.330.107.2012 Site U13761Expedition 330 Scientists2Background and objectivesSite U1376 (alternate prospectus Site LOUI-7A) on Burton Guyot (Fig. F1) was the fifth site drilled during Integrated Ocean Drilling Program (IODP) Expedition 330. This site and seamount have an estimated age of ~63–65 Ma, slightly older than Site U1375 on Achernar Guyot. As with Site U1375, new age data from Burton Guyot will fill an important gap in the age-versus-distance relationship of the Louisville Seamount Trail, providing information pivotal to reconstructing past plate motion and the motion of the Louisville hotspot. Burton and Achernar Guyots have structural bases <30 km in diameter, making them two of the smaller volcanoes in the Louisville Seamount Trail. Site U1376 on Burton Guyot was targeted in the middle of this small edifice (Fig. F2), away from the guyot’s shelf edges and any packages of dipping volcaniclastics on its flanks, the latter of which were targeted at Sites U1372, U1373, and U1374. The summit of Burton Guyot is characterized by two pedestals on its northeastern and southeastern ends, between which Site U1376 was placed at 1503 m water depth (Fig. F2). Burton Guyot shows no evidence of tilting. Side-scan sonar reflectivity and 3.5 kHz subbottom profiling data indicate that Site U1376 is covered with <10 m of pelagic sediment, and seismic reflection profiles suggest that this central part of Burton Guyot has a <110 m thick sequence of reflectors (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 down to 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. Drilling and logging were successfully accomplished after drilling to 182.8 meters below seafloor (mbsf) and carrying out the planned logging program. Coring was particularly successful, with an average recovery of 76% in igneous basement. 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 (Courtillot et al., 2003; Wessel and Kroenke, 1997) 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. Accurate paleomagnetic inclination data for the drilled seamounts are required 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 U1376 were planned for use in 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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