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doi:10.2204/iodp.proc.330.104.2012 Site U13731Expedition 330 Scientists2Background and objectivesSite U1373 (alternate prospectus Site LOUI-6A) on Rigil Guyot (28.6°S Guyot) was the second site completed during Integrated Ocean Drilling Program (IODP) Expedition 330 (Fig. F1) and the first of two sites drilled on Rigil Guyot (Sites U1373 and U1374), the second of five seamounts drilled in the Louisville Seamount Trail. At an inferred age of ~73 Ma, Site U1373 is one of the older seamount targets and is only a few million years younger than Site U1372 on Canopus Guyot to the northwest. If the Louisville hotspot experienced a paleolatitude shift similar to the recorded ~15° southern motion of the Hawaiian hotspot during the Late Cretaceous, this shift is expected to be largest for the oldest seamounts in the Louisville Seamount Trail. Rigil Guyot was determined to be a good target because it shows no evidence of tilting or significant posterosional volcanism. Because Rigil Guyot is only slightly younger than Canopus Guyot, a paleolatitude shift of Site U1373 is expected to be similar to that of Site U1372 and will strengthen our determinations of the Louisville paleolatitude at the old end of the trail. This volcanic edifice is part of a small cluster of two guyots (oriented east–west at 28.6°S and 28.7°S) and one small seamount to the south (28.8°S) (Fig. F2). Rigil Guyot itself consists of a single volcanic center that is 40 km long and 36 km wide; however, two small (perhaps posterosional) pedestals remain on the western portion of its summit. Site U1373 was placed on the summit plain close to the northern shelf edge at 1447 m water depth (Fig. F2). Side-scan sonar reflectivity and 3.5 kHz subbottom profiling data indicate that Site U1373 is covered with <10 m of pelagic sediment, and seismic reflection profiles (Koppers et al., 2010) show that this site is characterized by a 110 m thick section of reflectors (volcaniclastics?) dipping toward the south and overlaying igneous basement. Site U1373 on Rigil Guyot was an alternate site selected because operations for Site U1372 on Canopus Guyot were cut short (see “Operations” in the “Site U1372” chapter [Expedition 330 Scientists, 2012b]). To increase our prospects for determining the most precise paleolatitude on the old end of the Louisville Seamount Trail, we resorted to alternate Site LOUI-6A on Rigil Guyot, which formed likely only 3 m.y. after Canopus Guyot. 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 material and 350 m into igneous basement. A full downhole logging series was planned, including the standard triple combination and Formation MicroScanner-sonic tool strings, the Ultrasonic Borehole Imager tool, and the third-party Göttingen Borehole Magnetometer tool. However, upon tagging the seafloor before starting Hole U1373A, a vibration-isolated television (VIT) camera survey clearly showed cobble fields covered by a patchy sediment blanket. A better spot with more sediment cover was selected using the VIT camera, but it became clear upon spudding the hole that a hard-ground entry had to be made. As a result, little to no soft pelagic sediment was recovered, and coring went straight into consolidated volcaniclastics. Basaltic basement was encountered at 33.9 meters below seafloor (mbsf). Because reentry using a free-fall funnel failed, Hole U1373A had to be abandoned at only 65.7 mbsf. No downhole logging could be carried out. 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. A sufficiently large number of these independent cooling units would allow high-quality estimates of paleolatitude to be determined, and any recorded paleolatitude shift (or lack thereof) could 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 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. Another important objective of Expedition 330 at Site U1373 was to use new paleolatitude estimates, 40Ar/39Ar ages, and geochemical data to decide whether the oldest seamounts in the Louisville Seamount Trail were formed close to the 18°–28°S paleolatitude determined from ODP Leg 192 basalt for the Ontong Java Plateau (Riisager et al., 2003) and whether this large igneous province was genetically linked to the Louisville hotspot or not. Such a determination would prove or disprove the hypothesis that the Ontong Java Plateau formed from massive large igneous province volcanism at ~120 Ma, when the preceding plume head of the Louisville mantle upwelling reached the base of the Pacific lithosphere and started extensive partial melting (e.g., Richards and Griffiths, 1989; Mahoney and Spencer, 1991). Finally, basalt and sediment cored at Site U1373 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 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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