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doi:10.2204/iodp.proc.330.108.2012 Site U13771Expedition 330 Scientists2Background and objectivesSite U1377 (prospectus Site LOUI-4B) on Hadar Guyot (168.6°W Guyot) was the sixth and final site completed during Integrated Ocean Drilling Program (IODP) Expedition 330 (Fig. F1). Hadar Guyot, the youngest seamount targeted during Expedition 330, has a measured 40Ar/39Ar age of 50.1 Ma (Koppers et al., 2011), similar to that of Koko Seamount in the Hawaiian-Emperor Seamount Trail. Hadar Guyot shows no evidence of tilting and, together with another small guyot (located northeast at 168.3°W) and one of the largest guyots (located southeast at 168.0°W), forms a cluster of seamounts in the Louisville Seamount Trail that formed on top of the Wishbone Scarp (Fig. F1). Hadar Guyot is the smallest seamount cored during Expedition 330, consisting of a single volcanic center with a base diameter of ~25 km. Like all Louisville Seamounts drilled, it has a flat summit plain, defining it as a guyot that at some point must have emerged above sea level as a volcanic island. Site U1377 was placed near the middle of this small edifice (Fig. F2), away from its shelf edges and any packages of dipping volcaniclastics on its flanks. The approach is similar to that used for Sites U1375 and U1376, but contrary to that used for Sites U1372, U1373, and U1374, which targeted in particular volcaniclastic rocks from the flank sequences. Side-scan sonar reflectivity and 3.5 kHz subbottom profiling data indicate that Site U1377 is covered with <8 m of pelagic sediment, and seismic reflection profiles suggest that this site is characterized by a 52 m thick section of dipping 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 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. Like at Site U1375 on Achernar Guyot, drilling at Site U1377 became difficult after instabilities were met in the uppermost part of the seamount formation, likely from the presence of loose clast-rich volcanic breccia in the sediment cover, which was only sparsely recovered (16% and 39% recovery in Holes U1377A and U1377B, respectively). Drilling reached 53.3 meters below seafloor (mbsf) in Hole U1377A and 37.0 mbsf in Hole U1377B, but because of time constraints and shallow penetration no logging was 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 (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 paleolatitude shift (or lack thereof) can be compared with that defined by 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. Such 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). However, because Site U1377 is located in close proximity to the Wishbone Scarp (Fig. F1), there was speculation that the recovered rocks might have a slightly different geochemical composition as a result of a possible step in lithosphere thickness across the scarp (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 putative homogeneity of the Louisville plume, its compositional evolution between 80 and 50 Ma, its potential temperatures, and its associated magma genesis, volatile outgassing, and magmatic 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 U1377 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 on 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 or other 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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