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doi:10.2204/iodp.proc.330.105.2012 Site U13741Expedition 330 Scientists2Background and objectivesSite U1374 (alternate Site LOUI-6B) was the third site completed during Integrated Ocean Drilling Program (IODP) Expedition 330 (Fig. F1) and the second of two sites (U1373 and U1374) drilled on Rigil Guyot. At a predicted age of ~73 Ma, this site 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 and early Cenozoic, this shift is expected to be largest for the oldest seamounts in the Louisville Seamount Trail. Because Sites U1373 and U1374 target two separate sequences of ancient lava flows on the same volcanic edifice and because Rigil Guyot is only slightly younger than Canopus Guyot, these three sites together are expected to significantly strengthen our determinations of the Louisville paleolatitude at the old end of the trail. Rigil Guyot shows no evidence of tilting and 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) topographic highs remain on the western portion of its summit plain along with a third, more obvious topographic high on the eastern part of the summit plain. Site U1374 was placed west of these two small topographic highs and near its western rift zone at ~1559 m water depth (Fig. F2). Side-scan sonar reflectivity and 3.5 kHz subbottom profiling data indicate that Site U1374 is covered with <10 m of pelagic sediment, and seismic reflection profiles suggest that this site is characterized by a 110 m thick section of volcaniclastics dipping toward the west and overlying igneous basement. Site U1374 was an alternate site requested during Expedition 330 to allow more flexibility while drilling Rigil Guyot. It was argued that if we encountered borehole instabilities comparable to those at Site U1372 on Canopus Guyot (see “Operations” in the “Site U1372” chapter [Expedition 330 Scientists, 2012b]) or if drilling at Site U1373 had to be abandoned for other unforeseen reasons, then we could divert to Site U1374 to continue drilling Rigil Guyot and still complete our most important scientific objective. Because reentry using a free-fall funnel failed for Hole U1373A and the first site at Rigil Guyot did in fact need to be abandoned, Hole U1374A was spudded about 5.6 nmi away on the western portion of its summit plain. 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. After successful drilling to 522 meters below seafloor (mbsf), the full logging program was carried out. Coring was particularly successful, with a record-breaking 88% average recovery 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 paleolatitude shift (or lack thereof) can be compared with seamounts in the Hawaiian-Emperor Seamounts 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 paleolatitude 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, which contrasts with 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 U1374 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 U1374 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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