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Preliminary scientific assessment

Expedition 330 to the Louisville Seamount Trail was a record-breaking hard rock expedition with an exceptionally high recovery of rocks of surprising freshness considering their age and long-term submarine residence. In total, 1114 m of sediment and igneous basement at five seamounts was cored, and 806 m was recovered (average = 72.4%) (Table T3). At Site U1374 on Rigil Guyot, 522 m was drilled with a record-breaking 87.7% recovery. At all sites, most of the cored material is characterized by low degrees of alteration, providing a large quantity of samples of mostly well-preserved basalt containing, for example, pristine olivine crystals, fresh volcanic glass, unaltered plagioclase, and in one case mantle xenoliths and xenocrysts.

Extensive description and shipboard measurements of the recovered cores and downhole logging (at two sites) revealed some surprises about how the Louisville volcanoes were constructed. Although these flat-topped guyots once were volcanic islands like the Hawaiian and Easter Islands, drilling results provided only sparse signs of subaerial volcanism. Rather, primarily shallow to deeper submarine volcanic sequences were recovered after drilling through the thin sedimentary covers that now overlie these volcanoes. The Louisville volcanoes thus seem to have been smaller islands that remained above sea level for only a relatively short time before wave erosion planed off the upper part of the volcanoes to make them into flat-topped seamounts (i.e., guyots) at the end of their constructional phase. In the case of Site U1376 on Burton Guyot, evidence was found of an extensive algal reef and posterosional volcanism following the initial subsidence of this formerly volcanic island.

The good core quality provides a wide range of valuable seamount material for shore-based studies, including

  • Many different kinds of unaltered alkaline basalt;

  • Melt inclusions in fresh olivine crystals;

  • Volcanic glass found on water-quenched contacts of lava flows and as part of hyaloclastite breccia;

  • High-Mg olivine, clinopyroxene, and plagioclase phenocrysts;

  • Carbonate, zeolite, and celadonite alteration minerals; and

  • Various micro- and macrofossils.

The large quantity and excellent quality of the recovered sample material will allow all of the scientific objectives of this expedition to be addressed, including

  1. Constraining the paleolatitude history of the Louisville hotspot between 80 and 50 Ma;

  2. Reconstructing the age systematics along the Louisville Seamount Trail;

  3. Characterizing the geochemical evolution of the Louisville mantle source;

  4. Testing the relation between the Louisville hotspot and the 120 Ma Ontong Java Plateau;

  5. Determining the degree, potential temperature, and depth at which partial melting occurred for Louisville magmas;

  6. Adding crucial paleoceanography and paleoclimate data at 40°–50°S paleolatitudes in the Southern Ocean; and

  7. Exploring the unique geomicrobiology and fossil microbial traces in the igneous basement of the Louisville Seamounts.

Paleolatitude record of the Louisville hotspot

The main objective of Expedition 330 was to core deep in the igneous basement of several seamounts in the Louisville Seamount Trail in order to establish a record of the past motion of the Louisville hotspot between 80 and 50 Ma. This record will show whether the Louisville hotspot has undergone a large ~15° shift in paleolatitude, similar to the shift documented for the Hawaiian hotspot (Tarduno et al., 2003). If the Louisville and Hawaiian hotspots did not move in concert over time, it instead will indicate a considerable interhotspot motion between both primary Pacific hotspots, as predicted by mantle flow models (Steinberger et al., 2004; Koppers et al., 2004). Comparison of the Louisville and Hawaiian hotspots thus is of fundamental importance in determining whether these two primary hotspots have moved coherently or not, to understanding the nature of hotspots and convection in the Earth’s mantle, and to evaluating the possibility of true polar wander.

Accurate determination of the paleolatitude record for the five seamounts drilled and comparison of these records to the current ~50°–51°S location of the Louisville hotspot requires recovery of a sufficient number of time-independent lava flows at each drill site. Preferably, these lava flows should have erupted over a period of perhaps tens of thousands to a few million years in order to effectively average out paleosecular variation of the Earth’s magnetic field. Drilling during Expedition 330 resulted in a large amount of in situ lava flows, pillow basalts, and other types of volcanic products, such as autobrecciated lava flows, intrusive sheets or dikes, and peperites. In particular, the deeper holes on Canopus and Rigil Guyots, the two oldest seamounts drilled in the Louisville Seamount Trail, resulted in adequate numbers of in situ lava flows with (for now) eruption ages estimated to be ~75–77 and 72–73 Ma, respectively. Remarkably, all drill sites also recovered large quantities of hyaloclastites, volcanic sandstone, and basaltic breccia, which in many cases show consistent paleomagnetic inclinations when compared to lava flows bracketing these units, as shown by 9267 good-quality remanent magnetization measurements taken at 2 cm spacing from archive-half cores and by experiments on an additional 409 discrete shipboard samples that passed quality-control filters. However, most importantly, almost without exception the half-core data are in good agreement with the discrete measurements using either alternating-field or thermal demagnetization. For Site U1374 on Rigil Guyot a magnetic polarity reversal was also observed in the cored sequence, providing antipodal inclinations for both the normal and reversed polarity intervals cored. Overall, this is very promising for determining a reliable paleolatitude record for the Louisville Seamounts, but in order to achieve that goal detailed postexpedition paleomagnetic experiments need to be carried out on multiple (>4 per flow) discrete samples taken from all in situ lava flow units and from any other suitable lithology that may also reliably retain directional information. This will provide a more accurate and precise estimate of these paleolatitudes, particularly after a range of rock magnetic experiments have been carried out to study the character of the remanent magnetization held within both the basaltic lava flows and the large quantity of volcaniclastic sediments.

Importantly, analysis of the paleolatitude record for the Louisville hotspot will be supplemented by downhole logging data collected with the Göttingen Borehole Magnetometer (GBM). This third-party tool (Steveling et al., 2003) was run twice in Hole U1374A at Rigil Guyot and once in Hole U1376A at Burton Guyot, collecting continuous three-component magnetic data, together with the tool’s rotation history, using three built-in optical gyros. The data quality was significantly improved by inserting a truly nonmagnetic aluminum sinker bar directly above the GBM to isolate the tool from other magnetic parts higher up in the tool string. This provided a high-precision record of the magnetic field inside the borehole that with the continuously recorded rotation history of the GBM can be accurately reoriented and translated into geographic coordinates, allowing for in situ determination of inclinations and declinations of the seamount formations. This set of measurements will provide an independent record of the paleolatitude history of the Louisville hotspot and unique estimates of the past virtual geomagnetic pole positions of the Pacific plate on which the Louisville Seamounts formed.

Age systematics along the Louisville Seamount Trail

Radiometric dating of Louisville Seamount rocks using 40Ar/39Ar geochronological techniques is a crucial objective of Expedition 330 because it will provide the necessary time framework for determining the volcanic history of individual seamounts and the age progression along the Louisville Seamount Trail. In turn, this age information will aid in determining an accurate paleolatitude history for the Louisville hotspot and will be crucial in evaluating the possible relative motion between different hotspot systems. However, high-precision age measurements are necessary to resolve, for example, the total time of volcanic activity captured between the lowest and highest in situ lava flows at each drilled seamount site. During Expedition 330 a large amount of relatively unaltered basalt was recovered with K2O concentrations between 0.3 and 1.4 wt% and low weight loss on ignition of mostly <3%. The 40Ar/39Ar geochronology of this basalt will therefore likely yield ages with a precision of 0.2–0.4 Ma (2σ), which will allow the duration of and potential time gaps between multiple eruptive units at each single drill site to be resolved.

Paleontological evidence from calcareous nannofossils and planktonic foraminifers and the occurrence of macrofossils in the sediments overlying the igneous basement provide strong indications that the cored lava is as old as that predicted by previous studies (Koppers et al., 2004) and in some cases apparently even older. This means that in all cases the main constructional phase of the drilled seamount volcanoes can be age-dated, which in turn will provide the age-progressive timing required for detailed analyses of the paleolatitude record, refinement of the age progression along the Louisville Seamount Trail, and geodynamic modeling of the past motion of the Louisville mantle plume relative to the Hawaiian hotspot.

Geochemical evolution of the Louisville Seamounts

Expedition 330 also aimed to provide an improved understanding of the magmatic evolution and melting processes that have produced the Louisville volcanoes. Existing dredge data 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) and that the Louisville volcanoes might be typified by an entirely alkalic shield-building stage, in contrast to the characteristic tholeiitic shield stage of the Hawaiian-Emperor volcanoes (Hawkins et al., 1987). Shipboard chemical analyses show that the drilled rocks of the Louisville Seamounts have all alkalic or transitional compositions and that indeed tholeiitic basalt was not encountered at any of the drill sites. Trace element shipboard data show that the drilled volcanic rocks fall within the compositional fields defined by previous studies on dredge samples, thus reinforcing the remarkably homogeneous character of this primary hotspot. However, a full range of shore-based analyses including isotope studies is needed to further characterize this unique hotspot system and to define the “true” compositional heterogeneity in the mantle source from which Louisville magmas have been generated.

Fresh olivine phenocrysts were recovered at four of the six Expedition 330 drill sites, which will allow measurement of 3He/4He, an important noble gas isotope ratio that has never before been measured for the Louisville Seamount Trail but which may indicate whether this hotspot has a shallow- or deep-mantle origin. The fresh olivines will also allow melt inclusions trapped in these phenocrysts to be analyzed for primitive magma compositions and to provide insights into the mantle sources of these Louisville magmas. Because these inclusions often span a range of compositions wider than those exhibited by groundmass glass or bulk rock (Frezzotti, 2001; Danyushevsky et al., 2002), melt inclusion studies will complement the bulk-rock analyses of Louisville basaltic rocks. In addition, melt inclusions may preserve initial magma volatiles and the degassing path undertaken by the magma (Wallace, 2005). Finally, the Louisville basalts are an excellent candidate for determining Mg-Fe compositions of olivine phenocrysts and melt inclusions therein, which in turn may yield information about the source temperatures by relating the Mg/Fe ratio of olivine directly to that of the liquid from which it crystallized (Putirka et al., 2007).

Relation between the Louisville hotspot and the Ontong
Java Plateau

The Ontong Java Plateau is proposed to have been formed by the initial plume-head phase of the Louisville hotspot (e.g., Mahoney and Spencer, 1991; Tarduno et al., 1991). Results from Expedition 330 postexpedition studies will provide a much more rigorous test of the potential genetic relationship between Louisville and the formation of the Ontong Java Plateau by looking for conjunctions in the Louisville and Ontong Java paleolatitude histories and geochemical signatures. Even though this is a secondary objective that can be addressed only after the three primary objectives described above have been resolved, this test nevertheless will provide key insights in the mantle plume debate, especially for the plume-head–plume-tail model.

Paleoceanography and paleoclimate at high southern paleolatitudes

During Expedition 330 several intervals of carbonate were cored from the Louisville Seamounts, in particular at Site U1376 on Burton Guyot, where a ~15 m thick algal limestone reef was cored (66% recovery). On three of the other four seamounts drilled during Expedition 330, condensed pelagic limestone intervals were also recovered, but these did not exceed 30 cm in thickness. Nonetheless, these limestones provide valuable insights in the paleoclimate record at high (~50°) southern latitudes since the Cretaceous, in particular because the well-preserved Site U1376 limestone was likely formed toward the end of the Cretaceous or in the early Paleogene. It therefore might provide fundamental constraints on the ancient sea-surface temperatures and climate transitions in the greenhouse interval that are characteristic of this time period. Paleolatitudes, timing of formation, and eventually the drowning of such carbonate banks may provide evidence for a temperate climate during past warm periods at high latitudes in the southern Pacific Ocean, where paleoclimate data are mostly lacking (Premoli Silva et al., 1995; Wilson et al., 1998; Jenkyns and Wilson, 1999).

Geomicrobiology and fossil microbial traces

During Expedition 330, more than 60 microbiology samples were collected from four seamounts ranging in age between 80 and 50 Ma and from a maximum depth of 516 mbsf. This collection of igneous basement samples for microbiology is the largest of any hard rock expedition, most of which have focused particularly on young mid-ocean-ridge settings. Expedition 330 thus provides an excellent opportunity to study both living and extant microbial residents in the old subseafloor volcanic rocks that make up the Louisville Seamounts. Differences in microbial population between overlying (pelagic) sediments and volcaniclastic layers and the basaltic basement are of great interest, as is variation between different kinds of lava flows, with depth into the seamount structures, and between seamounts of different age. Because of the high number of fresh volcanic glass occurrences at most Expedition 330 drill sites, the search for and study of microbial fossil traces will provide new information on the activity of and boring patterns generated by glass-metabolizing microorganisms (Thorseth et al., 1995; Fisk et al., 1998; Furnes et al., 2001) in the largely unstudied seamount subsurface environment.