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doi:10.2204/iodp.pr.330.2011

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, of which 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 core material was characterized by low degrees of alteration, providing us with 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 measurement of the recovered cores and downhole logging (at two sites) revealed some surprises about how the Louisville volcanoes were constructed. Even though these flat-topped guyots once were volcanic islands, like the Hawaiian and Easter Islands, the drilling results provided only sparse signs of subaerial volcanism. Rather, after drilling through the thin sediment covers that now overlie these volcanoes, primarily shallow to deeper submarine volcanic sequences were recovered. The Louisville volcanoes thus seem to have been smaller islands or 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, or "guyots," at the end of their life cycles. 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

• Melt inclusions in fresh olivine crystals,

• Volcanic glass found on the water-quenched contacts of lava flows but also as part of hydrovolcanic hyaloclastite sediments,

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

• Carbonate, zeolite, and celadonite alteration minerals,

• Various micro- and macrofossils, and

• Many different kinds of unaltered alkaline basalts.

The large quantity and excellent quality of the recovered sample material allow us to address all the scientific objectives of this expedition and beyond, 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 genetic relation between the Louisville hotspot and the 120 Ma Ontong Java Plateau,
5. Determining the degree, potential temperature, and depth of partial melting 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 within the seamount igneous basement.

Paleolatitude record of the Louisville hotspot

The main objective of Expedition 330 was to core deep in the igneous basement of several Louisville seamounts to establish a record of the past motion of the Louisville hotspot between 80 and 50 Ma, which would show whether the Louisville hotspot has undergone a large ~15° shift in paleolatitude, as has been documented for the Hawaiian hotspot (Tarduno et al., 2003). If the Louisville and Hawaiian hotspots did not move in concert over time, this would indicate that both primary Pacific hotspots show considerable interhotspot motion, 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 if we are to determine whether these two primary hotspots have moved coherently or not and understand the nature of hotspots and convection in the Earth's mantle.

Accurate determination of the paleolatitude record for the five drilled Louisville seamounts and comparison of these records to the current ~51°S location of the Louisville hotspot requires the recovery of a sufficient number of time-independent lava flows at each drill site. Preferably, these lava flows also should have erupted over a geological period of maybe tens of thousand to a couple of millions of years in order to effectively average out paleosecular variation of the Earth's magnetic field. Drilling during Expedition 330 resulted in a large number of in situ lava flows, pillow basalts, or other types of volcanic products, such as auto-brecciated 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 ~78 and 73 Ma, respectively. Remarkably, all drill sites also recovered large quantities of hyaloclastites, volcanic sandstones, and basaltic breccias, which in many cases show paleomagnetic-consistent 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 our quality-control filters. However, most importantly, almost without exception the half-core data are in good agreement with the discrete measurements using either AF or thermal demagnetization. For Site U1374 on Rigil Guyot we also observed a magnetic polarity reversal in the cored sequence that provided 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 us with a more accurate and precise estimate of these paleolatitudes, in particular 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, the analysis of the paleolatitude record for the Louisville hotspot will be supplemented by downhole logging data collected with the 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 us with 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 VGP 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. 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 it will be crucial in evaluating the possible relative motion between different hotspot systems. However, high-precision age measurements are required to be able 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 number of relatively unaltered basalts were recovered with K₂O concentrations between 0.3 and 1.4 wt% and low LOI, mostly <3%. The ⁴⁰Ar/³⁹Ar geochronology of these basalts will therefore likely yield ages with precisions ranging between 0.2 and 0.4 Ma (2σ) that will allow us to date and resolve the duration of, and the potential time gaps between, multiple eruptive units for each single drill site.

Paleontological evidence from calcareous nannofossils and planktonic foraminifers and the occurrence of macrofossils in the sediments overlaying the igneous basement provided strong indications that the lavas cored were as old as predicted by previous studies (Koppers et al., 2004) and in some cases apparently even older. This means that in all cases we will be able to age date the main constructional phase of the drilled seamount volcanoes, which in turn will provide us with 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 Seamount Trail 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, indeed, the drilled rocks of the Louisville Seamounts have all alkalic or transitional compositions, and tholeiitic basalt was encountered at none of the drill sites. Trace element shipboard data show that the drilled lavas also 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 are needed to further characterize this unique hotspot system and to define the "true" (lack of) compositional heterogeneity in the mantle source from which the Louisville magmas have been generated.

Fresh olivine phenocrysts were recovered at almost every Expedition 330 drill site, 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 an upper or lower mantle origin. The fresh olivines will also allow us to carry out analyses on melt inclusions trapped in these phenocrysts to reveal primary magma compositions and to provide insights into the mantle sources of these Louisville magmas. Because these inclusions are often found to span a range of compositions wider than those exhibited by groundmass glass or bulk rock (Frezzotti, 2001; Danyushevsky et al., 2002), these 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, Louisville alkalic basalts are excellent candidates 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 the olivines directly to that of the liquid from which they crystallized (Putirka et al., 2007).

Relation between Louisville hotspot and the Ontong Java Plateau

The Ontong Java Plateau (OJP) 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). Expedition 330 drill sites will provide a much more rigorous test of the potential genetic relation between Louisville and the formation of the OJP by looking for conjunctions in the Louisville and OJP 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 only some condensed pelagic limestone intervals were recovered, but these did not exceed 30 cm in thickness. Nonetheless, together these limestones provide valuable insights in the paleoclimate record at high ~50° southern latitudes since Mesozoic times, in particular because the well-preserved Site U1376 limestone was likely formed toward the very 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 these 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, >60 microbiology samples were collected from four seamounts ranging in age between 80 and 50 Ma and having 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 within 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 keen 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 in most Expedition 330 drill sites, the search and study of microbial fossil traces will provide new facts 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.

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