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doi:10.2204/iodp.pr.330.2011

Introduction and background

Understanding the nature of mantle plumes is a critical goal of modern Earth sciences. The extent to which hotspots conform to the Wilson-Morgan fixed plume hypothesis (Wilson, 1963; Morgan, 1971) fundamentally constrains the assumptions used in models of mantle convection. To date, studies of the Hawaiian-Emperor Seamount Trail have dominated our thinking about hotspot volcanism, and, as a consequence, models for the construction and evolution of intraplate volcanoes, plate motion, and hotspot motion are strongly biased toward the Hawaiian hotspot. Without comparable data from any other important hotspot trail, many key questions remain unanswered. The Louisville hotspot trail (Fig. F1) is one of only three primary hotspots in the Pacific (together with Hawaii and Easter; Courtillot et al., 2003) and has great potential for providing these answers. New results from Integrated Ocean Drilling Program (IODP) drilling of the Louisville Seamount Trail during Expedition 330, together with existing data and future drilling of other hotspot trails in the Atlantic and Indian Oceans, will provide the best available opportunity to assess the importance of the motion between hotspots (or groups of hotspots) and true polar wander. These data, in turn, will provide some of the most valuable information on Earth's convection regime and a crucial calibration of current mantle flow models and global plate circuit reconstructions.

Hotspots and the (deep) mantle plumes that are presumed to be their underlying cause (e.g., Morgan, 1971) are essential features in contemporary geodynamic models of the Earth's mantle. One of the attributes that frequently has been assigned to mantle plumes is their fixity in the mantle, which contrasts starkly with the overlying plates that move at rates of up to 100 mm/y. However, plume theory does not demand fixity (e.g., Steinberger and O'Connell, 1998; Koppers et al., 2001), and paleomagnetic evidence collected during Ocean Drilling Program (ODP) Leg 197 from four Emperor seamounts (Detroit, Suiko, Nintoku, and Koko) indicates that the Hawaiian hotspot has moved at a speed similar to that of plate motion for tens of millions of years (Kono, 1980; Petronotis et al., 1994; Tarduno et al., 2003, 2009; Duncan et al., 2006). Three-dimensional mantle convection computations confirm this notion by generating mantle plumes with long, narrow thermal conduits that are anchored (deep) in the mantle. Nevertheless, these computations show plume migration rates of 10 mm/y and higher, primarily depending on the assumed viscosity contrast between the lower and upper mantle and the configuration of the subducting plates (Lowman et al., 2004).

These observations raise critical questions. Is the rapid motion documented for the Hawaiian hotspot an isolated event or does it happen to other hotspots as well? If so, do these hotspots move in a sufficiently coherent fashion that subsets can be used as a moving reference frame for reconstructing past plate motions? To address these questions we need to distinguish between the following geodynamic end-member models:

1. The three primary hotspots in the Pacific (Hawaii, Louisville, and Easter) move coherently over geological time and thus show minimal interhotspot motion (Wessel and Kroenke, 1997; Courtillot et al., 2003).
2. These primary hotspots show very different motions, resulting in increased interhotspot motions, as predicted by mantle flow model calculations (Steinberger, 2002; Steinberger et al., 2004; Koppers et al., 2004; Steinberger and Antretter, 2006; Steinberger and Calderwood, 2006).

The first model predicts that the motion of the Louisville hotspot should be equivalent to the 15° of southern motion documented for the Hawaiian hotspot between 80 and 50 Ma. The second model predicts an essentially eastward motion for the Louisville hotspot over the last 120 m.y., with a maximum shift in paleolatitude not exceeding 6° between 80 and 50 Ma and the present day, depending on various assumptions used in the mantle flow models (Fig. F2). Both models can be tested by drilling the Louisville Seamount Trail during Expedition 330 and accurately determining the paleolatitudes (from detailed paleomagnetic measurements on individual lava flows) and ⁴⁰Ar/³⁹Ar age dates for four seamounts between 80 and 50 Ma. For this purpose Expedition 330 provided a direct comparative test that mimics Leg 197 drilling in the Emperor Seamounts as closely as possible (Tarduno et al., 2003; Duncan and Keller, 2004; Duncan et al., 2006, 2007) by targeting guyots equivalent in age to Detroit (76–81 Ma), Suiko (61 Ma), Nintoku (56 Ma), and Koko (49 Ma) (Fig. F1).

Although determining the paleolatitudes in context of a high-resolution ⁴⁰Ar/³⁹Ar age framework was the main objective of Expedition 330, this project also afforded the opportunity to constrain the eruptive cycle and geochemical evolution of typical Louisville volcanoes. The Hawaiian and Louisville hotspots have been labeled as primary hotspots in the Pacific on the basis of the presence of obvious linear age progressions, long-lived and continuous volcanism, large buoyancy fluxes, and high (in the case of Hawaii) ³He/⁴He ratios (Courtillot et al., 2003; Koppers et al., 2003). Such hotspots are theorized to represent the locales at which plumes are rising from deep in the mantle, perhaps from near the core/mantle boundary (Clouard and Bonneville, 2001; Davaille et al., 2002; Courtillot et al., 2003). Unlike earlier studies that postulated that many (or even all) hotspots represent plumes originating from the core/mantle boundary, Courtillot et al. (2003) have argued that most hotspots arise from relatively shallow levels and that only a small number of primary plumes ascend from the core/mantle boundary. Nonetheless, among the primary hotspots of the Pacific Ocean, some marked differences in geochemistry and volcanic evolution are apparent. For example, the almost exclusive recovery of alkali basalts in the Louisville Seamount Trail (Hawkins et al., 1987; Vanderkluysen et al., submitted) raises the question of whether Louisville volcanoes have an alkalic shield-building phase instead of the tholeiitic shield-building phase that is a trademark of Hawaiian volcanoes. One possibility is that the shield stage of a Louisville volcano reflects systematically lesser amounts of partial melting than in the Hawaiian case, caused by deeper melting under an always uniformly thicker lithosphere. In addition, isotopic and trace element data from Louisville suggest a long-lived and remarkably homogeneous mantle source (Cheng et al., 1987; Hawkins et al., 1987; Vanderkluysen et al., submitted). If these volcanoes have entirely alkalic shield-building phases that are isotopically homogeneous over 80 m.y., this has major implications for how we think volcanism works for the Louisville Seamount Trail and intraplate volcanism in general.

Mantle geodynamics and hotspot motion

Recently it has been proposed that, because of large-scale mantle flow, plume conduits may become strongly tilted at times (e.g., Steinberger and O'Connell, 1998), which possibly may explain the fast southward hotspot motion observed for the Hawaiian hotspot (~40 mm/y) between 80 and 50 Ma (Tarduno et al., 2003). Such strong tilts may happen if the conduit is affected by a lower mantle return flow between cold downwellings associated with subduction and large-scale upwellings in the neighborhood of "superplumes." Alternatively, the capturing, bending, and releasing of the Hawaiian mantle plume by an ancient ridge system may also explain these observations (Tarduno et al., 2009). For the Hawaiian hotspot, all of these ingredients are present—a zone of past subduction to the north, a large-scale upwelling related to the Superswell to the south (Tarduno et al., 2003), and the waning of the Kula-Pacific ridge system to the north of the hotspot—allowing a possibly captured Hawaiian plume to quickly return to its original straight position in a dominantly southward flow (Tarduno et al., 2009). However, because the Louisville hotspot lies south of the Superswell, the closest subduction system has always been located to its west, and no spreading center was located north and in close proximity for most of its geological history, this pattern is not compatible with a similar rapid southward motion for the Louisville hotspot.

Modeling by Steinberger et al. (2004) shows the expected results of this configuration, whereby the Hawaiian hotspot indeed moves 15° south since 80 Ma, whereas the Louisville hotspot is predicted to have moved in an easterly direction between 130 and 60 Ma, and only ~2.5° south since 60 Ma (Fig. F2A). In these models a large-scale mantle flow field is first calculated from mantle density heterogeneities (as derived from seismic S-wave speed anomalies) by applying a radial mantle rheology structure (with a lower mantle assumed to be more viscous) and by using tectonic plate motions as boundary conditions (Steinberger and O'Connell, 2000; Steinberger and Calderwood, 2006). Within the modeled mantle flow field, then, a vertical plume conduit is inserted that gets advected over time, resulting in the sometimes strong tilting of mantle plumes and the drifting of hotspots (Steinberger and O'Connell, 2000; Steinberger and Antretter, 2006). Advection dominates the motion of a plume at depths where it rises relatively slowly in comparison to the overall mantle flow field, typically in the lower mantle. As a result, the hotspot motions seem in many cases (including Louisville) to be similar to the predicted horizontal flow in the mid-mantle, at which depth the transition occurs from motions dominated by advection to a more vertical motion dominated by the buoyant rising of the mantle plumes (Steinberger, 2000). These characteristics remain the same regardless of whether mantle plumes originate at the core/mantle or 670 km boundary layers. In spite of the large uncertainties in the data and the assumptions on which they are built, these mantle flow models provide an excellent basis for placing the geological data derived from the Louisville and other seamount trails into a more complex geodynamic context.

Mantle flow models using the modeling approach of Steinberger et al. (2004) thus show a largely eastward motion for the Louisville hotspot, which is very different from the ~15° southward motion for Hawaii during the same time interval. This longitudinal motion for the Louisville hotspot is followed by only a minor latitudinal shift over the last 60 m.y. (Fig. F2B, Model 5 in Fig. F2C). These models, however, show large variations in their predictions depending on the assumptions made, such as plume initiation age, root depth, viscosity structure, plume buoyancy, plume rising speed, plate motion history, and mantle viscosity. Antretter et al. (2004) and Steinberger and Antretter (2006) have considered the possible effects of these assumptions in more detail for the Louisville hotspot and predict paleolatitude shifts between almost none and ~8° to the south, as integrated over the last 80 Ma. However, the majority of their model runs (see Fig. F2C for six representative examples) show latitudinal motions for Louisville that are significantly less than those observed for Hawaii. Also, for models that show a faster Louisville hotspot motion, the increased motion always is more eastward, in a direction away from the subduction zone and toward the spreading ridge.

The shifts in paleolatitude of the Pacific hotspots also can be estimated by transferring plate motions from Indo-Atlantic hotspots to the Pacific plate using global plate circuits, assuming the fixity of the Indo-Atlantic hotspots. For example, Doubrovine and Tarduno (2008a, 2008b) showed that the Late Cretaceous to Paleogene apparent polar wander path (APWP) transferred from the paleomagnetic data of the Atlantic-bordering continents to the Pacific plate is consistent with the paleolatitude shift as observed in the Hawaiian-Emperor Seamount Trail. Shifts in paleolatitude of the Louisville hotspot can be predicted by the same approach (Fig. F3), although uncertainty still remains in the global plate circuit, which may go through East and West Antarctica (EANT-WANT; Cande et al., 1995) or, alternatively, through the Lord Howe Rise (LHR; Steinberger et al., 2004). Also critical is the current location of the Louisville hotspot, which still is a matter of debate because of its faint expression at the younger end of this seamount trail (Lonsdale, 1988; Raymond et al., 2000; Wessel and Kroenke, 1997; Koppers et al., submitted). All plate reconstructions (using different combinations of plate circuits and present-day hotspot locations) yield predictions that become significantly different from the position of the Louisville chain prior to 45 Ma (Fig. F3). As with the mantle flow models, a large longitudinal shift is apparent at 78 Ma when comparing the oldest seamounts in the trail to the reconstructed position for Chron C33, which all plot markedly west of the Louisville Seamount Trail. It is interesting to note that the predicted potential southward shift in these plate tectonic reconstructions seems less pronounced for the oldest seamounts (3.8° and 4.0°, depending on the plate circuit used), is largest (8.9° and 9.8°) for ~53 Ma seamounts (which actually indicates a possible 4°–5° northward motion of the plume between 80 and 50 Ma), and is nonexistent for seamounts younger than ~45 Ma.

The above geodynamic and plate tectonic models thus provide us with different predictions for the latitudinal history of the Louisville hotspot. These models were groundtruthed during IODP Expedition 330 and should allow us to distinguish between the following:

1. That the Louisville hotspot shows a pronounced southward motion (up to 15°) that is comparable to the southward motion of the Hawaiian hotspot, providing evidence for a common motion of the mantle underlying the Pacific plate with respect to Earth's spin axis;
2. That the Louisville hotspot shows an insignificant cumulative latitudinal shift (<2°–6°), supporting the Steinberger et al. (2004) and Steinberger and Antretter (2006) mantle flow models that predict minimal latitudinal motion and a pronounced easterly longitudinal motion; or
3. That the Louisville hotspot shows a variable (but significant) latitudinal motion between 80 and 50 Ma in combination with a large longitudinal shift, reconciling the observations made with global plate circuit models.

By comparing paleolatitudes from the paleomagnetic measurements on cored basalt flows and high-precision ⁴⁰Ar/³⁹Ar ages for the Hawaiian and Louisville Seamount Trails, the drilling results from Expedition 330 will offer strong constraints for one of these possibilities. Finding a large latitudinal shift between 80 and 50 Ma clearly would indicate that current assumptions in the mantle flow models are wrong. Finding no appreciable shift, on the other hand, would indicate significant interhotspot motion between the Hawaiian and Louisville hotspots and a stronger local control on the mantle flow regime.

Paleolatitude measurements of the Louisville Seamount Trail can also be used for comparisons of interocean motion between hotspots in the Pacific, Indian, and Atlantic Oceans. In previous studies, large discrepancies of up to 20° in motion have been reported between hotspot groups in the Pacific and Atlantic/Indian Oceans (Cande et al., 1995; DiVenere and Kent, 1999; Raymond et al., 2000). These observations have led to models of large-scale motion of the mantle underneath each ocean or to explanations involving true polar wander, which is defined as a coherent shift of the entire mantle relative to the spin axis (Goldreich and Toomre, 1969; Gordon, 1987; Besse and Courtillot, 1991, 2002; Torsvik et al., 2002). Drilling of the Louisville Seamount Trail helped to expand the necessary global paleolatitude and ⁴⁰Ar/³⁹Ar age databases that are needed to evaluate the above scenarios. In fact, Expedition 330 is collecting paleolatitude data for a time interval similar to that sampled in the Emperor Seamount Trail during Leg 197 (Kono, 1980; Tarduno et al., 2003), and in the future additional data may be collected from ocean drilling expeditions to the Chagos-Laccadive and Ninetyeast Ridges in the Indian Ocean and Walvis Ridge in the Atlantic Ocean. These expeditions may provide us with a state-of-the-art paleolatitude database covering five major hotspot trails in three oceans, all between 80 and 50 Ma in age. At that point it will be clear whether the observed paleolatitude shifts in hotspots are best explained by a coherent motion of all hotspots relative to the spin axis, by a coherent motion of hotspot groups within each ocean domain (but with a relative motion between these groups), or by incoherent motion of all individual hotspots.

Age relations along the Louisville Seamount Trail

Using high-resolution ⁴⁰Ar/³⁹Ar age data, Koppers et al. (2004) found that the age progression for the Louisville Seamount Trail is overall nonlinear (blue squares in Fig. F4A). Site survey ⁴⁰Ar/³⁹Ar age data from the SO167 and AMAT02RR expeditions underline the nonlinear character of the Louisville age progression (Fig. F4A), yet they also show a very systematic age-progressive trend from ~20 to 80 Ma (O'Connor et al., submitted; Koppers et al., submitted). Deviations from earlier reported ages (Watts et al., 1988) become noticeable around 35 Ma and are most significant toward the old end of the trail, where some guyots are now dated ~15 m.y. older than previously reported (Koppers et al., 2004). For now, and pending new age results from Expedition 330, the age progression for the Louisville Seamount Trail seems best approximated by the purple line in Fig. F4A, which simply envelops the oldest ages and follows the model of Koppers et al. (2004). Interestingly, new age information from the SO167 dredges now reveals a more complex age distribution in the older portion of the Louisville Seamount Trail, with some seamounts showing ages considerably younger than the age-progressive volcanism, which is the first evidence of late-stage or rejuvenated volcanic activity for this trail (O'Connor et al., submitted). Understanding the evolution of a typical Louisville seamount through state-of-the-art ⁴⁰Ar/³⁹Ar age dating (Fig. F4B) therefore will be crucial in evaluating models predicting relative motions between hotspots. For example, Wessel and Kroenke (2009) examined the temporal variations in the geographical separation between the Louisville and Hawaiian hotspots, as measured by any change in their great-circle distance over time. By considering both the geometry and the ⁴⁰Ar/³⁹Ar ages of the seamounts constituting these two primary seamount trails, they concluded that the Louisville hotspot may have moved several degrees to the south relative to Hawaii before ~55 Ma because congruent seamounts in both trails formed at a larger great-circle distance from each other during that time. However, they also showed that the Louisville and Hawaiian hotspots kept a very constant separation after that episode of interhotspot motion. Between 55 Ma and the present day, both hotspot systems seemingly have been moving in tandem, without any significant interhotspot motion for a prolonged period of time.

Geochemical evolution of the Louisville hotspot

The construction and geochemical history of an intraplate seamount often is envisioned to resemble that of a typical Hawaiian hotspot volcano (Clague and Dalrymple, 1988). There is, however, little empirical evidence for a similar evolutionary sequence in the Louisville Seamount Trail. Essentially all igneous rocks dredged from the Louisville Seamount Trail are alkalic basalts, basanites, or tephrites containing normative nepheline (Fig. F5A) (Hawkins et al., 1987; Vanderkluysen et al., submitted). In addition, isotopic and trace element data from this seamount trail suggest a long-lived and remarkably homogeneous mantle source equivalent to the proposed "common" components FOZO (Focal Zone) or C (Fig. F5B) (Cheng et al., 1987; Hawkins et al., 1987; Vanderkluysen et al., submitted). The minor variations in major and trace elements appear to be controlled mostly by variable extents of melting and fractional crystallization but with little influence from mantle source heterogeneities (Vanderkluysen et al., submitted; Beier et al., submitted).

This raises important questions that will be addressed by geochemical studies of the samples cored during Expedition 330. For example, do Louisville volcanoes evolve through geochemically distinct shield, postshield, and rejuvenated stages, similar to Hawaiian volcanoes? And, if so, is the dominant shield stage characterized by eruption of tholeiites or, quite the opposite, alkalic lavas? Tholeiites generally represent greater amounts of partial melting than do more alkalic lavas. One possibility is that the typical shield stage of a Louisville volcano reflects a systematically lesser amount of partial melting than in the Hawaiian case. It is also possible that dredging may have sampled only later stage lavas that in many locations cover the shield-stage flows. Interestingly, the least alkalic lavas obtained by dredging are from Osbourn Guyot near the Kermadec Trench, where extensive faulting may have exposed older shield-building lavas (Hawkins et al., 1987). The basement penetration achieved during Expedition 330 allowed direct sampling of the waning part of the shield-building stage, providing key constraints on the geochemical and magmatic evolution of the Louisville volcanoes in this primary hotspot system.

Hawaiian shield-stage lava flows also possess a wide range of isotopic and incompatible element compositions, which is even greater when data for postshield and posterosional lavas are included. In contrast, isotope and incompatible element ratios (e.g., Zr/Y and Nb/Y) for the Louisville Seamount Trail are surprisingly homogeneous (Fig. F5B). Another important difference between the Louisville and Hawaiian-Emperor Seamount Trails may be the age of the underlying seafloor at the time of volcano formation. It has been assumed that all Louisville volcanoes generally erupted onto seafloor that was ~40–50 Ma (Lonsdale, 1988; Watts et al., 1988; Lyons et al., 2000). This assumption includes the older (northwestern) portion of the Louisville Seamount Trail that formed close to the Osbourn Trough paleospreading center, which ceased its activity between 115 and 121 Ma (Downey et al., 2007). In contrast, only ~10 m.y. age differences between the seafloor and seamount formation are observed for the oldest Hawaiian seamounts; however, these age differences are larger than ~100 m.y. for the younger Hawaiian volcanoes (Keller et al., 2000; Caplan-Auerbach et al., 2000). Lithospheric thickness therefore seems to have been less variable for the Louisville hotspot than for Hawaii, in particular, because oceanic lithosphere tends to thicken more slowly after ~40 m.y. (e.g., Stein and Stein, 1993). Lithospheric thickness is a key control on partial melting because it determines the minimum depth of the top of the melting column and limits the extent of decompressional melting that occurs in the upwelling mantle (e.g., McKenzie and Bickle, 1988). Other things being equal, greater amounts of partial melting occur under thin lithosphere, and, in a mantle containing isotopically and chemically distinct lithologies, different mantle components are likely to begin to melt at different depths (Sun and Hanson, 1976; Ellam, 1992; Phipps Morgan and Morgan, 1999; Hoernle et al., 2000; Niu et al., 2002; Ito and Mahoney, 2005; Devey et al., 2003). For the Emperor Seamounts much of the observed isotopic and chemical variation may be related to changing proximity to a spreading center and related changes in lithospheric thickness (Keller et al., 2004; Regelous et al., 2003). In contrast, for most of the Louisville Seamount Trail, the limited isotopic variation is likely to reflect a remarkably homogeneous plume source or relatively uniform melting conditions over 80 m.y. This model, however, contradicts a recent study using seismic refraction that provided the first two-dimensional tomographic image of the internal structure of the oceanic lithosphere beneath seamounts (Fig. F6). This tomographic image for one of the oldest Louisville seamounts clearly delineates a downward-flexed Mohorovicic seismic discontinuity (MOHO; by as much as 2.5 km) that can only be explained by an elastic plate model in which this particular seamount was emplaced upon ~10 Ma oceanic lithosphere (Contreras-Reyes et al., 2010). This latter outcome agrees with the ocean crust age model for the Louisville Seamount Trail region presented in the global compilation of magnetic isochrons that went into the Müller et al. (2008) map. In this map the ocean crust is only ~10 m.y. older than the 78 Ma Osbourn Guyot, the oldest seamount in the Louisville Seamount Trail, but this age difference systematically increases and becomes ~70 m.y. for the 50 Ma Louisville seamounts.

Louisville as a primary hotspot

Courtillot et al. (2003) have argued that most hotspots arise from relatively shallow levels and that no more than three primary plumes ascend from the core/mantle boundary in the Pacific Basin: Hawaii, Easter, and Louisville. These authors suggest several criteria by which primary plumes may be assessed, the chief geochemical criterion being high ³He/⁴He ratios in hotspot lavas. Although agreement is not universal (e.g., Meibom et al., 2003), high ³He/⁴He ratios are considered by the great majority of scientists to be a sign of a deep-mantle origin (e.g., Allègre et al., 1983; O'Nions, 1987; Farley and Neroda, 1998). Hawaiian basalts, for example, have ³He/⁴He values as high as 35 R_A (where R_A is the atmospheric ³He/⁴He ratio), and even higher values have been reported for samples from Iceland. In comparison, mid-ocean-ridge basalts (MORB) typically have values of only 7–10 R_A (e.g., Graham, 2002). No He isotope data have been published for the Louisville Seamount Trail. Another geochemical indicator of a deep-mantle origin is high ¹⁸⁶Os/¹⁸⁸Os, which is interpreted by some workers to signify Os derived from the outer core. Only a few studies of Os isotopes in oceanic hotspot lavas have yet been performed, but anomalously high ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios have been discovered in at least some of the primary hotspots, such as Hawaii (e.g., Brandon et al., 1999). Although the interpretation of the elevated Os isotope ratios is disputable (e.g., Smith, 2003), combined studies of Os and He isotopes have the highest potential to reveal any deep-mantle signature in oceanic lavas. Drilling during Expedition 330 has recovered a high number of basalt flows with relatively unaltered olivine crystals and oxide minerals that will be used to successfully test these geochemical criteria and to prove or disprove that the Louisville hotspot is a primary hotspot. If these tests indicate that Louisville does not have a deep (lower)-mantle origin, this outcome will place limits on the mantle flow models by forcing a shallower root for its mantle plume. Although geodynamic modeling likely cannot resolve the depth of origin between primary and secondary hotspots, geochemical results possibly can on the basis of characteristically high ³He/⁴He and ¹⁸⁶Os/¹⁸⁸Os deep-mantle plume ratios.

Relation between the Louisville hotspot and the Ontong Java Plateau

The Ontong Java Plateau (OJP) has been proposed to be a large igneous province that is the product of the Louisville hotspot's initial plume-head phase (e.g., Richards and Griffiths, 1989; Mahoney and Spencer, 1991; Tarduno et al., 1991). However, existing isotopic data for Louisville dredge samples (Cheng et al., 1987; Vanderkluysen et al., submitted) offer no strong support for such a connection (Mahoney et al., 1993; Tejada et al., 1996). Ontong Java and Louisville samples have similar age-corrected Nd and Sr isotope values, but the Louisville lavas have significantly higher Pb isotope ratios than Ontong Java basalts (Fig. F5B), and the difference is more than can be accounted for by ingrowth of radiogenic Pb in the mantle source between 120 and 80 Ma (Vanderkluysen et al., submitted). Expedition 330 drill sites will provide a much more rigorous test of any geochemical connection between the OJP and the Louisville Seamount Trail to more definitively prove or refute this possible genetic relation.

Mantle temperatures of the Louisville hotspot

Comparable to other primary hotspots (e.g., Hawaii and Samoa), potential temperatures of the Louisville mantle plume sources are expected to be 100°–300°C higher than the 1350° ± 50°C temperature of an upper-mantle MORB source (Putirka, 2008; Courtier et al., 2007). Fresh olivine-phyric basalts were encountered at all Expedition 330 drill sites. Because we drilled into the alkalic (shield) lavas of the Louisville Seamounts, these rocks are excellent candidates for determining the Mg-Fe compositions of olivine phenocrysts and melt-inclusions therein. In turn, these compositions will 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 (e.g., Putirka et al., 2007). The challenge here is to make these determinations on the most Mg-rich olivines that also come closest to the parental magma compositions, a task that may be more complicated for the Louisville Seamounts because all samples studied so far are relatively evolved and typically alkalic. However, if Louisville volcanism is typified by alkali basalt shield building, and not by Hawaiian-style tholeiites, it may be the case that no true picritic lavas will be encountered, as was the case during Leg 197, where olivine-rich basalts were found after drilling >250 m into the Emperor Seamounts (Duncan et al., 2006).

Melt inclusions and volatiles in volcanic glasses

Melt inclusions also provide key insights into the "true" (lack of) compositional heterogeneity in the mantle source from which the Louisville magmas have been generated. Because melt inclusions are small (tens to a few hundreds of micrometers in diameter) volumes of melt trapped in phenocrysts, they can remain isolated from differentiation and alteration that affect the external melt, thereby preserving more primitive liquid compositions compared to those determined from groundmass glass or bulk-rock analyses (Sobolev, 1996; Kent, 2008). If trapped in an early crystallizing phase, such as olivine, they may even reveal primary magma compositions and can provide insights into the mantle sources of these magmas (Saal et al., 1998; Hauri, 2002). These inclusions often span a range of compositions wider than those exhibited by the groundmass glass or bulk rock, when they are being trapped at different stages in the evolution of the magma (Frezzotti, 2001; Danyushevsky et al., 2002). Melt inclusion studies will therefore complement bulk-rock analyses of the Louisville basaltic rocks because potential heterogeneities are more likely to be recorded by melt inclusions. In addition, if these melt inclusions are trapped before or during volatile degassing, suitable crystalline host phases (without cleavage or any other imperfections) may act as pressure vessels, which can isolate the trapped melt from pressure changes affecting the bulk magma. Melt inclusion volatiles therefore may preserve the initial magma volatile concentration and speciation and the degassing path undertaken by the magma (Wallace, 2005).

Hydrothermal and seawater alteration

Seamounts are believed to rival mid-ocean-ridge flanks in terms of the total mass of seawater that has been fluxed through their basement. They therefore are likely to play a critical role in regulating crust-ocean chemical exchange fluxes. In this regard, ocean-ridge-flank systems have been studied by several authors (Alt and Teagle, 1999, 2003; Bach et al., 2003), but the alteration and veining history of seamounts has not yet been studied in any detail. During Expedition 330, five submerged seamounts having crustal ages between 50 and 80 Ma and only thin sediment covers were drilled, providing a unique opportunity for investigating the exchange fluxes between these seamounts and the ocean. Whole-rock geochemical studies as well as Sr, O, H, and C isotopic analyses of carbonate veins and alteration minerals will provide the basis for reconstructing the seawater-rock interaction through the lifetime of hydrologic activity on these seamounts. Because of the thin sediment cover on the Louisville Seamounts it is likely that seawater access to these volcanic basements has been long term, and thus they are excellent targets for assessing the magnitude of carbonate vein formation in aging oceanic crust and its role as a global CO₂ sink.

Geomicrobiology and fossil microbial traces

Since the 1990s, microbiologists on ODP and IODP expeditions have documented the presence of microbial life in deeply buried sediments and the basaltic basement (Fisk et al., 1998; Parkes et al., 1994). Active microbial life has been detected as far below the seafloor as 1626 m (Roussel et al., 2008), and the introduction of molecular biology into marine ecology has led to great advances in our understanding of microbial life below the seafloor (Cowen et al., 2003; Inagaki et al., 2006; Mason et al., 2010). To this day, however, the microbiology performed on ODP and IODP expeditions has concentrated on sediments, with the notable exception of expeditions to the Juan de Fuca Ridge (Cowen et al., 2003) and the Atlantis Massif (Mason et al., 2010), both of which sampled younger than 3.5 Ma oceanic crust. Stable isotope evidence (Rouxel et al., 2008) and microbial fossils (Fisk et al., 1998) indicate that there is a subsurface biosphere in older basement rocks as well. The microbial fossil traces found there are believed to be due to the boring activity of microorganisms that mostly colonized the volcanic glass to which the bacterial cells and filaments are connected (Thorseth et al., 1995; Fisk et al., 1998; Furnes et al., 2001). A different type of boring activity is recorded in carbonates precipitated in the veins, vesicles, and void space of volcanic rocks (Peckmann et al., 2008; Eickmann et al., 2009; Ivarsson et al., 2008). These carbonate cements have enclosed filaments that very closely resemble budding and branching microbial structures. Expedition 330 provides an excellent opportunity to study both living and extant microbial residents within the old, 50–80 Ma subseafloor volcanic rocks that make up the Louisville Seamounts. Differences in microbial population between the overlying (pelagic) sediments and volcaniclastic layers and the basaltic basement are of keen interest, as is variation between lava flows with depth into the seamount structures and between seamounts of different age.

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