SCIENTIFIC OBJECTIVES

The new paleomagnetic data should also allow for the construction of an improved Pacific apparent polar wander path (APWP). In addition to its utility in the study of Pacific plate kinematics, the APWP can provide the basis for improved paleogeographic reconstructions important for paleoclimate studies. Such reconstructions should aid in the use of proxy climate data used to define past latitudinal gradients (e.g., Huber et al., 1995; Zachos et al., 1994), and they may serve as a more stable reference frame than that based on fixed hotspots (Cottrell and Tarduno, 1997b).

Through our drilling approach (obtaining time-averaged paleomagnetic data at each site), we can also address other aspects of the geomagnetic field through Late Cretaceous to early Tertiary time. To fully understand the nature of the geomagnetic field, global data are required. Progress in our understanding of the geomagnetic field over the past 150 m.y. is hindered by the lack of sufficient high-resolution data from the Pacific plate. By targeting sites where a secular variation record can be obtained in basalt, significant advances can be made in our understanding of the time-averaged geomagnetic field and its intensity for Late Cretaceous to early Tertiary times.

2. Investigate The Time-Averaged Late Cretaceous to Early Tertiary Geomagnetic Field
The need for high-resolution paleomagnetic data to constrain this history reaches far beyond the paleomagnetic community. Recent advances in modeling that have produced realistic simulations of the geodynamo (e.g., Glatzmaier and Roberts, 1995) highlight one need for paleomagnetic constraints on model parameters. The nature and history of the time-averaged geomagnetic field is a major topic of interest for many scientists interested in studies of the Earth's deep interior (SEDI). A full description the past field requires data from the Pacific Ocean basin, because of potential longitudinal components. The geomagnetic field at radius r, colatitude , and longitude can be described by the gradient of the harmonic potential as

.

The Gauss coefficients g_l^m and h_l^m describe the size of spatially varying fields. For the present field and models of the Late Cretaceous to early Tertiary field the axial dipole term (g₁⁰) is overwhelmingly dominant. Therefore, other terms will not greatly affect the accuracy of data used to test hotspot motion hypothesis. However, the data obtained can be used to better constrain the Gauss coefficients of the past field. For nonzonal terms (m 0; i.e., those terms varying with longitude), data from the Pacific basin are essential because of its sheer size; no global description of the field can be considered complete without data from the region.

Whereas the general importance and need for Pacific data are generally appreciated, the methods used to summarize past data prior to modeling (spherical harmonic analysis) have been given less consideration. For the early Tertiary and Late Cretaceous, plate motion cannot be neglected as they can for analyses of data over the past 5 m.y. (Constable, 1992) but instead the data must be first rotated into a common reference. The few analyses that have tried to incorporate data from the Pacific (principally older seamount results) have relied on a fixed hotspot frame of reference; hence previous estimates of Gauss coefficients may contain considerable errors if the hotspot motion hypothesis is correct.

Interestingly, these analyses show a dramatic change in the Gauss coefficients (a change in sign) during the critical Late Cretaceous to early Tertiary interval we have targeted for study (Livermore et al., 1984) (Fig. 7). Therefore, the data collected at the sites proposed for study can simultaneously address the hypothesis of hotspot motion and the reality of this change in sign of the spatially varying Late Cretaceous-early Tertiary geomagnetic field.

3. Investigate Late Cretaceous to Early Tertiary Geomagnetic Paleointensity
When compared with the considerable success of studies that utilize directional data derived from paleomagnetic measurements, work devoted to understanding the past intensity of the geomagnetic field has advanced more slowly. However, the long-term variations of paleointensity are essential for a complete description of the field, as well as for understanding the long-term magnetic signature of ocean crust. One reason progress has been slow is related to selection criteria needed to ensure reliable paleointensity determination. The preferred method of paleointensities measurement, Thellier-Thellier double heating experiments of basalts (Thellier and Thellier, 1959; as modified by Coe, 1967), often encounter problems due to chemical alteration during heating. Significant recent progress has been made by studying basaltic glass (Pick and Tauxe, 1993), which shows ideal magnetic properties. The available DSDP and ODP sites having basaltic glass have now been analyzed (Juarez et al., 1998), so further progress requires additional drilling (Fig. 7).

The coring we propose has the potential to yield several reference sites for Late Cretaceous-early Tertiary paleointensity. Because we propose to sample a significant number of flow units at each site cored, the chances of obtaining a time-averaged paleointensity value at our site are greatly increased. Even if basaltic glass is not recovered, recent advances in paleointensity measurements measured on single plagioclase crystals (Cottrell and Tarduno, 1997a; Cottrell and Tarudno, 1999) may allow considerable new paleointensity data to be recovered for the Late Cretaceous to early Tertiary interval. Magnetic inclusions contained within such feldspars have been shown to yield paleointensity data less affected by experimental alteration (Cottrell and Tarduno, 2000). We hope to explore whether time-averaged estimates of paleointensity (Tarduno et al., 2001) can be obtained through the investigation of multiple lava flows at each site.

4. Source and Melting History of the Hawaiian Hotspot
Hotspots are of continuing interest to mantle geochemists because they provide "windows" into parts of the mantle that lie beneath the upper mantle source region for midocean ridges. An observed range of distinct mantle compositions offers the means to investigate such important issues as the geochemical evolution of the mantle, temporal and spatial scales of mantle convection, and lithosphere-mantle interactions. No hotspot has been more intensely examined from a geochemical perspective than Hawaii through compositional studies of lava sequences from the islands at the southeast end (e.g., Chen and Frey, 1985; Garcia et al., 1998) to dredged and drilled rocks from about 30 sites along this prominent and long-lived lineament (e.g., Lanphere et al., 1980; Clague and Dalrymple, 1987; Lonsdale et al., 1993; Keller et al., 2000).

As an example, the Sr-isotope ratios of tholeiitic basalts from the Hawaiian hotspot track show a systematic trend through time (Fig. 8). These ratios are approximately constant along the Hawaiian Ridge (out to the 43-Ma bend) then decrease steadily northwards along the Emperor seamounts to Suiko. This decrease has been attributed to a decrease in distance between the hotspot and the nearest spreading ridge (Lanphere et al., 1980). Only the tholeiitic basalts from the shield phase of volcano construction show this trend, because only these magmas appear to have escaped contamination with the oceanic lithosphere (Chen and Frey, 1985). Keller et al. (2000) have extended this analysis to Detroit and Meiji Seamounts, and find that Sr-isotope ratios continue to decrease northward, with a minimum value at Detroit well within the range of compositions for Pacific midocean ridge basalts (MORB). This composition (confirmed with other isotopic and elemental ratios) is unprecedented in the Hawaiian hotspot-produced volcanism to the south but is consistent with the interpretation from plate reconstructions that the hotspot was located close to a spreading ridge at ~80 Ma. The seamount magmas, then, appear to be derived from a mixture of plume- ("enriched") and predominantly asthenosphere ("depleted") mantle sources. The plume end member is more like the "Kilauea" than the "Koolau" component of the modern hotspot.

A consideration of the age of seafloor surrounding the northern Emperor Seamounts (e.g., Mammerickx and Sharman, 1988) suggests a spreading ridge was (e.g., Mammerickx and Sharman, 1988) close to the Hawaiian hotspot at ~80 Ma. In other locations where a plume is close to a ridge (e.g., Galapagos, Easter, and Iceland), the isotopic compositions of hotspot products extend toward MORB values. Several processes may lead to this effect: the nearby spreading ridge could have provided a higher temperature and lower viscosity/density regime leading to significant entrainment of aesthenosphere within the rising plume. Thinner lithosphere near the ridge would promote a longer melting column in the plume leading to greater entrainment and homogenization of geochemical heterogeneities. Also, younger hotter lithosphere may be more readily assimilated by the ascending plume melts. Thus, the thickness of the lithosphere could determine how much asthenosphere contributes to hotspot volcanism. Finally, a change in the isotopic characteristics of the plume itself through time cannot be ruled out. The (deep mantle?) region where the Hawaiian plume acquires its geochemical characteristics has probably not been homogeneous and static. But the degree of geochemical variability at given sites within the Emperor seamounts has not been established on the basis of the few analyses reported so far.

Geochemical data for lava flows from the Emperor seamounts sites will be produced to document the compositional and thermal characteristics of mantle sources and melting conditions of the early history of the Hawaiian hotspot. Major and trace element abundances will place limits on the depth and extent of melting and track magma evolution (fractionation, contamination) to the surface. Such data will also categorize magmas as tholeiitic, alkalic, or post-erosional for comparison with Hawaiian islands construction. Isotopic work (Sr, Nd, Pb, and Hf isotope ratios, parent-daughter measurements of whole rocks, and He for glasses and fresh olivine if these are recovered) will identify mantle-source components. Other studies, such as volatiles in glasses, are planned depending on suitable material.

Knowledge of the physical volcanology of the lava flows at Emperor seamount sites is important for understanding the mechanisms and time scales of eruptions. Studies of the physical characteristics of historic lava flows at Hawaii have led to the means of linking outcrop-scale observations to important eruption parameters, such as flow volume, velocity, viscosity, and distance from source. From the recovered core and logging records, we will measure flow thickness, direction, structure, vesicularity, crystallinity, and estimate the duration of intervals between flows. This information will be integrated with evidence for eruptive environment (submarine vs. subaerial, volcano flank vs. summit) and secular variation measurements from the paleomagnetic studies to estimate timescales for the recovered sections.