The primary goal of Leg 197 was to obtain accurate and precise paleolatitude and age estimates for each of the sites drilled. These data, when compared with fixed and moving hotspot predictions, form the basis of our paleomagnetic test. To accomplish our goal, we targeted moderate penetration of lava flow sections with the aim of obtaining an average of secular variation (>15 independent paleomagnetic inclination units) at each site.
Our objectives differed slightly from site to site. At Detroit Seamount, we hoped to improve the precision of prior paleolatitude estimates and, possibly, obtain new time-average paleolatitude data with ages different from those of Detroit Seamount ODP Site 884. At Nintoku Seamount and Koko Guyot we hoped to investigate the mechanisms for discrepancies between paleomagnetic data and predictions based on fixed hotspot models. Combined with data from Suiko Seamount (Kono, 1980), time-averaged paleomagnetic data of known age from these seamounts should allow us to test existing models and potentially develop new models for the generation of the Emperor Seamount trend and the Hawaiian-Emperor bend.
New paleomagnetic data from Detroit, Nintoku, and Koko Seamounts could also allow for the construction of an improved Pacific apparent polar wander path. In addition to its utility in the study of Pacific plate kinematics, a refined APWP could provide the basis for improved paleogeographic reconstructions important for paleoclimate studies. Such reconstructions are needed when proxy climate data are used to define past latitudinal gradients (e.g., Huber et al., 1995; Zachos et al., 1994). APWP data may serve as a more stable reference frame for Pacific plate reconstructions than one based on fixed hotspots (Cottrell and Tarduno, 1997b.)
Through our drilling approach (obtaining time-averaged paleomagnetic data at each site), we also hoped to address other aspects of the geomagnetic field through Late Cretaceous to early Teriary time (Fig. F5). For the present field and models of the Late Cretaceous to early Tertiary field, the axial dipole term is overwhelmingly dominant. Therefore, other terms will not greatly affect the accuracy of data used to test the hotspot motion hypothesis. However, the data obtained can be used to better constrain the Gauss coefficients of the past field. 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.
Although 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 can not be neglected, as it 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 targeted for study (Livermore et al., 1984). Therefore, we hoped that the data collected at the Leg 197 sites could 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.
When compared to 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 paleointensity measurement, Thellier-Thellier double heating experiments of basalt (Thellier and Thellier, 1959; modified by Coe, 1967), often encounters problems due to chemical alteration during heating. Significant recent progress has been made in studying basaltic glass (Pick and Tauxe, 1993) that shows ideal magnetic properties. The available DSDP and ODP sites where basaltic glass was sampled have now been analyzed (Juarez et al., 1998), so further progress requires additional drilling.
The Leg 197 drilling plan included the potential recovery of reference sites for Late Cretaceous–early Tertiary paleointensity. We planned whole-rock, basaltic glass, and single plagioclase crystal (Cottrell and Tarduno, 1997a, 2000a; Tarduno et al., 2001) approaches to analyze the recovered cores and to derive Late Cretaceous to early Tertiary interval paleointensity data through shore-based study.
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 mid-ocean 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 basalt from the Hawaiian hotspot track show a systematic trend through time (Fig. F6). These ratios are approximately constant along the Hawaiian Ridge (out to the 43-Ma bend) then decrease steadily northward along the Emperor Seamounts to Suiko Seamount. 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 basalt from the shield phase of volcano construction show this trend because only these magmas appear to have escaped contamination by the oceanic lithosphere (Chen and Frey, 1985). Keller et al. (2000) have extended this analysis to Detroit and Meiji Seamounts, and they find that Sr isotope ratios continue to decrease northward, with a minimum value at Detroit Seamount well within the range of compositions for Pacific mid-ocean-ridge basalt (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 aesthenosphere ("depleted") mantle sources. The plume end-member is more like the "Kilauea" than the "Koolau" component of the modern hotspot.
Plate reconstructions (e.g., Mammerickx and Sharman, 1988; Atwater, 1989) place a spreading ridge close to the Hawaiian hotspot at ˜80 Ma. In other locations where a plume is close to a ridge (Galapagos Islands, Easter Island, 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 and 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 degrees of partial melting and homogenization of geochemical heterogeneities (M. Regelous et al., unpubl. data). Also, younger, hotter lithosphere may be more readily assimilated by the ascending plume melts. Thus, the thickness of the lithosphere could determine how much aesthenosphere contributes to hotspot volcanism or how possible isotopic heterogeneities within the plume itself are expressed through partial melting. 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.
The Leg 197 study plan called for the generation of geochemical data from lava flows recovered from the Emperor Seamount sites to document the compositional and thermal characteristics of mantle sources and melting conditions of the early history of the Hawaiian hotspot. We planned to measure major and trace element abundances to place limits on the depth and extent of melting and track magma evolution (fractionation and contamination) to the surface. We also planned to use such data to categorize rocks as tholeiitic shield, alkalic postshield, or posterosional lavas for comparison with models of Hawaiian Islands construction. Shore-based isotopic work (Sr, Nd, Pb, and Hf isotope ratios and parent-daughter measurements of whole rocks and He for glasses and fresh olivine) and trace element analyses were planned to help identify mantle source components. Studies of volatiles in recovered glasses and melt inclusions in phenocryst phases were also planned, as well as microanalyses of opaque minerals (Fe-Ti oxides) that will reveal alteration and cooling conditions and aid in the rock magnetic and paleomagnetic investigations of the leg.
Knowledge of the physical volcanology of the lava flows at Emperor Seamount sites is important for understanding the mechanisms and timescales 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, relative eruption rate, and distance from source. We planned to measure flow thickness, direction, structure, vesicularity, and crystallinity in the recovered cores. We planned to integrate this information with evidence for an eruptive environment (submarine vs. subaerial and volcano flank vs. summit) and secular variation measurements from the paleomagnetic studies to estimate timescales for the recovered sections.