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Scientific objectives

Testing plume and plate models

The primary objective of drilling on Shatsky Rise was to sample relatively fresh igneous basement rocks at multiple sites on the plateau. Such samples would allow several important problems to be addressed.

1. Determine the basement age to constrain the time evolution of the plateau.

Along with the preexisting radiometric dates from Site 1213, geochronologic data will show whether Tamu Massif erupted in a short period, as is currently postulated. A short time span suggests a plume head–like eruption, whereas a longer time span may indicate lower rates of effusion inconsistent with a plume head. The age distribution will place constraints on the types of mechanisms that can explain the initial Shatsky Rise eruptions. In addition, dates from the other Shatsky Rise edifices, Ori and Shirshov massifs, will show whether these edifices were constructed at or near the time of ocean crustal formation and whether the northern part of Shatsky Rise shows an age progression.

2. Determine chemical and isotopic compositions of igneous rocks cored from Shatsky Rise.

Although the signature of a lower mantle source is still debated, it is widely expected that mantle plumes erupt igneous rocks with ocean island basalt (OIB) chemical composition and isotopic characteristics postulated to represent the lower mantle or portions of it, such as high ratios of 3He/4He (e.g., Courtillot et al., 2003). Patterns of geochemical variation across Shatsky Rise will also help establish the plateau formation mechanism. For example, variability could be consistent with evolution or zoning of the mantle source or, alternatively, geochemical characteristics could be homogeneous, as is the case for sampled portions of the OJP.

3. Determine the source temperatures and degrees of partial melting that produced Shatsky Rise lavas.

Estimation of source temperatures and the degree of partial melting could be critical to distinguish between an upper or lower mantle source or to test models of abnormal mantle fertility for Shatsky Rise lavas. In general, high degrees of partial melting would be expected for a plume head eruption that is associated with high mantle temperature, especially in locations where the lithosphere is thin (e.g., near a ridge-ridge-ridge triple junction). For example, partial melting of as much as 30% is interpreted for OJP basalts from phase petrology and the pattern of incompatible element abundances in igneous rock samples (Fitton et al., 2004). On the other hand, regions of abnormally high mantle fertility tapped by the triple junction could result in similarly large melt fractions and high melt extraction rates (e.g., Foulger and Anderson, 2005). Evidence for source temperatures above the ambient mantle temperature, however, may be an important indicator for the existence of thermal (deep-sourced) mantle plumes. Recently, Putirka (2008) proposed an improved method of olivine thermometry to estimate mantle source temperature. Herzberg et al. (2007) also proposed a method to estimate the mantle temperature by using major element composition and phase equilibria. Such approaches may be particularly powerful if combined with results from other studies, such as combined He and Os isotope studies (e.g., Brandon et al., 2007).

Large igneous province geology

1. Determine the physical volcanology of Shatsky Rise eruptions.

Shatsky Rise is a monster volcanic construct. Although they share many characteristics with the thousands of seamounts scattered across the Pacific plate, the Shatsky Rise volcanic edifices exhibit some important differences. One of the more notable is the slope of the rise volcano flanks, which is much lower (~1°) than those of typical seamounts (~5°) (Sager et al., 1999). Another is the apparent mantling of Tamu Massif's southwest flank by sills or sheet flows, as interpreted from Site 1213 data and the seismic character of acoustic basement. Both observations may be indicative of high effusion–rate eruptions. Thus the volcanic stratigraphy may provide important clues about the eruptions of Shatsky Rise igneous rocks. This stratigraphy will be developed from description of igneous rock cores and comparison with logging data.

2. Determine the magnetic polarity of Tamu Massif and paleolatitudes of Shatsky Rise.

From a study of the magnetic anomaly of Tamu Massif, Sager and Han (1993) concluded that the edifice is largely of reversed magnetic polarity and was therefore erupted in a short period of time during a period of reversed magnetic polarity. Although inconclusive owing to the small number of independent samples of the magnetic field and the low paleolatitude, the paleomagnetism of igneous samples from Site 1213 is consistent with reversed polarity (Tominaga et al., 2005). Paleomagnetic studies of Tamu Massif sites will establish whether other sites are also of reversed polarity, which would support the hypothesis that this massif formed in a short period of time. In addition, the Jurassic and Early Cretaceous paleolatitude of the Pacific plate is uncertain, so paleomagnetic samples from Shatsky Rise have the potential to help establish the paleolatitude of the rise and the Pacific plate.

3. Determine paleodepths of Shatsky Rise.

Evidence of subaerial or shallow water paleodepths for the summits of Shatsky Rise massifs will be useful for constraining models of plateau formation. Plume head models predict significant uplift associated with introduction of a large starting–plume head beneath oceanic lithosphere (e.g., Olson and Nam, 1986). Indeed, there is evidence that the Kerguelen Plateau formed landmasses that later subsided below sea level as they moved away from the plume head source (Wallace, 2002). In contrast, OJP basalts were emplaced well below sea level (Fitton et al., 2004). Seismic profiles of Shatsky Rise also imply that the summits of Tamu Massif and Shirshov Massif were above sea level at eruption and subsided to their present depths, but the prediction has yet to be confirmed by sampling. Samples collected from the rise summits during Expedition 324 will establish whether they were originally shallow or subaerial or were emplaced in a deepwater environment. Paleodepths will be established from a number of different indicators, such as microfossils, the types and structures of sediments, conditions of alteration, and measurement of volatile abundances in basaltic glass (e.g., Roberge et al., 2005).

4. Determine magma evolution and magma chamber processes at Shatsky Rise.

Geochemical studies of oceanic plateaus suggest that many compositional variations in lavas are controlled by magma evolution processes (fractional crystallization, magma mixing, assimilation, and reaction with cumulates) in large-scale magma chambers. Understanding magma evolution processes in large magma chambers will also help to understand the formation of the cumulate part of the lower oceanic crust. For example, a primary picritic magma must have lost 20%–80% minerals by fractionation in shallow magma chambers to produce OJP basalts (Fitton et al., 2004). Current geochemical data from Shatsky Rise, although sparse, also suggest basement rocks experienced significant fractionation before their eruption (Mahoney et al., 2005). However, the magma chamber scale and evolution mechanism of oceanic plateaus are not clear compared to well-studied MORB. Systematic basement sampling during this expedition could provide information about the magma evolution process from chemical variations of whole rocks. Examination of chemical zoning profiles of phenocryst phases (olivine, plagioclase, and clinopyroxene), recording physiochemical properties (temperature, pressure, and magma compositions) during their growth, can further contribute to reconstruction of the history of magma evolution.