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doi:10.2204/iodp.sp.324.2009

Scientific objectives

Primary objectives

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

The timing and duration of volcanism at Shatsky Rise is one of the primary data types that can constrain the origin of the plateau. The drilling plan calls for sampling at three sites on Tamu Massif, which is the hypothesized product of plume head eruption. Along with the preexisting radiometric date from Site 1213, geochronologic data will show whether Tamu Massif erupted in a short period, as is currently hypothesized. A short time span would imply a plume head–like eruption, whereas a longer time span may indicate lower rates of effusion inconsistent with a plume head. If diachronous ages are found, the distribution will give constraints on the type of mechanisms that can explain the initial Shatsky Rise eruptions. In addition, dates from the other Shatsky Rise edifices, the Ori and Shirshov massifs, will show whether these edifices were constructed at or near the time of crustal formation and whether the northern part of Shatsky Rise shows an age progression. New geochronologic data will come from studies of igneous rock samples acquired at the proposed sites.

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

Much of what we know about mantle source rocks and magma genesis are from interpretations of geochemistry and isotopic chemistry from rock samples. Although the signature of a lower-mantle source is still debated, in general it is expected that mantle plumes give rise to igneous rocks with ocean island basalt (OIB) composition and isotopic characteristics of the lower mantle. Furthermore, high ratios of ³He/⁴He are also taken by many as evidence for a lower-mantle source (e.g., Courtillot et al., 2003). Currently, such evidence is missing from the Shatsky Rise rock samples that have been studied, but this could result from a sampling bias because of the small number of samples that have been studied. If OIB chemistry or other mantle indicators are found in the rocks cored at the proposed new sites, the plume model will be supported or, conversely, if new drilling fails to produce evidence of lower-mantle involvement, the plate model will be strengthened. In addition, the patterns of geochemical variation across Shatsky Rise will also help establish the formation mechanism. Will new samples show geochemical variability consistent with evolution or zoning of the magma source, or will the geochemical characteristics be homogeneous, as is the case for Ontong Java Plateau? Will new drilling provide evidence for exotic, high-temperature igneous rocks, such as the high-Mg Kroenke-type basalts from Ontong Java Plateau (Fitton and Godard, 2004) that may indicate higher mantle temperatures associated with a plume source? These answers will come from geochemical and isotopic studies of the igneous rocks cored from the proposed sites.

3. Determine the source temperature and degree of partial melt 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 abnormal mantle fertility models for Shatsky Rise lavas. In general, high degrees of partial melt would be expected for a plume head eruption that is associated with high mantle temperature. For example, partial melting of 30% is interpreted for Ontong Java Plateau eruptions from phase petrology and pattern of incompatible element abundances from 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 similar high melt extraction rates (e.g., Foulger and Anderson, 2005). Evidence for source temperatures above ambient mantle, however, may be an important indicator for the existence of thermal (deep) 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. These approaches might be particularly powerful if combined with other methods such as combined He and Os isotope studies (e.g., Brandon et al., 2007).

Secondary objectives

1. Determine the physical volcanology of Shatsky Rise eruptions.

Above all, 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 the Tamu Massif 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 descriptions 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 a reversed magnetic polarity. Paleomagnetic studies of proposed 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. Paleomagnetic studies will be done mostly from basalt samples cored on Shatsky Rise; however, if the coring recovers sufficient numbers of oriented samples from the sedimentary section, those samples can also be used to determine paleolatitude. In that regard, new results will be important for comparison from paleomagnetic results from Berriasian sediments cored at Site 1213 (Sager et al., 2005).

3. Determine paleodepths of Shatsky Rise.

Plume head models predict significant uplift associated with introduction of a large starting plume head beneath oceanic lithosphere (e.g., Olson and Nam, 1986). The associated constructional volcanism also creates a much thicker crust than normal. Indeed, there is evidence that the Kerguelen Plateau formed subaerial landmasses that later subsided below sea level as they moved away from the plume head source (Wallace, 2002). Seismic profiles of Shatsky Rise also imply that summits of Tamu Massif and Shirshov Massif were above sea level at eruption and subsided to the present depth, but the prediction has yet to be confirmed by drilling. Sampling 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 examination of microfossils and measurement of volatile abundances in basaltic glass (if preserved) (e.g., Roberge et al., 2005). If the summits of Shatsky Rise were shallow marine or above sea level, the classic plume head model is supported or, conversely, if the basement tops consist of deep submarine basalt, other models such as melting of a wet spot and/or melting of recycled crust have to be considered (Schilling et al., 1980; Korenaga, 2005).

4. Determine magma evolution and magma chamber process of Shatsky Rise

Geochemical studies of oceanic plateaus suggest that compositional variations in extrusive lavas are probably controlled by magma evolution processes (fractional crystallization, magma mixing, assimilation, and reaction with cumulates) in a large-scale magma chamber. Understanding magma evolution processes in large magma chambers will also help to understand the formation of the cumulative part of the lower oceanic crust. For example, a primary magma lost 20%–80% minerals by fractionation in shallow magma chambers to produce Ontong Java Plateau basalts (Fitton et al., 2004; Sano and Yamashita, 2004). Current geochemical data from Shatsky Rise also show that basement rocks experienced significant fractionation before their eruption. However, the magma chamber scale and evolution mechanism of oceanic plateau are not clear compared to well-studied MORBs (e.g., Sinton and Detrick, 1992). Systematic basement sampling on this expedition could provide information about magma evolution processes by using chemical variations of whole rocks. Examination of chemical zoning profiles of phenocryst phases (olivine, plagioclase, and clinopyroxene) and recording physiochemical properties (temperature, pressure, and magma compositions) during their growth can further contribute to reconstruct the history of magma evolution.

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