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Preliminary scientific assessment

Plume versus plate

Many of the Expedition 324 objectives relate to the ongoing debate about the source of volcanism and whether the plume head hypothesis explains oceanic plateau formation. To inform this debate, it is critical to know how quickly Shatsky Rise formed and what the source of the magmas was. Three primary objectives addressing these issues are: (1) to determine the basement age to constrain the temporal evolution of Shatsky Rise, (2) to determine chemical and isotopic compositions of igneous rocks, and (3) to determine the source temperature and degree of partial melting that produced the plateau lavas. Achieving these three primary objectives would directly address the aim of the cruise: "testing plume versus plate tectonic models of ocean plateau formation." In order to address the primary objectives, we planned to core as many basement lava flows as possible in the allocated time at five sites on the Shatsky Rise (Figs. F2, F3). Basement was penetrated at four of the five sites: U1346 (52.6 m basement penetration), U1347 (159.9 m basement penetration), U1349 (85.3 m basement penetration), and U1350 (172.7 m basement penetration). Although basement lavas were not encountered at Site U1348, a thick volcaniclastic sequence was penetrated (~120 m) that can yield important data about submarine emplacement processes and weathering styles on Shatsky Rise.

To obtain reliable 40Ar/39Ar radiometric age data, the recovery of lavas of suitable freshness was crucial because submarine alteration can strongly modify the composition of rocks and seriously impact the K-Ar decay system. Shipboard petrographic studies and geochemical data from core samples from Sites U1347 and U1350 show that, in general, alteration has only slightly affected rocks from these sites, making them especially suitable for high-quality postcruise 40Ar/39Ar age determinations. The effects of alteration are more significant for most basement lavas from Sites U1346 and U1349, but some individual samples might be fresh enough to produce reliable age data given special treatment. Because basement lavas have been sampled from all three main massifs (Tamu, Ori, and Shirshov massifs), the age data will constrain the timing and duration of volcanism of the whole of Shatsky Rise and reveal whether the volcanism shows the expected age progression. Furthermore, with two sites each on Tamu and Ori massifs, it should be possible to determine whether each edifice was formed quickly, as expected for a plume head eruption, or over a longer time span, which would support less effusive, plate tectonics–related models.

Freshness of rock samples is also important for most chemical and isotopic studies, whose goals are to establish the original elemental compositions and isotopic characteristics of the rocks. Such data are crucial for determining the mantle source composition. Although the geochemical signature of any lower-mantle source is debated, in general it is expected that mantle plumes give rise to igneous rocks with OIB-type isotopic characteristics and incompatible element ratios that indicate enrichment of highly incompatible elements in the magma source relative to the source of most MORB. The well-preserved lavas from Site U1347 and U1350 are particularly suitable for the determination of isotopic ratios and chemical compositions in postcruise studies. Such data will be important for inferring mantle source compositions. Although the effects of alteration are more severe for lavas from Sites U1346 and U1349, many individual samples from these sites appear suitable for most geochemical studies, particularly for investigations involving ratios of immobile incompatible elements and alteration-resistant isotope systems (e.g., Sm-Nd and Lu-Hf) and several other isotope systems provided special treatment of the samples (e.g., analyses of mineral separates or microanalyses by laser ablation techniques). Most important, large amounts of fresh volcanic glass were recovered in several intervals at Sites U1347, and U1350, and a small amount was found at Site U1346. Fresh glass was also found in a continuous interval within the volcaniclastic succession of Site U1348 (intervals 324-U1348A-23R-1, 110–126 cm, to 23R-2, 1–8 cm). The recovery of fresh glass gives us the opportunity to determine volatile and noble gas contents and to conduct a number of high-quality isotopic and chemical studies. If OIB chemistry is found in the glass and/or high ratios of 3He/4He (generally assumed to indicate lower mantle origin) in mineral separates, the plume model will be supported; conversely, if no evidence of lower-mantle involvement is found, the plate model will be strengthened.

In order to estimate source temperatures and the degree of partial melting, it was important to core relatively primitive rocks with olivine phenocrysts. Such rocks, called picritic basalts, were recovered at Site U1349, providing an opportunity to estimate source temperatures in detailed postcruise studies. Although the picritic basalts have suffered from severe alteration, as well as accumulation of olivine and clinopyroxene, they show compositional similarities with picritic basalts from the OJP (i.e., Kroenke-type basalts). If the estimated potential temperature and degree of melting indicate an abnormally hot mantle, plume models will be strengthened. On the other hand, if the estimated values are consistent with those of ambient mantle (i.e., MORB-source mantle), plate models will be supported. Another important result of this drilling expedition is the recovery of spinel crystals in Site U1346 and U1349 basalts. Because spinel is crystallized from relatively primitive magmas, onshore studies of its composition may provide important information on the degree of melting of the mantle source.

Oceanic plateau evolution

Whereas the primary objectives are directly related to each other, secondary objectives have a wider variety. Preliminary scientific assessments for the secondary objectives are described as follows.

1. Determine the physical volcanology of Shatsky Rise eruptions.

Expedition 324 recovered basement lavas from two summit sites (Sites U1346 and U1349) and two flank sites (Sites U1347 and U1350). Lava flows at both summit sites are characterized by high (>40 vol%) vesicularity, implying that the eruptive environment was shallow marine or even subaerial. A subaerial eruption is likely for Site U1349 lavas because of the style of weathering and the recovery of a highly oxidized horizon, which is interpreted as a possible paleosol, directly above the highly vesiculated lavas. In contrast, lava flows encountered at the two flank sites are mainly pillow basalts and massive inflation units, frequently interbedded with volcaniclastic or marine sediment. One massive basalt unit at Site U1347 is a very thick (~23 m) homogeneous lava flow, which shows similar characteristics to continental flood basalt flows. Both pillow and massive basalts were also found during OJP drilling (Mahoney et al., 2001). The exceptionally high basement recovery (for RCB drilling) at many sites (averaging 39%–67%, with many individual cores reaching >90%) will allow a detailed examination of flow unit distribution and emplacement. Together with planned postcruise structural geological investigations and age determinations from calcite veins, an integrated model for the volcanological evolution of the rise can be constructed.

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

Sager and Han (1993) suggested that the magnetic anomaly of Tamu Massif was produced in a short period of time during a period of reversed magnetic polarity. Shipboard paleomagnetic study of a thick (159.9 m) lava pile at Site U1347 implies that the entire succession has the same magnetic polarity, which supports the model of Sager and Han (1993). Paleolatitude measurements of oriented samples from all sites lead us to conclude that Shatsky Rise was located on or near the paleoequator during eruption. Although these preliminary results need to be refined by further, more detailed postcruise studies, the general model of a very short term formation of the entire Shatsky Rise in a near-Equator position seem to hold.

3. Determine paleodepths of Shatsky Rise.

Seismic profiles of two massifs (Ori and Shirshov) show flat basement summits beneath the sediment cover, indicating wave erosion; although these massifs are all in >2000 m water depth today. This fact may be explained by significant lithospheric uplift before/during Shatsky Rise formation and later subsidence below sea level as the massifs moved away from the magma source, consistent with the plume model (e.g., Ito and Clift, 1998). If we assume that all four basement sites (Sites U1346, U1347, U1349, and U1350) subsided at rates similar to normal oceanic lithosphere, their calculated original eruption depths would have been shallow marine or above sea level (Fig. F26). Furthermore, Expedition 324 provided a number of other observations that imply shallow submarine and/or subaerial eruption of Shatsky Rise. Benthic foraminifers in sediments immediately overlying the igneous basements imply shallow paleowater depths of <500 m for Site U1346 and <200 m for Site U1347. At Sites U1346, U1347, and U1349, sediments just above the basement are interpreted as shallow-water bioclastic sandstones with volcanic clasts. The high vesicularity (>40 vol%) of basalts at two summit sites (U1346 and U1349) also indicate shallow submarine or subaerial eruption because erupted magma must have been oversaturated with water. Assuming that the primary water content of the magma was similar to that of MORB and OJP magmas (<0.5 wt%), we estimate that the volatilization depth must have been <300 m (Newman and Lowenstern, 2002). Assuming original eruption in shallow water (or in air) and taking into account the contribution of the sediment load (Crough, 1983), the estimated subsidence for the basement sites, except for Site U1350, is 3200–3400 m (Fig. F26). This value is in excellent agreement with the prediction (3000–3600 m subsidence) for 140–150 Ma normal oceanic lithosphere (Parsons and Sclater, 1997; Stein and Stein, 1992). Figure F26 also shows that Sites U1346, U1347, and U1349 were significantly shallower (2000–2500 m) during eruption time compared to normal mid-ocean ridges. Although an eruption depth for Site U1350 is undetermined so far, postcruise studies will determine dissolved CO2 content in the preserved fresh glass rims. Because of the strong pressure dependence of CO2 solubility in basaltic melts and because fresh glasses were recovered at almost all sites (U1346–U1348 and U1350), we can expect further constraints on the uplift and subsidence history of Shatsky Rise.

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

In order to determine crustal magma chamber and magma evolution processes, lava samples from several evolutionary stages are desired (from less evolved, Mg-rich compositions to more differentiated, Mg-poor compositions). Expedition 324 recovered samples ranging from picritic basalts with high MgO contents (as much as 15.6 wt%) to more differentiated tholeiitic basalts with low MgO contents (as little as 4.9 wt%). The picritic basalts were cored at Site U1349, and lower MgO tholeiitic basalts were sampled from the other three basement sites (U1346, U1347, and U1350). Phenocryst assemblages of the basement rocks correlate well with rock compositions; picritic basalts have only olivine phenocrysts with spinel inclusions and the tholeiitic basalts are olivine-bearing plagioclase-clinopyroxene-phyric basalts. This correlation will be further examined by more detailed postcruise studies, but it is expected that the recovered material is suitable to address magmatic evolutionary processes. For example, plagioclase phenocrysts with reverse zoning and oscillatory zoning are observed in several rocks from Sites U1347 and U1350. The zoning profiles will be investigated by postcruise studies and will help to constrain magma evolution processes.