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doi:10.14379/iodp.pr.350.2014

Preliminary scientific assessment

Operations went perfectly smoothly for most of the expedition. On the way to the main objective Site U1437 in the Izu rear arc, we drilled for 1 day at Site U1436 (IBM-4GT) in the Izu fore arc. The goal of this effort was to drill a 150 m deep geotechnical test hole for potential future deep drilling (5500 mbsf) at proposed Site IBM-4 using the Chikyu. This goal was achieved. Furthermore, the core from this site was used to test and refine a new descriptive scheme for volcaniclastic rocks. Core from Site U1436 yielded a rich record of Late Pleistocene explosive volcanism, including distinctive black glassy mafic ash layers that may record large-volume eruptions on the Izu arc front. The expedition then proceeded to the main objective site on the rear arc, Site U1437, where drilling went very well. Site U1437 had excellent core recovery in Holes U1437B and U1437D, and we succeeded in hanging the longest casing in the history of JOIDES Resolution scientific drilling (1085.6 m) in Hole U1437E. Recovery declined and drilling slowed in the lower part of Hole U1437E, so it began to look unlikely that we would reach our very optimistic target depth of 2100 mbsf, although if drilling rate did not slow appreciably, we hoped to reach ~2000 mbsf. Unfortunately, after drilling to 1806.5 mbsf and pulling out of the hole for a routine bit change, the fiber optic cable on the reentry camera failed, so we were unable to reoccupy the hole for continued drilling; furthermore, Hole U1437E could not be logged. We have requested that one of the other IBM expeditions return to Hole U1437E while it is still in good condition and do the downhole logging. We decided to make use of the ~4 days left to us by returning to Site U1436 to drill three additional holes (Holes U1436B, U1436C, and U1436D), in an attempt to get better recovery of the black glassy mafic ash layers and enclosing sediments. This was highly successful.

The primary scientific objective for Site U1437 was to characterize “the missing half of the subduction factory.” Numerous ODP and Integrated Ocean Drilling Program sites had previously been drilled in the arc to fore-arc region, but this was the first site to be drilled in the rear part of the Izu arc. Rocks collected by dredges from the surface of rear-arc volcanoes were shown to be different from arc-front rocks, and drilling in the “missing half” was proposed in order to fully characterize this asymmetry and determine if it developed early or late in the ~52 My history of the arc system. This is important for determining whether the asymmetry is an inherent feature of arcs, for making mass balance and flux calculations for crustal evolution, and for testing models of intracrustal evolution. Volcaniclastic rocks in the fore arc had previously been shown to provide a faithful record of arc evolution (e.g., Gill et al., 1994; Straub, 2003; Bryant et al., 2003), and recent improvements in microanalytical techniques (e.g., single crystal grains or glass fragments) makes this approach more promising than ever. To this end, the expedition was highly successful. However, the temporal record might be shorter than expected, as discussed below.

The deposits drilled at Site U1437 were in general much finer grained than expected, making shipboard geochemical analysis more difficult; clasts were not large enough to be analyzed individually by ICP, except in lithostratigraphic Units VI and VII, so bulk analysis was required for most units. However, shore-based microanalytical techniques will overcome this problem. Alteration resulted in destruction of fresh glass from the top of Unit III downward (except for the black glassy lapilli tuff in Unit VII), so shipboard geochemical interpretations below Unit II relied on immobile elements to infer types of magmas represented (e.g., rhyolite versus andesite versus basalt) and source types (e.g., arc front versus rear arc). Shore-based geochemical analysis will therefore need to rely more on minerals, which are less altered than the glass for most samples taken deeper than ~750 mbsf.

Site U1437 is much more mud rich than expected; ~60% for the section as a whole and 89% in the thickest lithostratigraphic unit (Unit I, 433 m; Table T4); this obviously lessens the opportunities for studying the chemistry of volcanic clasts and grains. Furthermore, the volcaniclastic fraction of the section (40%) is relatively fine grained, composed half of ash/tuff and half of lapilli tuff of generally fine grain size (clasts < 3 cm); volcanic blocks were only sparsely scattered through Units VI and VII, which in turn make up only ~20% of the recovered material. The fine grain size of the tephra makes it more likely that many of them provide a record of arc-front (rather than rear arc) volcanism and limits the scope for studying submarine volcanic eruption processes because most of the record is relatively distal. However, shore-based geochemical techniques are expected to allow better distinction of volcanic source areas than was possible shipboard. Additionally, future shore-based geochemical work will be aimed at understanding the provenance of the mud/mudstone, which accumulated at unusually high rates for such fine-grained material.

Due to compaction and alteration, the lowest biostratigraphic datum was ~850 mbsf and the lowest paleomagnetic datum was at ~1300 mbsf (~9 Ma). Both a nannofossil assemblage constraint and a preliminary U-Pb zircon date generated shortly after the expedition indicate an age range of ~11–15 Ma for the depth interval 1389–1403 mbsf. The section is much younger than predicted from seismic interpretation of relations between the basin fill and the bounding volcanoes. Lithostratigraphic Unit II forms a reflector that was interpreted to correlate with the Manji Volcano (dated at ~6.5 Ma), but that unit is only ~4.3 Ma (Fig. F24). The Miocene/Oligocene boundary (~23 Ma) was inferred to lie at ~1250 mbsf, but strata at that depth are much younger (~8.5 Ma). Thus, the section dated so far provides a shorter temporal record than expected. The age of the lowest ~25% of the section (Units VI and VII) will not be conclusively known until shore-based radiometric dating of the volcaniclastic rocks is complete.

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