IODP

doi:10.14379/iodp.sp.352.2013

Scientific objectives

1. Obtain a high-fidelity record of magmatic evolution during subduction initiation by coring volcanic rocks down to underlying intrusive rocks, including radiometric and biostratigraphic ages.

Recent advances in studying the IBM fore arc document important vertical compositional variations within the volcanic sections. We know that the IBM fore arc exposes rocks that formed when this subduction zone began at ~52 Ma (Stern and Bloomer, 1992; Ishizuka et al., 2011). Reagan et al. (2010) documented that the volcanic succession exposed in the inner trench wall of the southernmost Mariana fore arc comprises a volcanic succession that changes from MORB-like tholeiites at the base (FAB) through increasingly arc-like basalts to boninites near the top. They inferred that the 450–700 m sections cored at Sites 458 and 459 in the Mariana fore arc sampled the transition between the FAB and boninite successions. Similar successions are common in ophiolites, which are increasingly recognized as fossil fore arcs (Stern et al., 2012; see below). The significance of this simple succession has not hitherto been appreciated because of a lack of direct information on fore-arc volcanic stratigraphy, mainly because this was not a priority for dredging and diving. The results of Reagan et al. (2010) provide the first reconstruction of this stratigraphy, and this dredging and diving in the Bonin fore arc was undertaken to see whether a similar magmatic stratigraphy was present there. In the area of Sites BON-1A and BON-2A, the results of Ishizuka et al. (2011) support the conclusions of Reagan et al. (2010). Drilling and coring this volcanic succession will provide a crucial test of this hypothesis by providing a more continuous section. It is also important to further constrain the rates at which the fore-arc magmatic succession was emplaced. Evidence so far available indicates that this sequence takes ~7 m.y. to form during subduction initiation, after which magmatic activity retreats ~200 km to the ultimate position of the arc magmatic front. Recovered cores should provide more material for U-Pb zircon, 40Ar/39Ar, and biostratigraphic age determinations.

2. Use the results of Objective 1 to test the hypothesis that fore-arc basalt lies beneath boninites and to understand chemical gradients within these units and across their transitions.

We expect to find a thick section of FAB at the base of the Bonin fore-arc volcanic succession and a thinner sequence of arc-like and boninitic lavas at the top. To understand the significance of these vertical variations, we need to know how the transition from one magma type to the next takes place: is it a step-function, or is there a slow transition from one magma type to the next? If it is a transition, we need to know whether it is continuous, gradual, and progressive or whether it is accomplished by alternations of one magma type with another. Within the main FAB sequence, we need to know whether there is any evidence that the subduction component increases with stratigraphic height and thus time. A key related question is whether the boninites vary in any systematic way upsection, for example from high-Ca boninite at the base to low-Ca boninite near the top. The nature of these transitions and variations provide important constraints for how mantle and subducted sources and processes changed with time as subduction initiation progressed.

3. Use drilling results to understand how mantle melting processes evolve during and after subduction initiation.

Assuming that we are able to accomplish Objectives 1 and 2, we will use the results to better understand how the mantle responds to subduction initiation. For example, a thick basal FAB succession indicates that adiabatic decompression is the most important process at the very beginning of subduction initiation in the IBM system, and an upper section of boninites indicates that flux melting was important just before the transition into normal arc magmatism. Whatever information is obtained from the cores will be used to construct more realistic geodynamic and petrologic models.

4. Test the hypothesis that the fore-arc lithosphere created during subduction initiation is the birthplace of supra-subduction zone ophiolites.

Much has rightly been made of the highly successful efforts of IODP and its precursors in establishing the architecture and crustal accretion processes associated with mid-ocean ridges of varying spreading rates and linking these to ophiolites. As discussed earlier, however, it now appears that most ophiolites form when subduction begins and are preserved as fore-arc crust until they are obducted. One testable hypothesis is that ophiolites that formed during subduction initiation can be recognized by a volcanic stratigraphy that varies from MORB-like at the base to arc-like or boninitic near the top, similar to the sequence that we expect to recover from the IBM fore arc. Most ophiolites are not well enough preserved or studied to infer volcanic chemostratigraphies, but some are (e.g., Mesozoic ophiolites such as Pindos, Mirdita, Semail, and Troodos and Ordovician ophiolites of the northeast Appalachians and Norwegian Caledonides). Some of these have volcanic stratigraphies that are similar to those of the IBM fore arc. Results from Bonin fore-arc drilling will allow us to prepare a more detailed volcanic chemostratigraphy expected for subduction initiation, which will in turn allow more detailed comparisons with these ophiolites and encourage geoscientists to try to reconstruct the magmatic stratigraphies of other ophiolites.