Subduction Initiation Investigated by Drilling of the Izu-Bonin-Mariana (IBM) Forearc

The theory of plate tectonics has plates created at mid-ocean ridges and destroyed at subduction zones, above which lie chains of volcanoes known as island arcs. Subduction zones have been likened to a factory (the 'Subduction Factory') whereby materials (mantle, oceanic crust and sediment) are fed into the factory, processed within the factory (by dehydration and melting), and create range of products (magmas, ore deposits). There are many 'health and safety' issues, including earthquakes, tsunamis and explosive volcanoes. Because much of this 'Subduction Factory' is located underwater and sub-surface, the International Ocean Discovery Program (IODP) has put together three marine drilling Expeditions (350-352), each two months long, aimed at a better understanding of how the subduction factory works. These Expeditions are all located in the ocean south of Japan known as the IBM (Izu-Bonin-Mariana) region, which is a key natural laboratory for understanding subduction processes. Each Expedition has a particular goal and Expedition 352 (the focus of this proposal) has the remit to investigate how subduction starts (i.e. how the 'Factory' opens) and the products of this subduction initiation process. Julian Pearce, the PI of this proposal, is one of its two co-chief scientists.

The rocks formed at the earliest stages of subduction are particularly well exposed in the inner walls of the deep ocean trenches south of Japan and it is here, in the Bonin Trench, that Expedition 352 plans to drill. Dredging has already demonstrated at the base is made up of an oceanic crust-like sequence with mantle rocks at its base, overlain by gabbros, and then by dykes and submarine lavas. These date back to c. 52Ma and are believed to represent the first stage of subduction initiation, when one plate sinks, peels back and creates a zone of extension. This is overlain by a set of geochemically-distinctive volcanic rocks which are believed to represent the transition from spreading to arc volcanism, and finally by more conventional island arc volcanic rocks marking the point at which true subduction starts, i.e. when the subducting plate has a horizontal as well as vertical component. Estimates indicate that it took about 8 m.y. between subduction initiation and the start of true subduction. Interpretations, and testing of hypotheses for subduction initiation, are however, limited by the fact that there is no reference section, and so we aim to obtain that by drilling of the full 1-1.5 km thick lava sequence.

Many studies will be carried out on the drilling ship and in the laboratory. As co-chief scientist, the PI has joint responsibility for synthesising the results, in this case to provide the geological community with a type sequence of subduction initiation volcanism which can be used for comparison with inferred subduction initiation sequences in volcanic rocks now exposed in old geological sequences in mountain belts and elsewhere. In addition, he has specific responsibility for analyzing three isotopes (of Pb, Hf and Nd) for use as tracers of the subduction initiation process, and for interpreting the results. For this he aims to work with Rex Taylor (co-I), who has worked in the IBM region himself and also runs an isotope laboratory that not only analyses Hf and Nd isotopes but also specializes in high precision analyses of Pb isotope ratios. The subduction process is complicated, with elements having a number of potential sources: mantle above the subduction zone, subducted crust and various types of subducted sediment. Pb is a particularly good tracer of subducted sediment, while Hf-Nd are good tracers of the provenance of the mantle and temperature of the system. By analyzing these isotope ratios in samples taken from bottom to top of the lava sequence, we aim to document the how subduction variables change during the transition from subduction initiation to true subduction.

A principal 'economic' beneficiary of this Expedition is the mineral resource sector. Subduction zone magmas are the source of many of the world's metal resources, including porphyry copper deposits which host most of the world's copper as well as a suite of other metals including base metals, Au, Mo and Re. By improved understanding of the generation and crystallization of subduction zone magmas achieved through the Expedition 350-352 combination, we expect to improve ore deposit models for this setting and therefore make a positive impact on mineral exploration. As far as the UK, is concerned, this is significant as many major mineral resource companies are based here (including SRK in Cardiff).

More specifically for Expedition 352, subduction initiation settings are the locations of formation of economic or strategically-important podiform chromite deposits, some of which have significant PGE (platinum group element) concentrations. Though these are much smaller than stratiform chromites, they provide ores of greater metallurgical importance and provide a strategic resource in case of proplems with South African supplies. These deposits likely form when mantle peridotite formed as a residue of melting during spreading immediately post subduction initiation subsequently reacts with magmas feeding the earliest arc edifices. By studying the ore metal contents and oxygen fugacities of lavas within the stratigraphic sequence, our Expedition will help identify the optimium period of chromite and PGE deposition within the context of the initiation and early evolution of subduction zones. In addition, volcanogenic massive sulphide deposits form on the sea floor during the initial extensional event and these are the sources of Cu, Au and other metals within the uplifted forearcs that form many ophiolite complexes on land. Through studying how magmatic, tectonic and hydrothermal processes interact in a subduction initiation setting, we again expect to improve models for the formation of this type of deposit, specifically by identifying what controls potential ore-forming hydrothermal events within the subduction initiation lava stratigraphy.

A second sector comprises consultancies and institutions (including national surveys) involved in geological mapping and regional geology interpretations. A particular aim of this Expedition is to provide criteria for the field and geochemical identification of volcanic sequences form during and immediately after subduction initiation. Rocks formed during subduction initiation are particularly common in mountain belts, where they can be represented as ophiolite complexes and related rock units. Recognising such complexes and linking them to subduction initiation provides a means of identifying the creation of a new plate boundary and possibly a period of global tectonic change. This in turn should lead to enhanced tectonic and geological reconstructions. For example, impact on the British Geological Survey might include its interpretation of the UAE-Oman ophiolite (recent consultancy in Fujairah and contribution to a future Oman Drilling Program). As with enhancements in mineral exploration, these impacts are not quantifiable but part of the progressive improvement in predictive capability that will be needed to find resources and make more sophisticated geological reconstructions in the future.

The expedition also has potential impact In terms of the environment and education. Notably, subduction zones are particularly important as locations for many of the Earth's greatest hazards (earthquakes, tsunamis, explosive volcanoes) and this in turn enhances their interest for both students in geology/geography and the general public. The fact that Expedition 352 involves drilling on the side of one of the deepest ocean trenches also adds to its interest.

The subsequent 'Pathways to Impact' section documents how the outcomes of the Expedition will be communicated to these potential users.