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doi:10.2204/iodp.sp.350.2013

Abstract

The spatial and temporal evolution of arc magmas within a single oceanic arc is fundamental to understanding the initiation and evolution of oceanic arcs and the genesis of continental crust, which is one key objective of the International Ocean Discovery Program Initial Science Plan. The Izu-Bonin-Mariana (IBM) arc has been a target for this task for many years, but previous drilling efforts have focused mainly on the IBM fore arc and the magmatic evolution of the volcanic front through 50 Ma. Rear-arc IBM magmatic history has not been similarly well studied in spite of its importance in mass balance and flux calculations for crustal evolution, in establishing whether and why arc-related crust has inherent chemical asymmetry, in testing models of mantle flow and the history of mantle depletions and enrichments during arc evolution, and in testing models of intracrustal differentiation.

Expedition 350 will contribute to the understanding of intraoceanic arc evolution and continental crust formation by drilling in the IBM rear arc to examine three phenomena:

1. Crust develops that is “continental” in velocity structure and seismically similar beneath both the volcanic front and rear arc but is heterogeneous in chemical composition.

2. Magmas at the volcanic front are rich in fluid-mobile recycled slab components that swamp the mantle, yet these magmas are so depleted in mantle-derived fluid-immobile elements that they are dissimilar to “average continental crust” in detail. This is less true in the rear arc where the diminished slab signature and lower degrees of mantle melting create crust that is more typical of the continents and allows the temporal history of the mantle source to be tracked more easily.

3. The crust beneath the rear arc is volumetrically more abundant than beneath the volcanic front.

To understand the evolution of the whole IBM crust, we will drill the Izu rear-arc region west of the modern volcanic front to recover a complete record of rear-arc volcanism from the present back to its likely inception in early Oligocene or Eocene times. The rear arc contains the record of the “other half” of the subduction factory output, and recovering that record is essential to the IBM drilling strategy.

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