OS026-10
The Off-Hawaii deep crust drilling on the North Arch as a pilot hole for future mantle drilling
Abstract:
Drilling a complete crustal section has been a major goal since the beginning of ocean drilling. It is one of the primary objectives of the mantle drilling project MoHole to Mantle (M2M) (Umino et al., 2012). Among the three M2M candidate sites, the Off-Hawaii site has the oldest crustal age (78–81 Ma) and formed at an spreading rate equal to the world average (8 cm/a). Furthermore, the proposed site is optimal to understand: 1. Whether magmatic underplating extended far off the axis of the Hawaiian plume; 2. the effects of North Arch volcanism on the Cretaceous aged lithosphere; and 3. the comprehensive record of giant landslides north of the Hawaiian Islands.
Estimated crustal temperatures of <50 °C at 2 km below the seafloor (which is within the habitability range of subseafloor life) and a relatively shallow water depth of ~4,300 m are advantages of the Off-Hawaii site compared to other oceanic regions of the same crustal age and spreading rate. The low temperatures and the nature of older crust with matured cementation will diminish drilling-induced fracturing and provide better drilling conditions than much younger alternative sites. Low borehole temperatures are also critical for long-term monitoring and observations by electronic instruments, which are generally restricted to less than 200°C.
This usage of the Chikyu will penetrate through the challenging section of the sheeted dike complex and the uppermost gabbros, as well as the seismic layer 2/3 boundary at 4.3 km-water depth. The Chikyu’s ‘Logging While Drilling’ tools will continuously obtain geophysical data from drilled intervals of low core recovery, enabling us to compare logging data with the recovered cores and restore original orientations during description on board.