The IODP Science Plan 2013–2023 sets a number of basic challenges for the next decade of planetary exploration. It recognizes that in order to understand the inherent connections between the Earth’s interior and its surface environment we must address fundamental questions about basic plate tectonic processes. Central among these questions are how seafloor spreading and mantle melting lead to the creation of oceanic lithosphere at mid-ocean ridges, and what controls the architecture of the ocean crust thus formed (IODP Science Plan Challenge 9). Constraining the composition, diversity, and architecture of the lower ocean crust and shallow mantle is critical to understanding the global geochemical cycle, particularly the exchange of heat, mass, and volatiles between the Earth’s interior, oceans, and atmosphere.

Crust formed at mid-ocean ridges extends across three-fifths of the Earth’s surface, comprising ~60% by area and ~30% by volume of the Earth’s crust. The ocean crust, as determined by seismic refraction studies, is typically ~6–7 km thick and apparently relatively uniform. The lower portion, corresponding to seismic Layer 3, is widely assumed to consist of gabbroic rocks formed in a magma chamber beneath the ridge axis. Layer 3 is separated from the mantle beneath by the Mohorovičić seismic discontinuity (Moho) and is conventionally regarded as the boundary between the igneous crust above and tectonized mantle peridotite beneath. Alternative interpretations are, however, possible: seawater penetration into peridotite causes partial alteration to serpentinite and reduces its velocity to that comparable to gabbro; hence, the seismic structure of Layer 3 in itself gives us little information as to its lithology. The Moho could equally represent an alteration front boundary between altered and unaltered peridotites and need not coincide with the crust/mantle boundary at all. Instead, the Moho could lie well within Layer 3. If this is the case, we know less about the architecture and composition of the ocean lithosphere than we thought we did and very much less about global magmatic, volatile, and heat budgets.

Until the advent of ocean drilling, the stratigraphy of Layer 3, the lower crustal layer, was largely unknown and widely misinterpreted. Even to date, only two penetrations of the lower crust >200 m have been attempted, and these ended no deeper than ~1508 m. Our direct knowledge, therefore, remains remarkably limited. What we have learned from deep drilling in the Indian Ocean and the Atlantic (e.g., Dick et al., 2000; Blackman et al., 2011), when compared to the shallow (~100–200 m deep) sections drilled at the East Pacific Rise (EPR; Gillis, Mével, Allan, et al., 1993; Gillis et al., 2014), is that the mechanisms of formation of the gabbroic lower crust at slow- and fast-spreading ridges differ profoundly (e.g., Sinton and Detrick, 1992; Coogan, 2014). At slow and ultraslow ridges, which represent ~60% of the global ridge system, tectonic stretching is recognized to be an integral part of the seafloor spreading process (e.g., Tucholke and Lin, 1994; Tucholke et al., 1998; Escartín and Canales, 2011). A consequence of this discovery is that tectonic windows exposing lower crust on the seafloor are widespread at slow and ultraslow ridges. In practice, they are the only places at which the crust/mantle boundary is likely to be accessible by drilling with current technology.