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doi:10.2204/iodp.pr.345.2014

Preliminary scientific assessment

Expedition 345 to Hess Deep Rift was remarkably successful, especially given the challenging operational conditions, which imposed less depth penetration and recovery than desired. Drilling into essentially rubble-covered fractured rock at water depths exceeding 4800 mbsl meant that our operational objective of recovering one or more 100–250 m sections was only marginally achieved. Instead, one single-bit 35 m hole and two shallowly cased reentry 110 m holes with 14%–30% recovery were drilled. This is a remarkable achievement on the part of the operations team.

Expedition 345 achieved its primary scientific goal of recovering the first drilled sections of primitive gabbroic rocks formed at a fast-spreading ridge. Drilling at Site U1415 provided the first confirmation of predictions that fast-spreading lower oceanic crust is layered. It also revealed a diversity of layering whose characteristics have similarities and differences to both layered sequences in ophiolites and layered mafic intrusions. Moreover, the core revealed significant unexpected mineralogical and textural diversity, some of which has been rarely observed in the lower oceanic crust elsewhere or in ophiolites. The core from Expedition 345 will be a reference section, for now the only reference section, for fast-spreading primitive ocean crust, which covers about half the Earth’s surface. In this way, we surpassed our scientific expectations, making Expedition 345 a milestone for ocean crustal research.

Expedition 345 was designed to address the following objectives:

• What is the origin and significance of layering?

• How is melt transported from the mantle through the lower crust?

• How, and how fast, is heat extracted from the lower plutonic crust?

• What are the fluid and geochemical fluxes in the EPR lower plutonic crust?

The primitive gabbroic core recovered at Site U1415 makes it possible to address all of these scientific objectives, as well as some unanticipated objectives, such as

• Determining the origin of orthopyroxene in primitive lower ocean crust gabbros,

• Determining how plate separation is accommodated in the crust given the evidence against high-strain flow in a partially molten or subsolidus state,

• Reconstructing parental/primary MORB melt compositions using recovered olivine-phyric basalts in Hole U1415N,

• Distinguishing an EPR versus Cocos-Nazca Ridge source for basaltic dikes that intrude the gabbroic lithologies, and

• Evaluating the fluid flux and timing of localized zones of brittle fracturing and cataclasis.

These scientific objectives will primarily be addressed using core from the two reentry holes and one 35 m deep single-bit hole at Site U1415. The relationship of each of these objectives to core is briefly commented on below.

1. What is the origin and significance of layering?

Modally layered lower crustal gabbros have been central to our models for the ocean crust since the so-called “layer cake” model of the ocean crust was first developed (Anonymous, 1972). And yet to date, evidence for modally layered crust from drill core or samples recovered by other means has been sparse, comprising a single discrete sample and a small number of short drilled intervals (Perk et al., 2007; Dick, Natland, Miller, et al., 1999; Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 305/305 Scientists, 2006). That is why recovery of abundant modally layered gabbros from the lower ocean crust, in particular those formed at a fast-spreading ridge, is a tremendous achievement. Finally, we have unequivocal confirmation of a long-standing paradigm.

The Layered Series recovered in all holes display a remarkable variety, from simple modal layering to diffuse banding. Similarly, the recovered rocks display a spectacular variation in the development of the magmatic foliation. These differing types of magmatic foliation, magmatic layering, and their margins record complex histories of crystallization, melt migration, intrusion and mixing of melts, and deformation, to name a few of the contributing processes.

2. How is melt transported from the mantle through the lower crust?

Many aspects of the recovered primitive gabbros were unexpected and call into question our understanding of MORB crystallization at fast-spreading ridges. The modal mineralogy of the Site U1415 core, in particular the abundance of orthopyroxene, demonstrates that crystallization of MORB at fast-spreading ridges may not proceed as expected. The mineral assemblages’ growth and deformation textures show a remarkable complexity that implies that no single process can explain the crystallization of these primitive rocks. The types of processes that may be operative include simple cumulus crystallization, disequilibrium crystallization caused by under-cooling of a melt, and crystallization during melt extraction and compaction and migration of melt either pervasively or in channels. Imposed on these are magmatic foliations ranging from weak to strong.

3. How, and how fast, is heat extracted from the lower plutonic crust? What are the fluid and geochemical fluxes in the East Pacific Rise lower plutonic crust?

These questions are combined here as they can primarily be addressed with postcruise research. Recovery of fresh lithologies will allow for the calculation of cooling rates using geospeedmometric techniques and investigation of the highest temperature of fluid ingress using isotopic and thermodynamic techniques. The metamorphic assemblages and their relative age relationship in the Site U1415 core already provide some insight into the evolution of fluid-rock interaction. Some of this history is particular to the tectonic setting of Hess Deep, but much of the alteration is thought to be related to lower crustal cooling prior to tectonic disruption. Similar to primitive gabbros from slow-spreading ridges, primitive bulk compositions and the abundance of olivine lead to very different alteration histories, and it is expected that postcruise research will document the extent and nature of the resultant chemical exchange.

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