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doi:10.2204/iodp.proc.345.101.2014

Expedition objectives

The principal objective for drilling at the Hess Deep Rift was to test competing hypotheses of magmatic accretion and hydrothermal processes in the lower ocean crust formed at the fast-spreading EPR. These hypotheses make predictions that can only be tested with drill cores, including the presence or absence of systematic variations with depth in mineral and bulk rock compositions, presence or absence of modally layered gabbro, variation of strain/deformation, and the extent and nature of hydrothermal alteration and deformation (Fig. F3). Specific scientific questions that address these predictions are outlined below.

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

Melts erupting at mid-ocean ridges are rarely saturated in all four mantle phases at plausible segregation depths (O’Hara, 1968; Stolper and Walker, 1980). The upper mantle and crustal processes responsible for the evolution of mantle primary melts into primitive MORB are the subject of a great many studies in the geochemical literature (e.g., Kelemen et al., 1997). Melts are transported and aggregated by porous flow at both mantle and crustal levels; the latter process may be identified on the basis of textural and chemical evidence. The different mechanisms of igneous differentiation (e.g., fractional crystallization, equilibrium crystallization, assimilation, and porous reactive flow) strongly influence the chemical evolution of residual liquids and host cumulates. Melt transport through the lower crust is an important boundary condition of the crustal accretion models. Is melt transported largely by porous flow through the lower crust, or is it transported in dike-like brittle fractures (Kelemen and Aharonov, 1998)? Previous studies at Hess Deep suggest that the liquid line of descent of lower crustal magmas may significantly differ from established models of MORB petrogenesis (Coogan et al., 2002a). This involves a much earlier and much more important role for orthopyroxene, such that primitive orthopyroxene is found in deeper level gabbro and in greater abundance in the Hess Deep shallow-level gabbro (Site 894) than in either the Oman ophiolite or slow-spreading ridges (i.e., Legs 118 and 176 and Expedition 304/305). Mineral and bulk chemical data for the core will provide tests for these and potentially other mechanisms of igneous differentiation and melt transport in the base of the ocean crust.

2. What is the origin and significance of layering?

Modally and compositionally layered gabbroic rock is common in the lower crustal sections of ophiolite (e.g., Anonymous, 1972). A layered lower crust is therefore one of the key and nearly ubiquitous features of all models of fast-spreading lower crust. However, pronounced modal layering of the sort observed in major ophiolites has rarely been observed or sampled on the ocean floor. This may be due to the lack of drilling into the deeper levels of fast-spreading plutonic crust; commonly used sampling and observation methods are not ideal to detect such layering readily, the right areas have not been sampled, or pronounced modal layering is absent. If ophiolites indeed represent sections of normal mid-ocean ridge crust, we expected to drill significant thicknesses of layered igneous rock during this expedition. The nature and extent of layering is likely to have a strong bearing on the applicability of ophiolite-based accretionary models for the formation of the lower ocean crust.

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

It is generally accepted that hydrothermal fluids initially penetrate all levels of the plutonic crust along microfracture networks at high temperatures, with fractures sealing at 600°–800°C (Alt et al., 2010; Manning et al., 1996; Coogan et al., 2002a). Initiation of incipient cracking in the upper gabbro at Hess Deep overlaps the solidus temperatures of the most evolved lithologies, as recorded by magmatic amphibole (850°–925°C) (Gillis et al., 2003) and zircon (690°–790°C) (Coogan and Hinton, 2006) using plagioclase-amphibole and Ti-in-zircon thermometry, respectively. Whether this is the case in the lower plutonic crust, where more primitive lithologies dominate, is not known. Penetration of fluids at high enough temperatures could promote hydrous partial melting (Koepke et al., 2007) as observed in slow-spreading and ophiolitic environments. Whether and at what depths this process occurs in fast-spreading lower crust is not known. Cooling rates for upper gabbro sections from fast-spreading crust and equivalent sections from the Oman ophiolite are rapid (1,000°–60,000°C/m.y.) (Coogan et al., 2002b, 2007), indicative of significant convective cooling. Slower cooling rates in deeper level gabbro suggest that heat flow was largely conductive (Coogan et al., 2007); cooling rates for lower gabbro sections from fast-spreading crust are not known. Key questions that will be addressed include

• What is the rate of cooling with depth?

• Is hydrothermal flow along microfracture networks an effective mechanism to cool the lower crust?

• When is hydrothermal cooling initiated?

• Does fluid penetration occur at high enough temperatures to induce hydrous partial melting?

• Does the lower crust cool largely by conductive or convective heat transport? This question is intimately linked to the overall crustal accretion models, as the gabbro glacier model is conductively cooled, while the sheeted sill model requires convective cooling of the lower crust.

4. What are the fluid and geochemical fluxes in the East Pacific Rise lower plutonic crust?

Our understanding of the extent of fluid flow and reaction in the oceanic lower crust is presently very limited. Thermal models developed to test the crustal accretion models predict that advective cooling of the lower plutonic crust at or very close to the EPR would lead to a progressive decrease in the fluid flux and attendant fluid-rock interaction with depth, whereas more gradational conductive cooling over a broader timeframe would likely lead to lower fluid fluxes and more limited fluid-rock interaction (Fig. F3). Bulk rock Sr isotope data have constrained the time-integrated fluid fluxes for the upper crust (Bickle and Teagle, 1992; Gillis et al., 2005; Teagle et al., 2003; Kirchner and Gillis, 2012), and application of this approach will allow us to constrain fluid fluxes in the lower crust. Thermodynamic modeling predicts that high-temperature fluid flow and reaction would leave little macroscopic evidence (McCollum and Shock, 1998), which is supported by δ¹⁸O data for petrologically fresh samples (Alt and Bach, 2006; Gao et al., 2006). Thus, it will be critical to combine petrological and geochemical data to assess the extent of fluid-rock interaction in the lowermost plutonic crust. Key questions that will be addressed include

• How does the total extent of alteration vary with depth?

• How does fluid flux vary with depth?

• What is the extent of chemical exchange between the lower crust and seawater?

• At what temperature is fluid-rock interaction initiated?

• What is the role of magmatic fluids?

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