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doi:10.14379/iodp.sp.360.2015

Scientific objectives

Expedition 360 represents the first leg of the SloMo Project, which seeks to use the tectonic window into the lower crust at Atlantis Bank to recover the full lower section of the ocean crust, core through the igneous crust–mantle transition, determine if Moho represents a serpentinization front and, ultimately, core through the Moho (~5.5 km at an ultraslow-spreading ocean ridge). SloMo has two phases. Phase I uses the JOIDES Resolution to drill to 3 km to test the hypothesis that the Moho represents a serpentinization front and to recover the full lower crustal section and crust–mantle transition zone. Phase II will utilize the riser D/V Chikyu to drill down ~5.5 km through the seismic Moho itself. In order to achieve the Phase I objective, two expeditions will be needed. Expedition 360 will need to properly establish the borehole for Leg 2 and thereafter drill as deep as possible. Given optimal conditions, a depth of at least 1300 mbsf, with continuous coring, should be achievable.

Operations during Expedition 360 should allow us to address the following:

1. What is the igneous stratigraphy of the lower ocean crust?

The drilling will determine if the igneous, metamorphic, and structural stratigraphy found for Holes 735B and 1105A is laterally continuous across the wave-cut platform on Atlantis Bank. In combination with Holes 735B and 1105A and the existing surface mapping, this will provide a four-dimensional view of the lateral continuity and evolution of the lower crust and the process of emplacement in space and time.

2. How much mantle is incorporated into the lower crust?

An unanticipated finding in the cores from Hole 1309D at the Atlantis Massif in the north Atlantic was the incorporation of significant volumes of hybridized mantle peridotite (Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and Expedition. 304/305 Scientists; Blackman et al., 2011; Drouin et al., 2009) (Figure F6). Comparable screens of mantle rock were not found in either Holes 735B or 1105A. This could reflect differences in accretion due to different magma budgets, or it could simply be happenstance, whereby further drilling will encounter equivalent ultramafic horizons at Atlantis Bank.

3. What are the modes of melt transport into and through the lower crust?

Several different modes of melt transport were identified in the Hole 735B cores. These included small intrusive bodies of cumulates, larger intrusive units on the scale of hundreds of meters, anastomosing channels produced by focused flow and melt-rock reaction, and compaction of late iron titanium–rich melts into shear zones where they hybridized olivine gabbro to iron titanium oxide–rich gabbro and gabbronorite. The continuity and scale of these features cannot be determined from a single deep hole. Thus, drilling during Expedition 360 seeks to further document these features in a younger section of the massif.

4. How does the lower crust shape the composition of mid-ocean-ridge basalt (MORB)?

The complex stratigraphies seen in Holes 735B and U1309D (Figure F6) differ in significant ways. What common factors, however, influence the composition of MORB? Recent work on gabbroic sections from the EPR has shown that the lower crust acted as a reactive porous filter homogenizing diverse melts that were intruded into it and modifying their trace element contents prior to their eruption to the seafloor (Lissenberg et al., 2013). To what extent do such processes operate at slower spreading rates?

5. What is the strain distribution in the lower crust during asymmetric seafloor spreading?

Asymmetric spreading produced by detachment faulting is now recognized as one of three major accretionary modes at slow-spreading ridges (symmetric, asymmetric, and amagmatic rifting). Although each of these is important, little is known of how magmatism and tectonism interact, and hence how lower crustal accretion differs in these environments. At present, two deep holes in oceanic core complexes, Holes 735B and U1309D, allow us to determine strain distribution with depth. The distribution of strain documented in these two locations differs significantly, with far more intense crystal-plastic deformation being found in the upper 500 m of Hole 735B than in Hole U1309D. We cannot assess the nature, extent, and role played by high-temperature deformation during lower crustal accretion with isolated, one-dimensional sections. Offsetting and drilling to the north of Hole 735B will allow us to determine the continuity of the strain distribution from Hole 735B across the Atlantis Bank core complex and thereby assess the role and broader significance of synmagmatic deformation in this tectonic environment.

6. What is the nature of magnetic anomalies?

Magnetic anomaly “stripes” are detectable across the Atlantis II Transform transverse ridge, including the Atlantis Bank platform itself (Dick et al., 1991b; Allerton and Tivey, 2001; Hosford et al., 2003; Baines et al., 2007, 2008). Whereas most of the platform is reversely magnetized, as are the gabbro intervals of Holes 735B and 1105A (Kikawa and Pariso, 1991), a narrow normally polarized zone ~2 km wide is present near its northern end. This is equated to Anomaly C5r.1n (11.476–11.531 Ma) (Allerton and Tivey, 2001; Baines et al., 2008) (Figure F7). It is detectable on surface (Dick et al., 1991b; Hosford et al., 2003) and deep-towed magnetic profiles (Allerton and Tivey, 2001) and in the JR31 seabed drill cores; however, the precise location of the boundary in each case is slightly offset (Figure F7). Because the sea-surface magnetization signal is derived from a much greater volume of rock than the near-bottom signal in the cores, Allerton and Tivey (2001) deduced from the sense of offset that the magnetic anomaly boundaries are inclined. They modeled the southern limit of the anomaly as a planar boundary dipping ~25° toward the south, suggesting that the surface represented either an isotherm corresponding to the Curie point or the edge of an intrusion (Figure F7). Alternatively, the form of the southern edge of the anomaly may have been modified by faulting: extensive brittle deformation and cataclasis is observed in shallow drill cores from the location of the southern reversal boundary, probably corresponding to a northward-dipping normal fault at the kink/narrowing of the platform summit at 32°41.8′S. If so, the reversal boundary is likely to dip more steeply than the 25° proposed by Allerton and Tivey (2001).

An important objective of Expedition 360 is to drill through normal polarity Chron C5r.1n to determine the nature of its boundaries and what controls magnetic anomalies in plutonic rocks.

7. Is there life in the lower crust and hydrated mantle?

A primary objective of the SloMo Project is to determine the microbiology of the lower crust, the potential serpentinized mantle above Moho, and the underlying mantle, to address IODP Science Plan Challenge 6 “What are the limits of life in the subseafloor?” For this purpose, Expedition 360 will assess the microbiology of the upper portion of the lower crust.

8. What is the role of the lower crust and shallow mantle in the global carbon cycle?

Serpentinization and weathering of ultramafic rocks as well as alteration of basalts are known, under the right conditions, to cause the formation of carbonates. Such carbonates are present in the form of extensive veining in serpentinized peridotites at the southern edge of the Atlantis Bank platform (shallow drill Site JR31-BGS-12 and contingency drill Site AtBk4).

The extent of these reactions in the lower crust is, to date, largely unknown. Although we do not expect that carbon sequestration will be a significant process in the lower ocean crust based on Holes 735B and U1309D, the possibility exists that Expedition 360 may penetrate the base of the lower ocean crust and thus offer the first opportunity to determine if carbon sequestration is significant below it.

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