SCIENTIFIC OBJECTIVES

1. Test the prediction, from the correlation of spreading rate with decreasing depth to the axial melt lens, that gabbros representing the crystallized melt lens will be encountered at 900–1300 m subbasement (msb) at Site 1256.

The transition from sheeted dikes to gabbros has never been drilled, and this remains an important objective in achieving a complete or even composite crustal section. The dike–gabbro transition and the uppermost plutonic rocks are assumed to be the frozen axial melt lens and the fossil thermal boundary layer between magma chambers and vigorous hydrothermal circulation. Detailed knowledge of the dike–gabbro transition zone is critical to discerning the mechanisms of crustal accretion. The textures and chemistries of the uppermost gabbros are presently unknown but are central to understanding crustal construction; to date we lack samples that link gabbroic rocks to the overlying lavas, leading to the following questions:

These questions can be answered through petrographic and geochemical (major and trace elements) studies of gabbros (e.g., Natland and Dick, 1996; Kelemen et al., 1997; Manning et al., 2000; MacLeod and Yaoancq, 2000; Coogan et al., 2002a, 2000b) and the overlying lavas and their mineral constituents.

Some basic observations regarding the architecture of ocean crust, including the lithology, geochemistry, and thicknesses of the volcanic and sheeted dike sections and how these vary with spreading rate or tectonic setting, are not well known. Karson (2002) provides estimates of the thicknesses of lavas and sheeted dikes from crust generated at fast and intermediate spreading rates (600–900 m lavas and 300–1000 m dikes at Hess Deep; 500–1300 m lavas and 500 to >1000 m dikes in Hole 504B and Blanco Fracture Zone), but these are based mainly on tectonized exposures, where tectonic complexities increase uncertainties in the estimates. Results of Expeditions 309 and 312 will provide the thicknesses of these upper crustal units at Site 1256.

Studies of tectonic exposures of oceanic crust suggest that faulting and distributed zones of fracturing are common within sheeted dike complexes in crust formed at fast and intermediate spreading rates (Karson, 2002). In contrast, sheeted dike complexes in the Semail ophiolite in Oman exhibit little of such faulting and distributed fracturing (Umino et al., 2003). Drilling the sheeted dike complex at Site 1256 will enable evaluation of whether such faulting and fracturing in tectonic exposures are representative of oceanic crust or whether they may be related to their tectonic setting.

Most dikes in sheeted dike complexes in tectonic exposures of crust generated at intermediate and fast spreading rates and in Hole 504B in intermediate-rate crust generally dip away from the spreading axis, suggesting tectonic rotation of crustal blocks (Karson, 2002). Do such rotations occur in crust generated at superfast spreading rates, and are they similar, or is the crust less tectonically disrupted? A single drill hole may not conclusively answer this question but should provide important constraint.

Ground-truthing regional geophysical techniques such as seismics and magnetic measurements is a key goal of the IODP Initial Science Plan and related documents (e.g., COMPLEX). A fundamental question we will address in this experiment is how velocity changes within seismic Layer 2 and the Layer 2–Layer 3 transition relate to physical, lithological, structural, and alteration variations in the volcanic rocks, dikes, and gabbros. At Site 504 in crust generated at an intermediate-rate spreading ridge, the Layer 2–Layer 3 transition lies within the 1 km thick sheeted dike complex and coincides with a metamorphic change (Detrick et al., 1994; Alt et al., 1996), but is this representative of ocean crust and of crust generated at different spreading rates? Is the depth to gabbros shallower in crust generated at a superfast spreading rate, as predicted, and what are the relative thicknesses of volcanic and dike sections compared with crust constructed at slow or intermediate spreading rates?

Marine magnetic anomalies are one of the key observations that led to the development of plate tectonic theory, through recognition that the ocean crust records the changing polarity of the Earth's magnetic field through time (Vine and Matthews, 1963). It is generally assumed that micrometer-sized grains of titanomagnetite within the erupted basalts are the principal recorders of marine magnetic anomalies, but recent studies of tectonically exhumed lower crustal rocks and serpentinized upper mantle indicate that these deeper rocks may also be a significant source of the magnetic stripes. Coring a complete section through the sheeted dike complex allows evaluation of the contribution of these rocks to marine magnetic anomalies. Whether these deeper rocks have a significant influence on the magnetic field in undisrupted crust is unknown, as is the extent of secondary magnetite growth in gabbros and mantle assemblages away from transform faults. Sampling the plutonic layers of the crust will test the Vine-Matthews hypothesis by characterizing the magnetic properties of gabbros through drilling normal ocean crust on a well-defined magnetic stripe, away from transform faults.

Little information presently exists on the heterogeneity of hydrothermal alteration in the upper crust or the variability of associated thermal, fluid, and chemical fluxes. How these phenomena vary at similar and different spreading rates is unknown. Metamorphic assemblages and analyses of secondary minerals in material recovered by deep drilling can provide limits on the amount of heat removed by hydrothermal systems and place important constraints on the geometry of magmatic accretion and the thermal history of both the upper and lower crust (e.g., Manning et al., 2000; MacLeod and Yaoancq, 2000; Coogan et al., 2002a, 2000b). Fluid flow paths, the extent of alteration, and the nature of deep subsurface reaction and shallower mixing zones are all critical components of our understanding of hydrothermal processes that can only be tackled by drilling. These problems can be addressed by examining the "stratigraphy" and relative chronology of alteration within the extrusive lavas and dikes, by determining whether disseminated sulfide mineralization resulting from fluid mixing and a large step in thermal conditions is present at the volcanic–dike transition (as in Hole 504B and many ophiolites), and by evaluating the grade and intensity of alteration in the lower dikes and upper gabbros. The lowermost dikes and upper gabbros have been identified as the conductive boundary layer between the magma chambers and the axial high-temperature hydrothermal systems, as well as the subsurface reaction zone where downwelling fluids acquire black-smoker chemistries (Alt, 1995; Alt et al., 1996; Vanko and Laverne, 1998; Gillis et al., 2001). However, extensive regions of this style of alteration or zones of focused discharge are poorly known, and information from ophiolites may not be applicable to in situ ocean crust (Richardson et al., 1987; Schiffman and Smith, 1988; Bickle and Teagle, 1992; Gillis and Roberts, 1999). Drilling beyond the boundary between the lower dikes and upper gabbros will allow tracing recharge fluid compositions, estimating hydrothermal fluid fluxes (e.g., Teagle et al., 1998, 2003), and integrating the thermal requirements of hydrothermal alteration in sheeted dikes and underlying gabbros with the magmatic processes in the melt lens. Detailed logging of cores combined with geochemical analyses will enable determination of geochemical budgets for hydrothermal alteration (e.g., Alt et al., 1996; Alt and Teagle, 2000; Bach et al., 2003). Is there a balance between the effects of low-temperature alteration of lavas versus high-temperature hydrothermal alteration of dikes and gabbros? This is a critical check on global budgets for many elements (Mg, K, ⁸⁷Sr, U, and ¹⁸O) presently estimated from vent fluid chemistries, riverine inputs, and thermal models (e.g., review of Elderfield and Schultz, 1996).

Microbial alteration of volcanic glass decreases with basement depth at other sites (Furnes and Staudigel, 1999), but the temperature and depth limits to subbasement microbiological activity can be investigated by deep sampling and study of glass alteration or other geochemical indicators (e.g., Blake et al., 2001; Alt et al., 2003).