Scientific objectives

To date, the Superfast campaign has accomplished the significant initial operational objective of drilling a section of intact upper ocean crust down into gabbros. This is a major scientific and engineering achievement, but with only ~100 m penetration into a complicated dike/gabbro boundary transition zone, many scientific objectives of the Superfast mission remain unfulfilled. Before we outline specific objectives for operations in Hole 1256D it is informative to predict what might be encountered with deeper drilling at this site.

What rocks are to be expected with deeper drilling?

With deeper drilling, Hole 1256D will explore unknown territory; hence we must use geological analogs to predict what rocks underlie the current base of Hole 1256D. Field mapping from the lower crust of the Oman ophiolite (e.g., Kelemen and Aharonov, 1998; MacLeod and Yaouancq, 2000; Nicolas et al., 2000, 2009) and submersible studies and shallow drilling of EPR crust at Hess Deep (e.g., Francheteau et al., 1992; Gillis, 1995; Karson et al., 1992, 2002; Natland and Dick, 1996) may be useful in this respect. However, because of the difficulties of detailed mapping and sampling using submersibles and remotely operated vehicles and the lack of deep drilling of the dike/gabbro boundary in Hess Deep, the well-exposed outcrops in Oman arguably provide the best available guide to the upper plutonic section of fast spreading ocean crust. With detailed geological mapping from the dike/gabbro boundary down toward the harzburgites of the mantle (MacLeod and Yaouancq, 2000) in Wadi Abyad, Oman provides one of the most useful templates of what might be encountered with deeper drilling in Hole 1256D (see also Pallister and Hopson, 1981; Nicolas et al., 1996).

Similar to Hole 1256D, the rocks directly underlying the sheeted dikes complex in Wadi Abyad are varitextured gabbros and microgabbros with subordinate ferrobasalts. The varitextured gabbros are ~150 m thick and show extreme variability in texture, grain size, and chemical composition at centimeter to meter scale and are petrographically very similar to the gabbroic rocks described so far from Hole 1256D. The Oman varitextured gabbros also display a wide range of compositions from those similar to cumulate gabbros to compositions more fractionated than the overlying dikes and lavas (e.g., Mg# 38–82). An average composition weighted according to outcrop abundance indicates that this horizon (Mg# ~65; MacLeod and Yaouancq, 2000) is a pooled basaltic liquid sourced from deeper in the crust (Kelemen and Aharonov, 1998; MacLeod and Yaouancq, 2000; Sinton and Detrick, 1992), similar to the gabbros recovered from Hole 1256D.

The varitextured gabbros grade over a few meters into ~650 m of foliated gabbros with steeply dipping, prominent magmatic foliations oriented subparallel to the strike of the overlying sheeted dikes. These rocks show only weak modal variations. These foliated gabbros pass down into layered gabbros that make up the bulk of the plutonic section in Oman. These layered gabbros exhibit Moho-parallel modal layering defined by variations in the proportion of olivine, clinopyroxene, and plagioclase. Both the foliated and layered gabbros have "cumulate" compositions with high bulk Mg# (>75). Very low P2O5 concentrations (<0.02 wt%) indicate low (<5%) proportions of trapped intercumulus liquids. Similar but less detailed observations are recorded from Hess Deep (Francheteau et al., 1992; Gillis, 1995).

Within a few hundred meters of drilling, we anticipate breaching the current dike–gabbro transition zone followed by penetration through a narrow (100–200 m) zone dominated by varitextured gabbros. Below these rocks, we anticipate drilling into cumulate rocks, perhaps with strong subvertical magmatic foliations and only weak modal layering. Modally layered gabbros with subhorizontal cumulate mineral layers are predicted to occur within 1000 m of the current bottom of Hole 1256D.

Scientific questions

Specific scientific questions that will be addressed by deepening Hole 1256D a significant depth into gabbros include the following:

  1. What is the major mechanism of magmatic accretion in crust formed at fast spreading rates? Is the lower crust formed by gabbro glaciers or sheeted sills or by some mixed or unknown mechanism?

  2. How is heat extracted from the lower oceanic crust?

  3. What is the geological significance of the seismic Layer 2/3 boundary at Site 1256?

  4. What is the magnetic contribution of the gabbro layer? Can the magnetic polarity structure of the lower crust be used to constrain cooling rates?

1. Is the lower crust formed by gabbro glaciers or sheeted sills?

There are two principal models for the accretion of the lower crust at fast spreading mid-ocean ridges (Fig. F11); the "Gabbro Glacier" model (Henstock et al., 1993; Phipps Morgan and Chen, 1993) and accretion by "Sheeted Sills" (e.g., Boudier et al., 1996; Kelemen et al., 1997; Korenaga and Kelemen, 1997; MacLeod and Yaouancq, 2000). Gabbro glacier models postulate that the entire lower crust is formed by ductile flow of solid material downward and outward from a single shallow axial magma chamber. In contrast, sheeted sill models are based on the crystallization of gabbro throughout the lower axial crust, with negligible downward flow of solid material (Kelemen et al., 1997).

High-level mafic cumulate rocks that balance the fractionated compositions of the dikes and lavas are a predicted consequence of both the gabbro glacier and sheeted sills modes of accretion, and cumulates should occur within a few hundred meters of the dike/gabbro boundary. Sampling the cumulate section through drilling will allow us to test the relative importance of these end-member mechanisms of crustal accretion. A gabbro glacier mode of crustal accretion will result in specific chemical and structural consequences for the lower oceanic crust that will be observable in drill core (Fig. F12). One would predict (1) that the upper plutonics and the lower crust have similar compositions and (2) that there will be increasing amounts of strain and subsolidus deformation in rocks with depth in the ocean crust.

Test 1.1: The gabbro glacier models predict that there will be no variation in cumulate composition (e.g., Mg#) with depth.

If all solidification of the lower crust takes place in the shallow melt lens, then there should be no vertical variation in the composition of the cumulates (e.g., Mg# of mafic phases). However, if the stacked sills model is correct, then we should see a gradual increase in the Mg# of the mafic phases with increasing depth in the section or a random vertical repartition of variable Mg# values. Studies of high-level plutonic rocks in Oman do show chemical trends with depth over a scale of kilometers (e.g., MacLeod and Yaouancq, 2000; S. Miyashita., pers. comm.), although small-scale (meters) chemical layering at the very base of the Oman crust is not well correlated with depth (Korenaga and Kelemen, 1998). Chemical layering in cumulate rocks should put constraints on melt migration and the size of magma lenses. Studies of cyclic variations in mineral trace element concentrations in Oman suggest the size of melt lenses is on the order of meters to hundreds of meters (Browning, 1984; Korenaga and Kelemen, 1997). Modeling the effects of melt flow through porous media such as a chemically layered crystal mush indicate that trace element correlations should be obliterated by melt-crystal reaction in hundreds to thousands of years (Korenaga and Kelemen, 1998). In Oman, chemical layering is well correlated for different elements and minerals, suggesting that the upwelling of melt through a crystal mush at the ridge axis cannot have occurred (Korenaga and Kelemen, 1998). These careful petrological tests have not been applied to rocks sampled from in situ ocean crust. The presence or absence of geochemical trends in whole-rock and mineral chemistry will place important boundaries on mechanisms of melt migration and magma emplacement in the lower oceanic crust.

Test 1.2: Gabbro glacier models predict increasing strain with depth.

Formation of the lower crust from a single high-level melt lens requires very large increases in strain with depth in the crust (Henstock et al., 1993; Phipps Morgan and Chen, 1993) that manifest as stronger shape and lattice fabrics. Such high strains have not been reported from rocks sampled in Hess Deep, but to date textural and structural measurements to estimate the extent of strain have not been employed on rocks from intact sections of fast spread ocean crust. If we find no evidence for increasing strain with depth, this would favor the multiple sills model. However, increasing strain with depth has been reported for the uppermost part of the foliated gabbro section of the Oman ophiolite (Nicolas et al., 2009), suggesting that subsidence does occur from the upper melt lens. Together with the evidence for sills, these observations may support a mixed model (Boudier et al., 1996).

2. Is the plutonic crust cooled by conduction or hydrothermal circulation?

The balance between conductive and hydrothermal cooling is key to understanding the thermal structure of the ocean crust, as well as for estimating the magnitude of hydrothermal chemical exchanges between the crust and oceans. Were the gabbros, particularly cumulate gabbros, cooled by conduction or hydrothermal fluids (Maclennan et al., 2005; Manning et al., 1996)? Were hydrothermal interactions pervasive or restricted to veins, and did alteration occur at black smoker or higher temperatures (350°–800°C)? Or did most of the hydrothermal interactions occur later at subgreenschist facies conditions (e.g., prehnite, clays) some distance away from the ridge? How do alteration effects relate to physical properties and the seismic layering of the crust? Simple gabbro glacier models suggest much slower rates of cooling for the lower crust (~0.02°C/y) than those required to match recent seismic tomographic models and compliance results from the EPR (Crawford and Webb, 2002; Crawford et al., 1999; Dunn et al., 2001). Multiple sills models require deep near-axis hydrothermal cooling and rapid cooling rates (~0.1°C/y), otherwise large-scale remelting of the lower crust will occur (Chen, 2001). However, deep hydrothermal cooling may also occur in some geometries of gabbro glacier models. Although the extent of hydrothermal cooling of the lower crust must be closely linked to the mode of magmatic accretion, quantifying these rates of cooling is a separate, important, and independently achievable objective.

Test 2.1: Are cooling rates much greater than expected from conductive heat transfer in the cumulates?

Recent petrological studies of the Oman ophiolite and Hole 504B have developed techniques for estimating the cooling rates of dikes and gabbros (Coogan et al., 2002, 2005a, 2005b). The Ca in olivine geospeedometer developed and refined by Coogan and others will allow the robust estimation of vertical variations in cooling rate that are sensitive enough to identify departures from conductive cooling profiles. Li measurements of coexisiting igneous plagioclase and clinopyroxene provide an independent cooling geospeedometer (Coogan et al., 2005b).

Test 2.2: Quantify fluid evolution and fluxes through lower crust using trace elements and Sr and stable isotopic profiles.

Well-established petrologic and geochemical approaches will be used to characterize the nature and relative timing of hydrothermal exchange between seawater and the lower crust that complement the trace element cooling rate studies of magmatic minerals discussed above.

Mineral geothermometers, crosscutting vein mineral sequences coupled with trace element and strontium and stable isotopic measurements of whole-rock samples and mineral separates, can be used to establish the chemistry of fluids reacting with the lower crust (Gregory and Taylor, 1981; Bach et al., 2004; Coggon et al., 2004; Gillis, 1995; Manning et al., 1996; Teagle et al., 1998a, 1998b). By looking at 87Sr and 18O profiles away from hydrothermal mineral veins, we can establish the scale of fluid channeling in the lower crust (e.g., Bickle, 1992; Teagle and Bickle, 1993). The advection of seawater-derived tracers, particularly when whole-rock and mineral data are closely coupled, have proved useful for estimating time-integrated hydrothermal fluid fluxes (Bickle and Teagle, 1992; Gillis et al., 2005; Teagle et al., 2003).

3. What is the geological significance of the seismic Layer 2/3 boundary at Site 1256?

Understanding the seismic structure of the ocean crust requires the calibration of remotely obtained regional geophysical data against physical properties and petrological measurements of geological samples recovered from within deep ocean boreholes. Hole 504B remains the only site where the seismic Layer 2/3 boundary has been penetrated (e.g., Detrick et al., 1994). At that location, the change in seismic gradient clearly occurs within the dikes, and the Layer 2–3 transition reflects changes in bulk physical properties associated with an increased grade of hydrothermal alteration (albite + chlorite to amphibole + plagioclase). In Hole 1256D, gabbros have been recovered from crust clearly within seismic Layer 2 based on shipboard, wireline, and seismic refraction velocity measurements (Figs. F4, F7) (Swift et al., 2008). The depth of the Layer 2/3 boundary estimated during site survey seismic experiments at Site 1256 is between 1450 and 1750 mbsf. Drilling deeper at Site 1256 would provide a second test of the geological meaning of the seismic layering of the ocean crust, where the Layer 2–3 transition lies beneath the first appearance of gabbro.

4. What is the magnetic contribution of the gabbro layer? Can the magnetic polarity structure of the lower crust be used to constrain cooling rates?

Site 1256 was deliberately located ~5 km on the old side of the C5Cr–C5Bn magnetic reversal (Fig. F3). Preliminary interpretation of the downhole magnetic field indicates that the flow and dike section has reversed polarity. Interpretation of paleomagnetic samples has been severely hampered by drilling overprint. However, the downhole magnetic field is more diagnostic than analysis of samples for determining the average in situ magnetization of a particular crustal layer. Very preliminary modeling of the downhole field intensity suggests that the flow and dike layers contribute about two-thirds of the amplitude of the marine magnetic anomalies measured at the sea surface, mostly from the thicker flow units. In addition to quantifying the contribution of gabbros to the sea-surface anomalies, magnetic measurements from a significantly deepened Hole 1256D could help determine whether the middle crust cools quickly by convection or slowly by conduction. The blocking temperature at which magnetization becomes stable over geologic time is ~400°C. The position of the site indicates that the Earth's field changed from reversed to normal polarity 50–80 k.y. after most dikes and the lower extrusives formed. End-member models for hydrothermal circulation therefore predict very different observations of the polarity structure of the middle and lower crust. Models for deep, young hydrothermal circulation (e.g., Maclennan et al., 2005) predict that the reversed polarity should continue to near the base of the crust. In contrast, models with young hydrothermal circulation largely restricted to the more porous upper crust (e.g., Henstock et al., 1993) predict that normal polarity should be encountered within a few hundred meters of the base of high-volume circulation, potentially within the penetration of an additional single drilling leg.