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doi:10.2204/iodp.sp.345.2012

Background

Prior to the discovery of axial magma chambers in the late 1980s, general views of the geometry and evolution of the lower crust were dominated by a paradigm derived from studies of gabbroic bodies on land—the layered basic intrusions (Wager and Brown, 1967). These large igneous bodies form in the continental crust as large single magma pulses that crystallize more or less in situ, producing a wide range of magma compositions that can largely be traced to fractional crystallization with minor wall rock assimilation. The oceanic counterpart to these is the “infinite onion” model (Cann, 1974), whereby a single magma body with the approximate depth of the plutonic layer served as a reservoir for magmas, a source for the magmatic cumulates of the plutonic crust, and a feeder for the overlying sheeted dike complex.

Seismic reflection studies along fast- to intermediate-spreading ridges have dramatically changed our view of axial magma chambers (AMCs), revealing melt lenses <1 to 2 km below the seafloor that are ~1 km wide and a few tens of meters thick (Detrick et al., 1987; Hooft et al., 1997; Kent et al., 1990; Singh, 1998). The region underlying the AMCs is a low-velocity zone interpreted to be partially molten, containing <20% melt (e.g., Dunn et al., 2000). The internal structure of this region, such as the distribution of melt and its geometry, is not well constrained because, for example, of the uncertainties of estimating varying melt distribution. Locally, melt has been shown to pool at or below the Mohorovicic discontinuity (Moho), both on- and off-axis, and also within the lower crust off-axis (Garmany, 1989; Crawford and Webb, 2002; Durant and Toomey, 2009; Canales et al., 2009).

Two end-member models for crustal accretion have emerged from geophysical observations along fast-spreading ridges (see above) and geological evidence derived from the Oman and other ophiolites (e.g., Nicolas et al., 1988) (Fig. F2). In the gabbro glacier model (Henstock et al., 1993; Quick and Denlinger, 1993; Phipps Morgan and Chen, 1993), most crystallization occurs within a shallow melt lens and the resulting crystal mush subsides downward and outward by crystal sliding to generate the full thickness of the plutonic layer (Fig. F2A). The latent heat of the plutonic crust is largely lost to the overlying hydrothermal system on-axis. The sheeted sill model (Kelemen et al., 1997; Korenga and Kelemen, 1997, 1998; MacLeod and Yaouancq, 2000) predicts that almost all of the lower crust crystallizes in situ in a sheeted sill complex, such that melts pond repeatedly as they are transported through the lower crust, with crystallization occurring from the Moho to the AMC (Fig. F2B). This requires extensive hydrothermal cooling of the plutonic crust along the sides of the crystal mush zone to remove the latent heat of crystallization on-axis (Chen, 2001). Hybrid models have also been proposed, in which some crystallization occurs in the AMC and some in situ within the plutonic crust (Boudier et al., 1996; Coogan et al., 2002b; Maclennan et al., 2004). In fact, both end-member models require some portion of each process. In the gabbro glacier model, the melt lubricates subsiding crystals, allowing them to flow, would crystallize in the deeper crust. In the sheeted sill model, more rapid cooling at shallower levels in the crust requires some crystal subsidence to prevent the AMC solidifying (e.g., Maclennan et al., 2004).

The compositional framework for the crustal accretion models is largely based on the Oman ophiolite, where there is a significant chemical contrast between upper and lower gabbros (e.g., Pallister and Hobson, 1981; Coogan et al., 2002b). The lava sequence, sheeted dike complex, and upper gabbros have mafic phases and calculated melt compositions that are not consistent with direct derivation from liquids in equilibrium with the upper mantle (as expressed, for example, by their Mg#). In the lower gabbros, compositions are more primitive and range between those of the upper gabbros and melts in equilibrium with the upper mantle.

For comparison, upper gabbros from fast-spreading crust sampled at Hess Deep, Pito Deep, and ODP Site 1256 all show evolved compositions (Hékinian et al., 1993; Perk et al., 2007; Wilson et al., 2006; Koepke et al., 2011). At Pito Deep, however, nearly all the gabbros from >300 m below the base of the sheeted dike complex are much more primitive than at the same structural level at Hess Deep (Hanna, 2004; Perk et al., 2007). This compositional difference suggests that primitive, mantle-derived magma may be transported to shallow depths with little fractionation occurring along the way (Pito Deep) and that crystal fractionation and postcumulus reactions may produce evolved rocks at these depths as well (Hess Deep, Site 1256) (Perk et al., 2007). The new observations at Site 1256 and Pito Deep, in concert with data from Hess Deep, suggest that the competing accretion models may both be viable when spatial and/or temporal variability in magmatic processes along the EPR are considered (Coogan, 2007, in press).

The rate of cooling of the plutonic crust depends on the interplay between the addition of heat by magmatic processes (latent and specific heat of cooling) and heat loss through conductive and hydrothermal convective transport. Several theoretical studies focusing on the axial heat budget have investigated the viability of the lower crustal accretion models (Sleep, 1975; Morton and Sleep, 1985; Chen, 2001; Cherkaoui et al., 2003; Maclennan et al., 2004). Both accretion models, as well as hybrids of the two, have been shown to be viable (e.g., Cherkaoui et al., 2003; Maclennan et al., 2004); however, the lack of observational constraint on key input parameters (e.g., permeability) makes the results of these thermal models uncertain. For a more detailed discussion of the petrochemical and thermal consequences of the competing crustal accretion models, see the review article by Coogan (2007, in press)

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