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Focused Crustal Accretion at Slow-Spreading Ridges

A central paradigm of Ridge Interdisciplinary Global Experiments (RIDGE) program studies is the hypothesis that mantle flow, or melt extraction, or both, are focused in three dimensions toward the centers of magmatic ridge segments, at least at slow-spreading ridges such as the MAR. This is based on observations from ophiolites, with emphasis on the Oman ophiolite (Ceuleneer, 1991; Ceuleneer et al., 1988; Ceuleneer and Rabinowicz, 1992; Jousselin et al., 1998; Nicolas and Boudier, 1995; Nicolas and Rabinowicz, 1984; Nicolas and Violette, 1982), the theory that partially molten mantle may be subject to diapirism via Rayleigh-Taylor instabilities (Barnouin-Jha et al., 1997; Buck and Su, 1989; Crane, 1985; Jha et al., 1994; Parmentier and Phipps Morgan, 1990; Rabinowicz et al., 1984, 1987; Schouten et al., 1985; Sparks and Parmentier, 1993; Sparks et al., 1993; Su and Buck, 1993; Whitehead et al., 1984), the observation that peridotites are commonly dredged near fracture zones along slow-spreading ridges, but not near ridge segment centers (Dick, 1989; Whitehead et al., 1984), and gravity and seismic studies of the MAR suggesting thick crust near segment centers and thin crust at segment ends (e.g., Barclay et al., 1988; Kuo and Forsyth, 1988; Lin et al., 1990; Tolstoy et al., 1993; Tucholke et al., 1997). In addition to the possible role of mantle diapirism, various workers have proposed that melt transport may be focused in two or three dimensions, on the basis of theoretical work and field observations (e.g., Aharonov et al., 1995; Kelemen et al., 1998a, 1995a; Magde et al., 1997; Phipps Morgan, 1987; Sparks and Parmentier, 1991, 1994; Spiegelman, 1993; Spiegelman and McKenzie, 1987). Such focused melt extraction could operate, with or without focused flow of the upwelling mantle, to produce the observed focusing of crustal accretion toward the center of magmatic ridge segments.

The idea that focused mantle upwelling at the centers of magmatic ridge segments occurs only beneath slow-spreading ridges was formulated by Marc Parmentier and his students (e.g., Lin and Phipps Morgan, 1992; Parmentier and Phipps Morgan, 1990; Turcotte and Phipps Morgan, 1992) and is supported by seismic results from the recent Mantle Electromagnetic and Tomography (MELT) experiment along the fast-spreading southern East Pacific Rise, in which no focused mantle upwelling was detected (e.g., Forsyth et al., 1998; Team, 1998; Toomey et al., 1998). However, recent observations from Oman and the fast-spreading northern East Pacific Rise have called this into question (e.g., Barth and Mutter, 1996; Dunn and Toomey, 1997; Nicolas et al., 1996). Nevertheless, most investigators agree that slow-spreading ridges such as the MAR represent the best place to test general hypotheses for the mechanism(s) of three-dimensional focusing of crustal accretion.

Comparison of Models for Mantle Upwelling

In the literature describing theories of three-dimensionally focused mantle upwelling, the terms "focused" and "3-D" receive different definitions from different authors. Thus, Parmentier and Phipps Morgan (1990), who first presented the now-famous "phase diagram" for two-dimensional (2-D) vs. three-dimensional (3-D) mantle upwelling as a function of spreading rate and mantle viscosity, chose a detailed example that is indeed 3-D but that does not correspond well to observations of diapirs in the mantle section of the Oman ophiolite. In Parmentier and Phipps Morgan's (1990) example, the region of mantle upwelling at, for example, 40 km depth is ~200 km wide in a ridge-parallel section and widens upward; near the top it is almost as wide as their 300-km ridge segment. Along-ridge transport of upwelling mantle occurs gradually over the upper 60 km of the upwelling region.

In contrast, the interpretation of Jousselin et al. (1998), loosely based on observations from Oman, is that "at any depth above 50 km there is no vertical flow outside the narrow zone of subridge upwelling." They take the zone of upwelling to be cylindrical, with a diameter of ~10 km. Furthermore, in their interpretation, all corner flow (ridge parallel and ridge perpendicular) occurs in the upper 500 m of the upwelling region. More than half of the shallow mantle in their 25-km-long ridge segments is fed by horizontal flow in this 500-m-thick layer just below the base of the lithosphere. Such narrow pipes of upwelling mantle may be consistent with the physical models of Buck and Su (1989) (Su and Buck, 1993), which show very sharp focusing of mantle flow. Such features could conceivably have escaped seismic detection in the recent MELT experiment. However, if this is the geometry of mantle upwelling, then the amount of ridge-parallel horizontal transport of mantle material must be very large.

In the ensuing discussion, we take the Jousselin et al. (1998) geometry as the end-member example of 3-D focused mantle flow and passive corner flow to be the end-member example of 2-D mantle flow with no focusing. The Jousselin et al. (1998) scenario may seem extreme at first, but it does provide a clear description of an upwelling geometry that could produce a variation in igneous crustal thickness from ~10 km at a segment center to ~0 km near the segment ends, as interpreted on the basis of geological and geophysical observations in the 14° to 16°N region of the MAR. These observations are typical of the first-order features of slow-spreading ridges, which are thought to reflect three-dimensionally focused magmatic accretion.

In contrast, available 3-D physical models of diapiric mantle upwelling beneath ridges cannot account for these observations because the upwelling is not sufficiently tightly focused. As stated by Barnouin-Jha et al. (1997), "short wavelength segmentation of slow spreading centers requires some process not included in our models of mantle flow." This missing process might be tightly focused upwelling, as in the scheme of Jousselin et al. (1998) or focused melt migration.

Testing Hypotheses for the Mechanism(s) of Focused Crustal Accretion

Despite the difficulties with 3-D physical models, outlined in the previous section, the hypothesis that mantle flow, or melt extraction, or both, are focused in three dimensions toward centers of magmatic segments at slow-spreading ridges has essentially reached the status of accepted theory. However, these ideas have never been subject to a direct test. A strike line of oriented mantle peridotite samples extending for a significant distance within such magmatic segments offers the possibility of directly testing hypotheses for focused crustal accretion.

The primary aim of drilling in the 14° to 16°N area along the MAR is to characterize the spatial variation of mantle deformation patterns, residual peridotite composition, melt migration features, and hydrothermal alteration along axis. Published hypotheses for focused solid or liquid upwelling beneath ridge segments make specific predictions regarding the spatial variation of mantle lineation or the distribution of melt migration features, which can be tested by drilling.

Interpretation of Ductile Flow Fabrics in Mantle Peridotites

Models of focused solid upwelling require ridge-parallel, subhorizontal flow of residual mantle peridotites from segment centers to segment ends (Fig. F6A) (Barnouin-Jha et al., 1997; Buck and Su, 1989; Crane, 1985; Jha et al., 1994; Parmentier and Phipps Morgan, 1990; Rabinowicz et al., 1984, 1987; Schouten et al., 1985; Sparks and Parmentier, 1993; Sparks et al., 1993; Su and Buck, 1993; Whitehead et al., 1984). This is supported to some extent by patterns of mantle flow inferred from ductile fabrics in residual peridotites in the Oman ophiolite (Fig. F6B) (Ceuleneer et al., 1998; Ceuleneer et al., 1991; Ceuleneer and Rabinowicz, 1992; Jousselin et al., 1998; Nicolas and Boudier, 1995; Nicolas and Rabinowicz, 1984; Nicolas and Violette, 1982), although, as already noted previously, the scale of focused upwelling in Oman (~10 km) is different from that in current 3-D models of mantle diapirism (~100 km). Mantle flow direction may be determined by measurement of spinel shape fabrics (lineation at high strain is parallel to ductile flow), measurement of the orientation of olivine crystal shape fabrics relative to subgrain boundaries (subgrain boundaries are oblique to the long sides of elongate crystals, indicating the sense of shear), and measurement of olivine crystal lattice preferred orientation (olivine a-axes are aligned parallel to ductile flow directions at high strain).

Cores from a series of drill holes in mantle peridotite along a slow-spreading ridge axis can, in principle, be used to test the prediction that shallow ductile flow of residual mantle at the ends of segments is ridge-parallel and subhorizontal. There are two problems with this approach: (1) the core must be restored to a geographical reference frame, and (2) tectonic rotations of the peridotite that postdate ductile flow must be considered before the orientation of ductile fabrics can be interpreted in terms of large-scale mantle flow. Work on cores of partially serpentinized mantle peridotite from the East Pacific (Boudier et al., 1996) and the Atlantic (Ceuleneer and Cannat, 1997) have shown that they can be reoriented into the geographical reference frame using remanent magnetization (Hurst et al., 1997; Kelso et al., 1996; Kikawa et al., 1996; Lawrence et al., 1997; Richter et al., 1996). Where the magnetic inclination in the core after horizontal rotation is not parallel to the inferred magnetic inclination at the time of lithospheric formation, tectonic rotations may be inferred and then "removed." However, an important caveat is that the remnant magnetization in partially serpentinized peridotites is hosted in magnetite that is produced during serpentinization, so that tectonic rotations of the peridotite prior to serpentinization cannot be detected.

Accounting fully for possible tectonic rotations of exposed mantle peridotite is a daunting prospect, but there is hope for a definitive result for the following reasons. Magnetic susceptibility anisotropy data also may provide information about the tectonic stress field where magnetite grains become aligned following serpentinization. Tectonic rotations resulting from normal faults are likely to occur mainly around axes parallel to the ridge axis. Thus, subhorizontal ridge-parallel flow lineation is likely to be affected very little, if at all. Furthermore, rotations are likely to be away from the ridge axis, increasing the angle between lineations and the ridge axis. Thus, if ridge-parallel lineations are consistently observed, this can be taken as good evidence that shallow ductile flow of the mantle was indeed parallel to the ridge. In the best case, observation of systematically varying ductile flow lineations in mantle peridotite, ranging from nearly ridge-perpendicular lineation near segment centers to ridge-parallel lineations near segment ends could be taken as very strong evidence that focused mantle upwelling did occur near segment centers.

Interpretation of Chemical Variation in Mantle Peridotites

Models of focused crustal accretion predict different patterns of mantle depletion resulting from melt extraction as a function of distance from magmatic segment centers. For strongly focused 3-D mantle flow, there should be no variation in the degree of mantle depletion along axis, since all of the shallow mantle peridotites originate within a narrow, pipelike upwelling zone. For purely passive corner flow, with no other factors considered, again there should be no variation in depletion along axis. However, when passive flow is coupled with cooling of the ends of ridge segments against a fracture zone wall, then the degree of melting is predicted to decrease along axis away from segment centers. This has been termed the "transform edge effect" (Ghose et al., 1996; Langmuir and Bender, 1984; Magde et al., 1997; Phipps Morgan and Forsyth, 1988). Provided that melt extraction is equally efficient throughout the melting region, this variation in melt production should be observed in shallow mantle samples. If partial crystallization of melt migrating into conductively cooled mantle lithosphere occurs, forming "impregnated peridotites" (e.g., Ceuleneer et al., 1988; Ceuleneer and Rabinowicz, 1992; Dick, 1989; Elthon et al., 1992; Seyler and Bonatti, 1997), then this should occur primarily near fracture zones, enhancing the chemical signal of the transform edge effect in mantle peridotites. Furthermore, impregnated peridotites often preserve structural relationships indicative of the nature of melt migration. Impregnated peridotite samples from the western ridge-transform intersection (RTI) of the Kane Fracture Zone (Ishizuka et al., 1995) show evidence for migration of melts into localized ductile shear zones, suggesting that melt migration extended into the active transform fault.

In general, geochemists have searched for the transform edge effect in lavas, which is complicated by the difficulties of seeing through variations in crustal differentiation processes and mantle source composition. Detailed analysis of a suite of peridotite samples, collected from a single ridge segment at various distances from a fracture zone, could provide an independent evaluation of the presence and importance of the transform edge effect.

Interpretation of Melt Transport Features in Mantle Peridotites

Models of focused melt migration toward ridge segment centers predict various different spatial distributions and orientations of melt transport features. Before discussing the various predictions, we will introduce some of the melt transport features that can be recognized in mantle peridotite samples. For reviews of the literature on these features, please see papers by Nicolas (1986, 1990) and Kelemen et al. (1997, 1995a).

  1. Dunites are rocks composed almost entirely of the mineral olivine, with minor spinel; pyroxene generally forms <1% of these rocks. Dunites are present in tabular to cylindrical bodies in ophiolite peridotites. Few, if any, are tabular dikes filled entirely with magmatic olivine. Instead, most or all form by dissolution of pyroxene and crystallization of a smaller amount of olivine in olivine-saturated melt migrating by porous flow. Some have an origin entirely via focused porous flow, either in dissolution channels or within ductile shear zones, whereas others form in porous reaction zones around cracks. The relative importance of entirely porous vs. fracture-related origins for dunites is controversial but is not crucial to this science plan. The main point to be made here is that dunites commonly form in the region of adiabatic mantle upwelling beneath spreading ridges, though they also form within the region of transition between adiabatic upwelling and conductively cooled lithosphere.
  2. Pyroxenite and gabbro mantle dikes are highly elongate, generally parallel-sided features that almost certainly form as fracture-filling magmatic rocks. Their compositions, where they have been studied in detail, are indicative of crystal fractionation from magma that was cooling within conductive "lithosphere." However, this is debated, and Nicolas and co-workers have interpreted them to be representative of melt-filled fractures that form within the adiabatically upwelling mantle.
We now consider predictions of spatial distribution and orientation of melt migration features, with an emphasis on dunites formed within the adiabatically upwelling mantle. Most models predict that such dunites are transposed into a subhorizontal orientation in the shallow mantle, at least by 2-D corner flow and perhaps also by 3-D diapiric flow (dunites that are not subhorizontal may have formed in the region of transition from adiabatically upwelling mantle to conductively cooled lithosphere).
  1. If melt migration and crustal accretion are focused mainly because of diapirism, as proposed by Nicolas (1990), then no systematic variation in dunite abundance along the ridge axis is predicted. Furthermore, if melt-filled fractures form within mantle diapirs and these are represented by mantle dikes (Nicolas, 1990), then mantle dikes may be nearly vertical near segment centers and progressively transposed into a horizontal orientation toward segment ends.
  2. If melt migration and crustal accretion are focused mainly because of melt migration beneath permeability barriers parallel to the base of the lithosphere, as proposed by Sparks and co-workers (Magde et al., 1997; Sparks and Parmentier, 1991, 1994) and Spiegelman (1993), then dunites should be shallowest (and most commonly sampled by drilling) near the centers of segments.
  3. If melt migration and crustal accretion are focused mainly as a result of coalescing porous flow within the upwelling mantle, as proposed by Phipps Morgan (1987), Spiegelman and McKenzie (1987), Kelemen and co-workers (Aharonov et al., 1995; Kelemen et al., 1995a, 1995b, 1998a), and Daines, Zimmerman, and Kohlstedt (Daines and Kohlstedt, 1997; Kohlstedt and Zimmerman, 1996), then dunite abundance in the shallow mantle should increase toward segment centers. Thus, if porous flow mechanisms predominate in producing focused crustal accretion, then dunite abundance should increase toward segment centers, whereas if diapiric upwelling is the predominant reason for focused crustal accretion, then dunite abundance should be relatively constant along axis.
On a smaller scale, the detailed size/frequency and spatial distribution statistics of a large number of dunite veins in outcrops of mantle peridotite can be used as indicators of the geometry of melt extraction conduits (Kelemen et al., 1998a, Braun and Kelemen, in press). Dunites in mantle outcrops in the Ingalls and Oman ophiolite show a negative power-law relationship between size and abundance, with many small dunites and only a few large ones. This is consistent with the hypothesis that dunites form an interconnected channel network, in which many small conduits feed a few large ones. We hope that the systematics of the spatial distribution can be used to distinguish between dunites that originate as reaction zones around cracks and dunites that form entirely as a result of porous flow mechanisms. Drill core samples are ideal for this type of study, which would be a secondary goal of the proposed drilling in the 14° to 16°N area.

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