Journal of Conference Abstracts

Ultramafic and Gabbroic Rocks Exposed in the 23°N Region (Mid-Atlantic Ridge): A Record of Melt Migration and Crystallization in a Sub-Axial Root of Mantle Lithosphere?

Mathilde Cannat CNRS-URA 736, Université de Paris VI, 4 Pl. Jussieu, 75252 Paris Cédex 05, France

mac@ccr.jussieu.fr

Indraneel Ghose CNRS-URA 736, Université de Paris VI, 4 Pl. Jussieu, 75252 Paris Cédex 05, France

Catherine Mével CNRS-URA 736, Université de Paris VI, 4 Pl. Jussieu, 75252 Paris Cédex 05, France

On-axis and off-axis rock sampling in the 23°N region of the Mid-Atlantic Ridge shows that the emplacement of ultramafic and gabbroic rocks in the seafloor has been a common process there for the past 7 million years (Cannat et al., 1995). These rocks crop out in the south wall of the Kane transform valley (offset 150 km), in the western axial valley wall near a smaller offset (less than 10 km; Gente et al., 1995) axial discontinuities at 23°10'N and 22°10'N, and in the off-axis traces of these axial discontinuities (Fig. 1). The ultramafic rocks are variably depleted serpentinized harzburgites and lherzolites (Cr/[Cr+Al] in spinel ranging from 0.16 to 0.42; Ghose et al., submitted), with less common serpentinized dunites, clinopyroxenites and websterites. The gabbroic rocks are troctolites, olivine gabbros (volumetrically the predominant rock type), gabbros, gabbronorites, oxide-gabbronorites, olivine-gabbronorites and norites, as well as small volumes of leucocratic, commonly quartz-bearing, segregates. This range of gabbroic mineralogies, similar to that sampled at Site 735 in the Southwest Indian Ocean, corresponds to a wide range of mineral compositions (Mg/[Mg+Fe] in clinopyroxene: 0.59 to 0.90; plagioclase anorthite content: 30 to 86%), suggesting closed-system fractionation in isolated plutons. These various gabbroic rock types are found either as extensive outcrops (such as those drilled at ODP Sites 921-924; Fig. 1), or as impregnating lenses, dykes and dykelets intruding the serpentinized peridotites.

Fig. 1: Location of the ultramafic and gabbroic samples considered in this abstract on a morphological sketch map of the Mid-Atlantic Ridge in the 23°N area (Cannat et al, 1995) showing the crest of abyssal hills (thin lines) and the main scarps (slightly thicker lines). The axial valley is bounded by high scarps (heavy lines). Basins deeper than 4000m (shaded) outline the off-axis, commonly oblique traces of small-offset axial discontinuities (Gente et al., 1995).

Intrusive relationships between various gabbroic rock types and serpentinized peridotites are best illustrated in samples from the 200 meters deep hole drilled at Site ODP 920 (Cannat et al., in press). Similar intrusive relationships are observed in dredges and dive samples from the Kane transform and from dredges SDM 17 and SDM 4 in the off-axis traces of axial discontinuities (Fig. 1). The least evolved gabbroic assemblages form impregnation lenses of anorthitic plagioclase and magnesian clinopyroxene in the peridotites, similar to those observed beneath the gabbroic sequence in the Oman or Newfoundland ophiolites. Gabbroic rocks with more evolved mineralogies form dykes and dykelets, some of which are deformed with their host peridotite in centimeter- to decimeter-sized granulite to amphibolite facies ductile shear zones. It appears that all of these dykes and dykelets were emplaced into the peridotite prior to serpentinization. Chemical modifications in the serpentinized peridotites near gabbroic intrusions range from the formation of dunite screens, to cryptic variations in olivine Mg# and nickel content, in clinopyroxene sodium content, and in spinel Mg# and titanium content. These chemical modifications occur over variable distances from the dyke margins (one millimeter to over one decimeter), probably reflecting variable temperatures of the peridotite during melt circulation. There is a systematic relationship between the dyke's mineralogy and the width of these chemical reaction zones, suggesting that the crystallization of progressively more evolved gabbroic intrusions occurred during progressive cooling of the peridotites in the axial lithosphere.

Residual ultramafic rocks in the Mid-Atlantic seafloor are thought to represent fragments of the sub-axial mantle that have been tectonically exhumed, cooled, and extensively altered in the axial region. Because the axial region is also a magmatically active domain, it is not surprising that gabbroic intrusives should have been emplaced in these ultramafic rocks at various stages of their exhumation and cooling history. The point we wish to make here is that most of these gabbroic intrusives were probably emplaced below the crust, in a root of sub-axial lithospheric mantle. This hypothesis is based on thermal and mechanical models of slow-spreading ridges, suggesting that the axial lithosphere can be thicker than the crust modelled from gravity data, or measured from seismic data. This is particularly the case for the thin-crust ridge regions that are near axial discontinuities, where ultramafic and gabbroic rocks are exposed in the seafloor. If these models are correct, the temperature field in the crust of these regions is simply too cold for extensive melt-rock chemical interactions, such as those observed near the dyke margins in some of our samples, to occur. Although we have no constraint at this point on the proportion of magma that may have crystallized in the peridotites of the 23°N region before they reached crustal level, this phenomenon should be taken into account in discussions of the magma plumbing system at slow-spreading ridges. It also has potential consequences on the way we interpret the along-axis crustal thickness variations observed at these ridges.

References

Cannat, M. et al., Geology 23, 49-52 (1995).

Cannat, M., Chatin, F., Whitechurch, H. & Ceuleneer, G., Proceedings of the Ocean Drilling Program, Scientific Results 153, College Station, Texas (in press).

Gente, P. et al., Earth Planet. Sci. Lett. 129, 55-71 (1995).

Ghose, I., Cannat, M. & Seyler, M., Geology (submitted).