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This paper discusses the geological and geophysical data available on mid-ocean ridges with outcrops of serpentinized mantle peridotires, with the objective of better constraining the modes of emplacement of these rocks in the seafloor. Ridges with serpentinized peridotires outcrops are in most cases characterized by slow-spreading rates, and in every case by deep axial valleys. Such deep axial valleys are thought, based on geophysical constraints and on mechanical modelling results, to characterize ridges with a thick axial lithosphere. A predictable effect of a thick axial lithosphere is that it should prevent magmas from pooling at crustal depths in a long-lasting magma chamber: gabbroic magmas should instead form shortlived dike or sill-like intrusions. Samples from axial outcrops of serpentinized peridotires are often cut by dikelets of evolved gabbros which are interpreted as apophyses of such dike and sill-like intrusions. This observation leads to a discontinuous magmatic crust model, in which mantle-derived peridotites form screens for numerous gabbroic intrusions. This discontinuous magmatic crust is expected to form in magma-poor ridge regions, where there is not enough magma to produce a 440 7-km-thick magmatic crust, and where the uppermost kilometers of oceanic lithosphere therefore have to be at least partially made of tectonically uplifted mantle material. Because the dimensions of individual mantle-derived ultramafic screens may be smaller than seismic experiments detection limits, the discontinuous magmatic crust model discussed in this paper may produce a layer 3-type seismic signature, even without extensive serpentinization of its ultramafic component. It therefore provides an alternative to Hess's [1962] serpentinite layer 3 model, for the geological interpretation of seismic data from oceanic areas with frequent outcrops of deep crustal and mantle-derived rocks. INlRODUCTIONThe correlation of seismic velocity data from present-day oceans with ophiolitic stratigraphy suggests that the oceanic crust may be formed of an upper, 1 to 2 km-thick layer of basaltic lavas and dikes, rooted in 3 to 5 km of gabbros (papers from Penrose Conference on Ophiolites, Geotimes, volume 17, 1972; Figure 1 a). This model appears consistent with recently acquired seismic data from the fast-spreading East Pacific Rise [Detrick et al., 1987; Harding et al., 1989; Vera et al., 1990]. It does not account, however, for the exposure of gabbros and serpentinized peridotires on the seafloor along some other portions of the world's mid-ocean ridge system (Figure 2). These axial outcrops of deep crustal and mantle-derived rocks indicate that the oceanic magmatic crust is segmented, and in places discontinuous. This is currently interpreted as a result of amagmatic [Karson and Dick, 1983; Karson et al. 1987; Karson, 1991] or magma-poor [Dick et al., 1981; Dick, 1989; Mdvel et al., 1991] spreading; the diverging motion of the plates being accommodated by extensional deformation of the ridge axis' lithosphere. This paper u...

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Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers. The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast-spreading centres, but it has been difficult to image such magma chambers beneath slow-spreading centres, owing to rough seafloor topography and associated seafloor scattering. In the absence of any images of magma chambers or of subsurface near-axis faults, it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow-spreading ridges. Here we report the presence of a crustal magma chamber beneath the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge. The reflection from the top of the magma chamber, centred beneath the Lucky Strike volcano and hydrothermal field, is approximately 3 km beneath the sea floor, 3-4 km wide and extends up to 7 km along-axis. We suggest that this magma chamber provides the heat for the active hydrothermal vent field above it. We also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward-dipping faults cutting through the volcanic edifice, suggesting continuous interactions between tectonic and magmatic processes.

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Abstract. The comparison of segment lengths, relief, and gravity signature along the very slow spreading Southwest Indian Ridge (SWIR) between 49øE and 69øE suggests that the marked change in segmentation style that occurs across the Melville transform (60ø45'E) reflects a change in the modes of formation of the axial topography. We propose that the axial relief east of Melville is largely due to volcanic constructions that load the axial lithosphere from above. By contrast, the axial relief in segments west of the Melville fracture zone appears to be primarily due, as proposed for segments of the faster spreading Mid-Atlantic Ridge, to along-axis changes in the depth of the axial valley, and to partial compensation of negative loads (thicker lower crust and/or lighter upper mantle) acting within the plate, or at the bottom of the plate. In terms of geology, this means that the contribution of the uppermost, effusive, part of the crust to along-axis crustal thickness variations may be greater east of Melville than in other regions of the study area. Regional axial depths suggest that the ridge east of Melville is also characterized by a low melt supply and is underlain by cold mantle. A simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region, and the shallower region west of the Gallieni transform (52ø20'E), are of the order of 80øC and 4 km, respectively.

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