Journal of Conference Abstracts

Ultramafic and Gabbroic Rocks Exposed in the 23°N Region Mid-Atlantic Ridge: Penetration of Seawater in the Oceanic Lithosphere

Catherine Mével CNRS-URA 736, Laboratoire de Pétrologie, Univ. Paris 6, 4 Place Jussieu,

75252 Paris Cédex 05, France

cam@ccr.jussieu.fr

Pierre Agrinier Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe, 4 Place Jussieu,

75252 Paris Cédex 05, France

Mathilde Cannat CNRS-URA 736, Laboratoire de Pétrologie, 4 Place Jussieu, 75252 Paris Cédex 05, France

Eva Marion CNRS-URA 736, Laboratoire de Pétrologie, 4 Place Jussieu, 75252 Paris Cédex 05, France

In the 23°N region of the Mid-Atlantic Ridge (MAR), gabbros and serpentinized peridotites are commonly incorporated in the upper level of the crust. These rocks contain abundant secondary minerals which record their history of interaction with seawater derived fluids. The rocks discussed in this study have been drilled during ODP Leg 153 (Cannat, Karson et al., 1995b), dredged during the SEADMA cruise (Cannat et al., 1995a) or collected with submersible during the HYDROSNAKE cruise (Mével et al., 1991).

Massive gabbroic sequences are made up of troctolites, olivine gabbros, gabbros, gabbronorites, oxide-gabbronorites, olivine-gabbronorites and norites, as well as small volumes of leucocratic, commonly quartz-bearing, segregates. These rocks may be undeformed, plastically deformed or, more rarely, cataclastically deformed. Plastically deformed rocks vary from anhydrous flaser gabbros (granulite facies) to foliated amphibolites (amphibolite facies). In these rocks, seawater penetration is obviously related to shearing. Undeformed rocks are far less recrystallized. Metamorphic crystallizations are related to fracturing which allows fluid circulation. Cataclastically deformed samples are intensely recrystallized, predominantly under greenschist facies conditions or lower.

Oxygen isotope ratios of secondary minerals show evidence of interaction with a seawater derived fluid at high temperatures while magmatic phases retain their primary signature. Strontium isotope ratios of secondary phases (amphibole, epidote), of the order of 0.703, suggest that the reacting fluid was evolved with respect to seawater. Its composition may have been buffered by reaction with the overlying crust.

The sequence of events and distribution of secondary phases is similar to what has been described in Hole 735B gabbros. A notable difference is the greater abundance of greenschist facies cataclastic rocks, apparently associated with major low-dipping fault surfaces, such as on the western wall of the axial valley. This difference may only reflect sampling.

Peridotites are predominantly harzburgites, cross-cut by abundant gabbroic dykelets. They are massively serpentinized (50 to 100%), and the dykelets that cross-cut them strongly altered, with respect to the massive gabbros. Serpentinization occurred mostly under static conditions, and the original textures are well preserved. An apparent schistosity is sometimes created by a network of parallel fractures filled with serpentine. Oxygen isotope measurements on whole rocks and serpentine-magnetite pairs suggest that serpentinization occurred at relatively high temperatures, i.e. >300°C (Agrinier and Cannat, in press). Accordingly, the associated gabbroic dykelets display mostly greenschist facies paragenesis.

These observations point out that massive gabbros are far less altered, and record higher temperatures of interaction, than peridotites. However, it is important to stress that both gabbros and peridotite demonstrate that most interaction with seawater derived fluids occurred when rocks were still at some depth beneath the surface, and at temperatures above 300°C.

Geological observations, in accordance with indirect inferences for crustal thickness, show that serpentinized peridotites crop out in discontinuities between segments, where the magma supply at the surface is minimal. Spreading is accomodated by tectonics, as evidenced by the interruption of abyssal hills and the predominance of transverse structures (Cannat et al., 1995a). This highly tectonized environment is likely to favor seawater penetration in the peridotite which cools down as it ascends. It is only above the 500°C isotherm that serpentinization may start, but it then becomes catastrophic, and the associated small gabbroic intrusions are intensely altered as well.

On the other hand, massive gabbros sequences probably correspond to relatively large intrusions (over several hundred meters). These intrusions must provide heat at high levels in the lithosphere and accomodate stretching by magmatic and low stress crystal plastic flow. Interaction with seawater derived fluids starts along discrete fault zones during the progressive cooling of these gabbro bodies and continues through the ductile/brittle transition. However, open fractures are rapidly sealed, preventing extensive alteration, except in cataclastic zones.

References

Agrinier, P. & Cannat, M., Proc. ODP Sci. Res. 153 (in press).

Cannat, M., Karson, J.A., Miller, D.J. et al., Proc. ODP Init. Rep. 153 (1995a).

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

Mével, C. et al., Tectonophysics 53, 31-53 (1991).