Rock sampling during cruise Sl9: lateral variation of the ultramafic rocks composition

From the lower part of the northern scarp of the VTR thirty five dredge stations were carried out. They were spaced about 3 nautical miles apart along a sea floor spreading flow line, with the main goal of recording temporal variations in upper mantle composition. Sampling was done both west and east of the Nautile sections, i.e., west of 42:50'W and east of 42:30'W (Fig. 3a and b).

The western area include a set of 22 dredge stations, located between 43:35'W and 42:52'W, a stretch of 87 km equivalent to a time intewal of 3.9 My (from CANDE1995 time scale).

The eastern area included a set of 13 dredge stations, carried out between 42:36' W and 42:08' W, a stretch of 55 km corresponding to 2.5 My (from CANDE1995 time scale).

A shipboard description of the dredge hauls is reported in Tab. 1.

Serpentinized peridotites are the main rock type recovered at all sites deeper than 4000 m. They make up about 60% (by weight) of the total material recovered from all sites. Othet common lithotypes are gabbros (10%), dolerites (10%), basalts (5%), limestones and breccias (15%). FE-Mn films and crusts often cover the different lithotypes.

A preliminary decsription of the samples suggests systematic lateral variations in the structure and composition of the ultramafic unit exposed on the VTR. Some domains of the VTR basal unit are dominated by relatively undeformed porphyroclastic peridotites. In the other domains strongly deformed, mylonitic peridotites prevail, frequently containing amphiboles. These different domains reflect probably different processes of lithosphere emplacement ("magmatic" accretion versus "amagmatic" extension) at ridge axis at different times. In addition preliminary electron probe analysis of mantle equilibrated relict minerals (olivine, opx, cpx, and spinel) suggest variations in composition that may reflect variable degrees of depletion.

These preliminary data confirm our assumptions that the VTR will release important information on the temporal variations of processes of formation of the oceanic lithosphere at the axis of the Mid Atlantic Ridge. Analytical work is in progress with the aim of achieving this task.

Figure: .

Figure: .