Major and rare earth element concentrations of basalts beneath ODP Hole 118-735B (Table 3)

A wide-angle seismic experiment at the Atlantis II Fracture Zone, Southwest Indian Ridge, together with geochemical analyses of dredged basalt glass samples from a site conjugate to Ocean Drilling Program hole 735B has allowed determination of the thickness and the most likely lithological composition of the crust beneath hole 735B. The measured Na, composition of 3.3 +/- 0.1 corresponds to a melt thickness of 3 +/- 1 km, a result consistent with rare earth element inversions which indicate a melt thickness of between 1.5 and 4.5 km. The seismic crustal thickness to the north and south of the Atlantis Platform (on which hole 735B is located) is 4 +/- 1 km, and probably consists largely of magmatic material since the seismic and inferred melt thicknesses agree within experimental uncertainty. Beneath hole 735B itself. the Moho is at a depth of 5 +/- 1 km beneath the seafloor. The seismic model suggests that, on average. about 1 km of upper crust has been unroofed on the Atlantis Platform. However, allowing for the inferred local unroofing of 2 km of upper crust at 735B, the base of the magmatic crust beneath this location is probably about 2 km beneath the seafloor, and is underlain by a 2-3 km thick layer of serpentinised mantle peridotite. The P-wave velocity of 6.9 km/s for the serpentinised peridotite layer corresponds to a 35 +/- 10 vol% serpentine content. The Moho beneath hole 735B probably represents a serpentinisation front.