(Table 1) Basalt geochemsitry at DSDP Hole 72-516F

Basalts from Hole 516F, DSDP Leg 72 on the Rio Grande Rise are tholeiitic in character but differ from normal mid-ocean ridge basalts in the South Atlantic in higher concentrations of incompatible elements such as Ti, K, V, Sr, Ba, Zr, Nb, and light rare-earth elements and in lower concentrations of Mg, Cr, and Ni. They contrast with previously reported basalts from the Rio Grande Rise, which were highly alkalic in character. The Rio Grande Rise basalts from Hole 516F (age 84.5 Ma) are generally similar to basalts from the eastern end of the Walvis Ridge (80-100 Ma). It is suggested that they either originated, like the Walvis Ridge, from a mantle hot spot that is different from the present-day hot spot (Tristan da Cunha) and that has changed composition with time, or from a spreading center that was shallow and chemically influenced by the adjacent hot spot, similar to the present-day Mid-Atlantic Ridge near the Azores and Tristan da Cunha.

(Table 1) Apparent ages from 40Ar/39Ar heating steps for DSDP Hole 72-516F basalts

Submarine basalts are difficult to date accurately by the potassium-argon method. Dalrymple and Moore (1968) and Dymond (1970), for example, showed that, when the conventional K-Ar method is used, pillow lavas may contain excess 40Ar. Use of the 40Ar/39Ar step-heating method has not overcome the problem, as had been hoped, and has produced some conflicting results. Ozima and Saito (1973) concluded that the excess 40Ar is retained only in high temperature sites, but Seidemann (1978) found that it could be released at all temperatures. Furthermore, addition of potassium, from seawater, to the rock after it has solidified can result in low ages (Seidemann, 1977), the opposite effect to that of excess 40Ar. Thus, apparent ages may be either greater or less than the age of extrusion. Because of this discouraging record, the present study was approached pragmatically, to investigate whether self-consistent results can be obtained by the 40Ar/39Ar step-heating method.