Major oxide, trace element and rare earth element composition at DSDP Leg 92 holes

The chemical composition of the basement in DSDP Holes 597C, 599B and 601B is examined in this chapter. These holes were drilled during Leg 92 west of the East Pacific Rise (EPR) crest along 19°S. Hole 597C reached a depth of 91 m into the basement with a recovery of about 55%, so it is the most successful hole drilled in fast-spreading crust. On the other hand, only drilling breccia and some basaltic cobbles were recovered from Holes 599B and 601B.
In total, 60 samples of the least altered basalts from Hole 597C were analyzed. All these basalts were altered to some extent, as is evident from alteration-sensitive parameters. Oxidative seawater alteration influences the upper portion of basement, with considerable uptake of K, Rb, and H2O, loss of sulfur, and increased iron oxidation. The lowermost basement was affected by a non-oxidative alteration process, showing only enrichment in H2O; the other parameters have values typical of fresh basalts. Despite the alteration, the primary basaltic composition and variations can be established. According to their major and trace element data, the magmas appear to have undergone different degrees of high-level crystal fractionation. Several eruption episodes may have caused the observed downhole variations in Hole 597C.
Four basalt samples from Hole 599B show the same features as the basalts recovered from Hole 597C. The three basalt samples from Hole 601B, on the other hand, are more evolved and can be defined as Fe-Ti basalts, which are commonly described from young EPR crust.