Hole 504B - Costa Rica Rift
Leg 140 represented the second in a two-leg program to renovate and deepen Hole 504B. Leg 137
was designed to fish junk left after Leg 111 and conduct downhole logs and experiments. Leg 140
was intended solely to deepen the hole. Building on experience during Legs 111 and 137, more
durable bits were designed which allowed Hole 504B to be deepened by 378 m to a total depth of
2000.4 mbsf. The rocks recovered consist largely of fine-grained basalt dikes, similar to those
drilled during the latter half of DSDP Leg 83 and ODP Leg 111, which are virtually identical
chemically to the overlying pillow lavas and dikes and distinguished only by an igneous chemistry
indicative of high degrees of melting and depletion of the mantle source and an unusual degree of
homogeneity. A complex suite of mineral clots and xenoliths was present in many diabases,
recording the early conditions of magma crystallization at depth below the dike complex. Unlike
earlier legs, the dike margins were typically composed of fine-grained basalt, the average grain size
of the dikes was coarser, and medium-grained diabase was intermittently observed.
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