LEG 140

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.

The alteration mineralogy of the Leg 140 dikes is similar in most respects to that higher in the section, showing strong evidence of hydrothermal alteration under greenschist facies conditions. New features observed in Leg 140 rocks include common, strongly pleochroic and abundant amphibole and chlorite richer in magnesium than that in the overlying dikes, indicating a continued increase in alteration temperature with depth. The rocks also contain trace amounts of epidote replacing plagioclase at depths below 1814 mbsf, whereas near the top of the dike section, epidote only occurs in veins. In addition, relict olivine was observed and often abundant. Olivine higher in the dike section is entirely pseudomorphed, and its appearance in the Leg 140 dikes indicates locally low permeability during alteration beneath the ridge axis.

The rock densities at the bottom of Hole 504B are those predicted for layer 3. Moreover, the features indicating lower permeabilities, slower crystallization and cooling, and alteration at higher ambient temperatures in the Leg 140 dikes, are characteristic of those found near the base of sheeted dike sections in many ophiolites. The dike section is strongly depleted in zinc compared to MORB, and this zone could be the source of Zn in hydrothermal fluids and black smoker deposits at mid-ocean ridges. Rocks similarly depleted in Zn and other metals also occur at the base of the sheeted dikes in various ophiolites.