Hole 896A is a second deep basement hole within oceanic crust formed at the Costa Rica Rift, which penetrated ~290 m of the lava sequence. The lava sequence is dominated by pillow lavas and massive flows, although minor breccias and two this dikes were also recovered. The basalts are predominately plagioclase + olivine phyric theoleiites, although in one section of the hole (353.1–392.1 mbsf) clinopyroxene is also present as phenocrysts.
Geochemically, the basalts are strongly depleted, moderately evolved mid-ocean ridge basalts. all the basalts are similar to the Groups D and D' basalts recovered from nearby Hole 504B, located approximately 1 km to the north of Hole 894A. None of the Hole 896A basalts have chemical signatures similar to the more evolved Group M or T basalts recovered for Hole 504B.
A significant geochemical feature of the hole 894A basalts is that the drilled section can be divided into a number of discrete chemical units. The uppermost unit, Group A (<340 mbsf) has a relatively uniform composition, although the more primitive lavas occur at the tope of this unit. Below 340 mbsf, the chemostratigraphy is characterized by sawtoothed profiles, produced by fractionation (plagioclase + olivine + clinopyroxene) followed by replenishment of the magma chamber. Near to the base of the current drilled section, the lowermost unit (Unit E) is geochemically similar to Unit B, except that the more primitive lavas occur at the top of this unit. Unit E is separated from the remainder of the drilled section by a thin highly depleted unit (Unit D).
The new geochemical data suggest that magmatism at the Costa Rica Rift developed in a dynamic system, where melts were produced from a common source by similar degrees of melting, but higher level processes (i.e., fractional crystallization and or mixing) generated discrete magma batches. This variation in the chemistry of the individual magma batches probably reflects a difference in the crustal level of the individual magma chambers and /or the timing of replenishment of the magma chambers.
Date of initial receipt: 8 August 1994
Date of acceptance: 21 July 1995
Return to Contents of Leg 148
Return to Contents of Scientific Results