Geochemistry of Lavas from Hole 896A

Abstract

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 thin dikes were also recovered. The basalts are predominantly plagioclase + olivine phyric tholeiites, 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 896A. None of the Hole 896A basalts have chemical signatures similar to the more evolved Group M or T basalts recovered from Hole 504B.A significant geochemical feature of the Hole 896A 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 top 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.

Effect of hydrothermal ridge flank alteration on the in situ physical properties of uppermost oceanic crust

“…Potassium content is also lower in lava flows than in pillow basalts. The only exception is Hole 896A, where extensive alteration along rims of lava flows was observed, possibly caused by focusing of fluid flow along lava flow contacts [ Alt et al , 1993; Brewer et al , 1996]. …”

Igneous rocks of the Brook Street Terrane, New Zealand: Implications for Permian tectonics of eastern Gondwana and magma genesis in modern intra‐oceanic volcanic arcs

“…We have shown that the Bluff Complex is geochemically distinct from the bulk of the BST. With the exception of the relative enrichments in Ba, Sr, and Pb, the Bluff samples closely resemble ultradepleted basalts (D-MORB) recovered from ocean-floor drilling on the East Pacific Rise (Marsh et al 1983;Brewer et al 1996). Genesis involving a subduction fluid component is invoked to account for the Ba, Pb, and Sr anomalies, but the lack of enrichment in other fluid-mobile elements (K, U, Rb) confirms that these basalts are not typical arc rocks (Pearce & Peate 1995).…”

Architecture of the volcanic layer from the Costa Rica Rift, constraints from core‐log integration

“…Both whole‐rock and glass data have been analyzed from Hole 896A by the shipboard scientific party [ Alt et al , 1993; Brewer et al , 1996] and Fisk et al [1996], respectively. The whole‐rock chemistry was analyzed by X‐ray fluorescence spectrometry [ Brewer et al , 1996], and the glass data were analyzed by electron microprobe [ Fisk et al , 1996]. …”

“… Brewer et al [1996] identified five chemical units in the Hole 896A core (refer to Figure 11). (1) Unit A extends from 195.1 to 340 mbsf, and basalt samples have relatively uniform compositions.…”

“…(5) Unit E at 434 mbsf has similar chemical compositions to unit B, but the most primitive compositions occur at the top of this unit. Brewer et al [1996] noted that at depths >340 mbsf the concentrations of TiO 2 , V, Cr, and Ni show saw‐tooth patterns and suggested that units B to D may represent fractional crystallization of separate magma batches interspersed with magma chamber replenishment. In unit E the most primitive magmas occur at the top of the unit, suggesting that magma mixing is the dominant shallow‐level process [ Brewer et al , 1996].…”