Saponite and celadonite in layer 2 basalts, DSDP Leg 37

Abstract

Detailed examination of igneous core recovered during Leg 37 of the DSDP, has revealed that basalt diagenesis, an alteration effect resulting from low temperature interaction with seawater, is pervasive to depths of greater than 600 meters in layer 2. Phyllosilicates are among the main components of this low grade alteration effect and occur exclusively as saponite, celadonite and interlayered mixtures of these two phases. Saponites fall into two chemical groups as (a) Mg rich-Fe poor types characteristic of oxidative diagenesis and (b) Fe rich-Mg poor types characteristic of non-oxidative diagenesis. Celadonite is entirely restricted in occurrence to the oxidative diagenetic environment (oxidation halos) and rarely, if ever, occurs as a pure phase. In this respect, X-ray and electron microprobe analyses suggest the presence of interlayered smectite in some cases and iron-oxide contamination in others.

Phyllosilicates generated during oxidative diagenesis of basalt occur in discrete mineralogical zones, typically arranged in the sequence Fe³⁺-oxide, celadonite, saponite proceeding outwards from a vein. The generation of these zones are modelled in terms of an infiltration dominated metasomatic process and their occurrence reflects a gradual extraction of oxygen from inflowing seawater during progressive reaction with the massive crystalline basalt.

Mass balance calculations applied to the oxidative diagenetic process suggest that a source external to the immediate environment of the massive crystalline rock is required for Si, Fe, K, Mg, Ca, and Al, in order to account for the observed secondary minerals. While Mg, K, and Ca are indigenous to seawater, Si, Fe, and Al were probably enriched in the fluid during wall rock reactions in the fracture system.

It is suggested that diagenesis of layer 2 basalts results in the extraction of Mg and K from seawater, in enough quantity to contribute significantly to the chemical mass balance of the oceans.