Geochemistry and ages at DSDP Holes 81-552A, 81-553A and 81-555

Glauconite-rich sediments have been encountered at two horizons during drilling in the southwest Rockall Plateau. The younger of these horizons lies at the base of a deep-sea ooze sequence and is of early or middle Miocene age. Glauconite formed in situ during periods of nondeposition related to strong bottom-water currents, in water depths of as much as 2500 m - five times greater than previously accepted limits for glauconite formation. The older horizon, of early Eocene age, is a record of the major transgression coincident with the separation of Rockall and Greenland. Isotopic age dating of the Miocene glauconites gives results in relatively close accord with their biostratigraphic age. However, an Eocene (NP12) glauconite gives a highly discrepant date (36.5 m.y. ago). One possible explanation is that the Eocene glauconites have continued to evolve after burial by the diagenetic uptake of potassium from the surrounding mud matrix, a possibility denied to the Miocene glauconites by the relative scarcity of available potassium in the nannofossil-foraminiferal ooze matrix.

Mineralogy and geochemistry of alteration products in DSDP Leg 81 Holes

Alteration products of basalts from the four holes drilled during Leg 81 were studied and found to be characterized by the widespread occurrence of trioctahedral clay minerals (Mg smectite to chlorite). In some cases zeolites (analcite, chabazite) are associated with the saponite. A more oxidizing stage is marked by a saponite-celadonite association, presenting the geochemical characteristics of hydrothermal processes. Later stages of alteration are represented by palagonitization and subaerial weathering at two sites. These different alteration processes of basalts from Leg 81 record the paleoenvironment during the first opening stages of the Northeast Atlantic Ocean in the Paleocene-Eocene periods.

Geochemistry and minerals at DSDP Leg 81 Holes

Three phases of volcanism have been recognized in the lower Paleogene sequence of the southwest Rockall Plateau which are related to the onset of seafloor spreading in the NE Atlantic. The earliest, Phase 1, is marked by a sequence of tholeiitic basalts and hyaloclastites which form the dipping reflector sequence in Edoras Basin. Phase 2 is characterized by tuffs and lapilli tuffs of air-fall origin, ranging in composition from basic to intermediate. They were generated by highly explosive igneous activity due to magma-water interaction, and terminate at the level of a major transgression. Subsequently, volcanism reverted to tholeiitic basalt type, producing the thin tuffs and minor basalt flows of Phase 3. Alteration of the volcanic glass and diagenesis of the tuffs and lapilli tuffs has been considerable in many cases, with a large number of diagenetic mineral phases observed, including smectite, celadonite, analcime, phillipsite, clinoptilolite, mordenite, and calcite. Although calcite is the latest observed diagenetic cement, it nevertheless occurred relatively early, in one case totally preserving basaltic glass from alteration.