Geochemistry and minerals at DSDP Leg 65 Holes

Petrologic studies of mid-ocean ridge basalt (MORB) (e.g., Melson et al., 1975; Flower, et al., 1977; Byerly and Wright, 1978; Melson, 1979; Byerly and Sinton, 1980; Thompson, 1980) show that magmatic liquid-fraction trends are indicated by the compositions of fresh glass selvedges, usually, but not always, associated with pillow basalts. In contrast, whole-rock compositional variation will often reflect the complicating effects of syn- and post-eruptive phenocryst accumulation. Additional variation may be introduced by the reaction of basalts with seawater. While comparatively severe alteration of variable type was noted locally in the young basalts recovered across the mouth of the Gulf of California on Leg 65, most of the basalts were extremely fresh, making them ideal for studies of compositional variation.

Basal dolomitic sediments ofthe Tyrrhenian Sea

Basal dolomitic sediments were recovered at three drill sites in the Tyrrhenian Sea during Ocean Drilling Program (ODP) Leg 107 (Sites 650, 651, and 655). These sediments overlie the basaltic basement complex and are enriched in iron, and in some instances, also in manganese. The manganese enrichments, together with a very slight enrichment in trace transition elements, strongly suggest that the basal sediments have an affinity to deep-sea metalliferous deposits of hydrothermal origin. At Sites 651 and 655, the dolostones contain variable amounts of authigenic palygorskite, a Mgrich clay mineral. At Site 651, the basal sediments are 40 m thick and contain nonstoichiometric dolomite, sometimes Ca rich, but primarily Mg rich. The occurrence of Mg-rich dolomite with excess Mg up to 4% is unusual for the deep-sea environment; it may be associated with a hydrothermally driven flux of altered sea water through the directly underlying basement complex, which comprises basalt, dolerite, and serpentinized peridotite. Low-temperature alteration of the basement complex could produce solutions enriched in Mg. Oxygen-isotope equilibrium temperatures indicate that all of the studied dolomites formed under low-temperature conditions (i.e., < 70?C). The carbon-isotope compositions, together with the strong isotopic covariance, suggest that the Mg-rich dolomite precipitated more rapidly than the Carich dolomite. We suggest that the low-temperature, hydrothermal convection of Mg-rich solutions through the basal sediments in this back-arc basin environment (1) overcame kinetic problems related to the formation of massive dolostones, and (2) provided a mass-transport mechanism for dolomitization.

Mineralogy and geochemistry of sedimentary deposits at DSDP Leg 55 Holes

The four holes (including a re-entry hole) drilled at Site 433 allow determination of the sedimentary sequence of Suiko Seamount in the Emperor chain. The holes are in a small graben basin situated within a lateral lagoon on the seamount. The sedimentary deposits range from the Paleocene to the upper Pliocene and are not uniform and continuous. A major hiatus exists at the top of the lower Eocene reef sediment, below the lower and upper Miocene pelagic sediments. The depositional history and succession of environments are shown by mineralogical and geochemical changes in the sediments.

Mineralogy of ODP Leg 104 holes

The mineralogical and geochemical study of samples from Sites 642, 643, and 644 enabled us to reconstruct several aspects of the Cenozoic paleoenvironmental evolution (namely volcanism, climate, hydrology) south of the Norwegian Sea and correlate it with evolution trends in the northeast Atlantic. Weathering products of early Paleogene volcanic material at Rockall Plateau, over the Faeroe-Iceland Ridge and the Voring Plateau indicate a hot and moist climate (lateritic environment) existed then. From Eocene to Oligocene, mineralogical assemblages of terrigenous sediments suggest the existence of a warm but somewhat less moist climate at that time than during the early Paleogene. At the beginning of early Miocene, climatic conditions were warm and damp. The large amounts of amorphous silica in Miocene sediment could indicate an important flux of silica from the continent then, or suggest the formation of upwelling. Uppermost lower Miocene and middle to upper Miocene clay assemblages suggest progressive cooling of the climate from warm to temperate at that time. At the end of early Miocene, hydrological exchanges between the North Atlantic and the Norwegian Sea became intense and gave rise to an important change in the mineralogy of deposits. From Pliocene to Pleistocene, the variable mineralogy of deposits reflects alternating glacial/interglacial climatic episodes, a phenomenon observed throughout the North Atlantic.