(Table 1) Chemical analyses of rock samples from low atolls of the Western Indian Ocean

Compositions, structures, and microstructures of different types of phosphorites and poorly phosphatized rocks from low atolls in the near-equatorial part of the Western Indian Ocean are described. The rocks were examined under optical and scanning microscopes using microprobe techniques and etching of selected samples with weak solvents as well as with the help of chemical analyses. It is proved that phosphorites have been formed owing to the uneven phosphatization of primary carbonate rocks; degree of their phosphatization ranges from traces to 40% P2O5. In the phosphorites numerous organic remains were encountered; they included fragments of plankton, debris of tortoise shells, and coccoidal and filamentous bacteria-like formations. It is suggested that the phosphorites formed due to high local biological productivity over the outer edges of coral reefs and are not related to guano accumulation or to endoupwelling.

Composition of alkali picrites from the southwestern Greenland continental margin

The Labrador Sea is a basin with oceanic crust in its deep part. Bottom morphology of the Labrador Sea is rather complicated. Data of seismic profiling in this region indicate presence of numerous submarine mountains and hills, which are dominated by volcanic rocks. Some chemical and mineral characteristics of the rocks, in particular, high concentrations of alkalis and phosphorus, and presence of high-titanium augite, ilmenite, and devitrified glass enriched in K and Na, allow us to attribute them to K-Na subalkaline picrites typical for ocean islands, seamounts, and oceanic plateaus. Rocks of the K-Na subalkaline series usually form submarine basements and subaerial volcanoes of ocean islands, seamounts, and oceanic plateaus. Thus, the suggestion on formation of the highs on the continental crust is not confirmed by petrographic data, which require a refinement of the tectonic model of the northern part of the Labrador Sea.