ALBERT

Description: The origin of authigenic minerals on the ocean floor has been extensively discussed in the past with emphasis on two major processes; precipitation from solutions originating from submarine eruptions, and slow precipitation from sea water of dissolved elements, originating from weathering of continental rocks. It is concluded that in several marine authigenic mineral systems these processes overlap. A diagnostic principle is suggested, permitting a qualitative or semiquantitative discrimination between marine authigenic minerals crystallized from dissolved species, which have spent a long time in solution on the one hand, and the same minerals generated from solutions, near their source on the other. Extensive data are available for the manganese and iron oxide minerals forming manganese nodules. It is indicated on the basis of their composition and structure that many of the nodules found in the vicinity of the continents are made up essentially of manganese derived from continental weathering. In contrast to this group, all of the nodules found in the Pacific area of submarine vulcanism display the criteria for rapid precipitation near the source of solution. The distribution of barium minerals over the deep ocean floor is discussed.The same diagnostic principle is suggested for application to these solids, in order to discriminate between baryte and harmotome crystallized near the source of barium- rich, acidic vulcanites, and the same minerals formed from continental solution with passage through the biosphere. In the case of the authigenic aluminosilicates it is found that many of the framework elements (Si and particularly Al) have low passage time through solution, and the major fraction of these elements is consequently removed from solution in the vicinity of the eruptive source materials. Extensive modification of the crystal structures, however, takes place over long periods of time, adding particularly cations from sea water, and probably to some extent silica from siliceous fossils, which on their decay on the ocean floor appear to contribute to the silicate framework of growing zeolites. The marked fractionation of the rare earth ions between coexisting phases is pointed out, with discussion of the potential use of this phenomenon to indicate the processes of formation. The use of the hafnium/zirconium ratio as a tracer for the igneous source type is suggested, and the application of ideally imperfect tracers to establish the varying relative importance of volcanic versus halmeic source of marine minerals is discussed in general.