Evolution of the Mantle Fluids Composition in the Precambrian (on the example of the Ukrainian shield ferruginous formation)

Abstract

The basic world ferruginous formations evolve from 3.5 to 1.9 billion years and have three main peaks of manifestation: 3.2—2.7, 2.6—2.5, and 2.0—1.9 billion years ago. Three ferruginous formations, which correspond to these periods, are highlighted at the Ukrainian shield: ferruginous-siliceous-volcanogenic (FSV), ferruginous-siliceous-shale (FSS), and ferruginous-siliceous-carbonate (FSC). The source of the material for ferruginous formations, associated primary volcanogenic and chemogenic-sedimentary formations are magmatic melts of the thermal asthenosphere. The FSV Archean formation occurs in greenstone structures of granite-greenstone areas. The ferruginous rocks are jespers, consisting of a mixture of quartz-magnetite, volcanogenic silicate and carbonate (siderite and breinerite) layers. They occur mainly among amphibolites and komatiites, and obligatory contain ferruginous amphiboles and pyroxenes. Similar melts and fluids form in the presence of oxidized fluids (О2–, СО2, Н2О, SiO2), at high activity of Cl–, which provides miscibility of melts and fluids. Ferruginous-siliceous-shale formation is generated at the boundary of Archean and Paleoproterozoic. Lower and upper parts of productive strata are folded by the iron carbonates, and in the center quartz-magnetite and quartz-hematite interlayers are separated, alternating with each other. Separation into an independent phase of a water-silicate fluid carrying iron chloride complexes indicates an increase in the alkalinity of fluids and melts of the feeding asthenosphere while maintaining a high oxidizing potential. An increase in alkalinity is indicated by the presence of graphite in the accompanying shale formations. FSC formation is territorially connected with mantle fault zones of deep laying. Associates with igneous rocks (serpentinites) and shale formations (high magnesian and high alumina shales with graphite) formed in an alkaline environment (with the participation of carbonate-fluoride-sodium fluids) at a depth of at least 250 km, with a high oxidizing potential. Ferro carbonate rocks, including olivine and hematite, are formed from a carbonate phase separating from similar depths. A directed change in the ferruginous formations’ composition reflects the deep fluids and melts composition evolution and is associated with the self-organization of the Earth’s shells.