Abstract
The article represents an overview and generalization of the microprobe and thermomagnetic data on the distribution, content and composition of metallic iron in Pleistocene lacustrine sediments of the Darhad Basin, northern Mongolia, in the Upper Miocene sediments of Lake Baikal, epicontinental sediments of Miocene-Late Jurassic, and Early Cambrian ages in different regions of North Eurasia and in Miocene-Late Jurassic oceanic sediments of North- Western Atlantic. Iron particles percentage varies from ~10-5 to 0.05%, their distribution is bimodal with the distinct zero mode, and the accumulation of iron particles is inversely linked with the rate of sedimentation. Pyrite and pyrrhotite are widespread in the studied sediments, and the distribution of metallic iron does not depend on the presence of pyrite (i.e., on redox conditions). The variations in the Curie temperature of iron from 700°C up to 780°C. The nickel admixture in native iron forms three groups, 1) pure iron (main part of particles), 2) iron with mode 5-6% Ni (kamacite), and 3) Fe-Ni alloy with mode 50% Ni (rare), were found in all studied sections. The global pattern of the distribution and composition of the iron particles clearly indicates that their origin is associated with interplanetary cosmic dust. At the same time, the particles of Fe-Ni alloy are very rare, and their concentration does not correlate with the content of other iron particles. Most likely, the particles of Fe-Ni alloy are mainly due to impact events. Correlation between the concentrations of iron particles and terrestrial minerals (magnetite) is absent as a rule. Such correlation (r=0.3-0.7) testifies the redeposition of iron particles.
Highlights
Particles of metallic iron are often found in sedimentary rocks, but concentration is normally very low
We have studied the spreading and composition of metallic iron particles in the Pleistocene lacustrine sediments of the Darhad Basin, northern Mongolia, in the Upper Miocene sediments of Lake Baikal, in epicontinental sediments of Miocene, Oligocene, Eocene, Cretaceous, Late Jurassic, and Early Cambrian age in different regions of North Eurasia and in ocean sediments of the NorthWestern Atlantic [4,5,6,7,8,9,10]
According to Thermomagnetic analysis (TMA) and Microprobe analysis (MPA) data in all studied samples there are unevenly distributed throughout sediments: 1) magnetite and cationdeficient magnetite (Tc=575-650°C) is ubiquitous magnetic mineral, and main part of sample magnetization (M20); 2) iron hydroxides, such as goethite (Tc=80-150°C, which disappears on the second heating; its portion in M20 is 500°C, expressed as a “pyrite” peak caused by the oxidation of Fe-sulfides to magnetite [12,13]
Summary
Particles of metallic iron are often found in sedimentary rocks, but concentration is normally very low. The role of redeposition of iron particles is assessed from the correlation between the concentrations of metallic iron and terrestrial magnetite+titanomagnetite in the sediments.
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