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significant sources of these elements is mantle rare metals alkaline granites,
carbonatites, miaskites, agpaite nepheline syenites and metasomatites.
Metamorphic, placers deposits and, especially, exogenic sources, such as
mineralized natural waters and salt-water have an important value for
industrial production of many rare elements.
As judged by the discovered reserves of these metals (Solodov, 1980),
three basic metallogenic epochs of rare metal are being emphasized:
Sumian - on the boundary of Archean and Proterozoic (2,6-2,4 Ga); than
Grenville - in medial Riphean (1,2-1,1 Ga); and Kimmerian - in Mesozoic
(230-140 Ma) (Table 9, Figure 5). On the continents the early Proterozoic
rare elements deposits are located on Archean shields; middle Riphean -
within the limits of ancient platforms, and Mesozoic – on the Precambrian
middle missives, young platforms and less in geosynclinals belts. It is
necessary to note, that rare metal deposits more often meet on Laurasian
continents in comparison with Gondwanian, approximately in the
proportion of 3:1. This fact testifies to the irregular spatial dislocation of
these rare metals
on the planet, as well as uranium and thorium.
Меtallogenic epochs of rare metal are precisely dated in time and in
space to the periods of tectono-magmatic activity after a long period of
tectonic rest on the continents. The close geochemical
connection of the rare
lithophile elements and alkalines, especially potassium, testifies to the
unified deep source. On the opinion of V.N.Larin (1980), the juvenile
source of lithophile rare and alkaline metals, it is necessary to consider the
pyrolite layer of the mantle. Extraction of rare metals from pyrolite is
carried out by deep intertelluric fluid, the occurrence and character of which
is determined by degassing of hydrogen from the core of the Earth. The
increasing amount of rare metal in the areas of tectono-magmatic activity is
caused by accumulation in upper mantle the layer of pyrolite in connection
with the expansion of the Earth, appropriated fall of gravity, reduction of
gradient of pressure in mantle and transformation spinel-garnet in olivine-
pyroxene (pyrolite). It is accompanied by “lattice downthrou” of
isomorphous impurity of lithophile elements. At the establishing of
intertelluric fluidic flow these elements are exposed by "flotation " and are
involved in processes of formation rare metal ores in the upper parts of
terrestrial crust. Thereby, the consideration of evolution terrestrial crust and
upper mantle of the continents on the basis of the hypothesis of the
primordial hydridic Earth, allows us to come closer to the understanding of
the internal reasons determining legitimacies of the
distribution of lithophile
rare metals in time and in space.
5. 4. O c e a n i c o r e g e n e s i s. The lamination of external
geosphere of the Earth on the crust, pirolite and gipolite with rather contrast
distribution in them potassium, rubidium and, accordingly, radiogenic
strontium caused in the past time the contrast between the geochemistry
character of the allocation these elements in the
zone of sedimentation at the
depending from process of ocean formation, which resulted to consequent
disclosure of more and more deeper horizons of the planet. The carbonates
73
of calcium fix the isotope composition of strontium in the water, from
which they are fallen out, that allows to consider the evolution of the ratio
87
Sr/
86
Sr in the ocean’s water of the Earth.
The curve, describing the evolution of isotope composition of strontium
in the oceans on the time, has appeared rather singularity (Figure 6). Three
cardinal perturbations are clearly prominent on it, which, obviously, reflects
essential changes of the image of the Earth. The data show exponential
increasing of the relation
87
Sr/
86
Sr in waters, circumfluent of the planet
during the period from 3,0 up to 1,1 Ga years back. The extrapolation of
this curve untill now has resulted to the
modern value of the isotopic
relation of strontium in continental crust. However, in late Precambrian,
approximately on the boundary of middle and upper Riphean, this relation
sharply deviates from the exponential dependence (perturbation А) that
testifies to the appearance of a source with the low isotope attitude of
strontium exposed on extensive territory. According to V.N.Larin (1980), it
is, most probably, connected with the beginning of active ocean formation,
during which low potassium has emanated the tholeiite basalts in huge
amounts, what accompanied the formation of oceanic depressions. It is
possible to connect the sharp failure of the curve (perturbation В), coming
on the end of Paleozoic and beginning of Mesozoic, with the acceleration of
processes of ocean formation in Mesozoic era, when there was disclosure
the Atlantic and Indian young oceanic depressions alongside with
proceeding expansion of the existing Pacific Ocean.
The acceleration of the ocean formation, connected with expansion of
the Earth, by all means should cause the strain, and at the end the break of
mantle layer restite in middle part of the ocean couch, accompanied by rise
and output on the surface the again formed silicate mattress of the layer B,
arising by the way of metasomatism of the intermetalic layer C of the upper
mantle. The content of Rb in intermetalic connections of the layer C should
indicate initial concentration of this element on the planet and, hence,
should not be less, than in gipolite. In this connection, the sharp increase of
the relation
87
Sr/
86
Sr in oceanic carbonate sediments (perturbation С) is
explained. At the pressure of level 10 GPа and above, the formed silicate
mattress should be represented by spinel-garnet mineral association,
whereas at reduction of pressure will prevail ever more pyroxene-olivine
paragenesis. As far as the isomorphic capacity of lattices olivine and
pyroxene is much lower, than at garnet and spinel (in the attitude of
potassium, uranium and others lithophile elements), that the change of
mineral paragenesis, formed by metasomatic way in the bowels of mid
oceanic ridges, necessarily should be accompanied by increasing gab of
these elements. Just here it is possible that huge source which was caused
the late Jurassic perturbation C in geochemistry of potassium and strontium
in the zone of sedimentation at the bottom of oceans of the Earth (Larin,
1980).
Deep-water red clays of pelagian parts of the oceans along with the high
contents of ores elements have the sharply increased concentration of