Fluid+melt intrusions in lechatelierite from the Popigai suevites: A product of dynamic interaction melts and fluids during the shock melting of the target gneiss |
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| Authors: | S A Vishnevsky N A Gibsher N A Pal’chik |
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| Institution: | 1.Institute of Geology and Mineralogy, Siberian Branch,Russian Academy of Sciences,Novosibirsk,Russia |
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| Abstract: | The paper presents data on lechatelierite form suevites of the Daldyn Formation in the Popigai astrobleme. Some of the lechatelierite
samples show a complicated structure and contain block of diaplectic quartz glass and dynamic “intrusions” of glasses of types
I, II, and III. The glasses of types I and II abound in fluid inclusions and display evidence of partial homogenization with
lechatelierite. The glasses of type III are clearly separated from all other glasses but show evidence of dynamic interaction
with them in the molten state. Fluid inclusions in the glasses of types I and II are syngenetic but have notably different
densities from those of completely liquid or gaseous inclusions at 20°C. As is indicated by cryometric data, the liquid phase
of the inclusions is aqueous solution of low salinity (5–8 wt % NaClequiv). The bulk petrochemistry of the glasses of type I characterizes them as highly silicic (96.04 wt % SiO2 on average), with elevated K and Na concentrations (Na2O + K2O = 0.72 wt % on average), with 0.73 wt % Al2O3 (on average) and analytical totals 1.97 wt % less than 100%. The glasses of type II are also rich in SiO2 (91.51 wt % SiO2 on average) but contain a broader spectrum of concentrations of major oxides (totaling 5.53 wt % on average) and deficient
analytical totals (by 2.96 wt % on average). The glasses of type III are completely equal to impactites produced by melting
gneisses of the Popigai astrobleme. The glasses of type I are interpreted to be the intrusion products of the “early” highly
mobile and H2O-rich fluid+melt mixtures, whose protolithic material was K-Na feldspars of the target rocks. The derivation of these melts
was associated with the capturing of much silica and water at a highly mobile behavior of K and Na and an inert behavior of
Al. The glasses of type II were produced by the extensive mixing of silica and water at the limited involvement of apogneiss
melts, and these glasses are sometimes deficient in Al. The glasses of type III are usual mixed apogneiss melts. Excess silica
in the glasses of types I and II and their richness in water and deficiency in Al suggest impact anatexis and the selective
separation of components during their derivation; the parental fluid-melt mixtures of these glasses were derived from such
“hydrous” varieties of the target gneisses as diaphthorized and fractured rocks. The evolution and partial vitrification of
lechatelierite and the glasses of types I and II proceeded under residual shock pressures, as follows from data on the dense
(from ∼0.5 to 1 g/cm3) aqueous inclusions in these glasses, which suggest that the inclusions were captured in the glasses under pressures from
∼0.8 to 3.3 GPa. It follows that our lechatelierite samples have a complex multistage genesis, and their quenching facilitated
the preservation of “intrusions” of various stages of shock melting, including the products of the “early” impact anatexis
of the gneisses with the selective separation of components at the active participation of water. |
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