LIQUID IMMISCIBILITY IS A POSSIBLE REASON FOR THE ORIGIN OF THE ANCIENT CONTINENTAL CRUST

Origination of the continental crust is an important episode in the Earth’s history. But there is no clear conception for its origin. The most popular hypothesis on the appearance of acid and intermediate melts is a partial melting of ultrabasic material of primitive mantle. It can be due to a very small extent of partial melting of mantle material with following extraction of the melt from a huge volume of rock. Both reason and mechanism of this process isn’t clear. The fractional crystallisation of magmatic ocean is another way for acid melt origin. But the problem of extraction and segregation of last drops of liquid is remaining undecided anyway.

There are facts and conceptions sustaining that the Earth and planets were molten substantially or completely at the initial stage of its development. They are glasses and features of the melt crystallisation in meteorites, conception of magmatic ocean, and experimental evidence on iron core origin as a result of liquid separation (Marakushev, Besmen, 1983). Therefore the evolution of the Earth can be considered as evolution of completely molten body, and the formation of the Earth’s structure can be resulted from crystallisation and development of a primary melt.

Compositional changes of a primary melt after iron core separation is an important problem. If remove any part of iron from the chondritic melt, the residual silicate liquid will fall into the stable liquid immiscibility field. The compositions of two coexisting silicate liquids will pointed forward the pyroxenite and potassium granite in the pseudoternary diagram (Naslund, 1983). Gradual compositional change of the low-density liquid can lead to the origin of tonalite–trondhjemite–granite rock series. Appearance of this liquid at the early stage of Earth evolution can explain the origination of continental crust and the sequence of magmatic processes in the early history of the Earth.

This research is supported by RFBR grant 01-05-65060.