THE DISTRIBUTION OF ZR, HF, NB, TA, TH, U IN MINERAL-MELT EQUILIBRIA (REVIEW)

The destribution of Zr, Hf, Ta, Th, U in mineral - melt equilibria is considered on the base of 200 equilibria in natural volcanic samples a nd 38 experimental studies. Distribution coefficients of rare lithophy le elements in rockforming and accessory mineral - melt equilibria are estimated and the recommended values are presented for numerical geoc hemical models. Crystallisation conditions of rare element bearing pha ses (zircon, rutile, monazite etc.) are systematically ascribed. It is shown that substantial variations of nonpaired element ratios (Zr/Nb, Ta/Th etc.) are peculiar to the crystal fractionation process whereas the geochemical pairs (Zr/Hf, Nb/Ta) are variable in the crystallisat ion of the accessory mineral phases. In the crystal fractionation of n atural magmatic melts the combined distribution coeffecient values for Zr, Nf, Nb, Ta are not below O.n with the probable increase of the la tter value up to > 1 at the final stages of rock formation.