DISTRIBUTION OF RARE LITHOPHILE ELEMENTS (ZR, HF, NB, TA, TH, U) AND THEIR RATIO VARIATIONS IN THE FRACTIONATION PROCESS OF ALKALINE-BASALTIC OCEANIC ICELAND SERIES OF THE SOUTHERN ATLANTIC (GRAN-CANARIA, ST-HELEN, TRISTAN-DA-CUNHA)

The geochemistry of rare lithophyle elements in volcanic series of oce anic icelands is studied in concern with the problems of the heterogen eity of mantle oceanic sources. Substantial variations of Zr/Hf and Nb /Ta ratios coupled with constant Th/U values are reported. The variati ons of Zr/Hf are not related to the differentiation extent or some cer tain magmatic source of corresponding rocks. Two maxima: normal (35-45 ) and high (60-80) are observed at the frequency curve. Similar distri bution is characteristic of volcanics of other volcanic icelands. Dist ribution coefficients of lithophyle elements are calculated for main m inerals of studied volcanics. It is found that Zr distribution coeffic ient in clinopyroxene - melt system is below that of hafnium. The esti mated values of Nb and Ta distribution coefficients in titanomagnetite - melt system is much lower than the former of low alkaline volcanite s. Rayleigh equation was used for the description of rare element frac tionation in volcanic series and in crystal fractionation. The hypothe sis of ''portion'' fractionation is advanced for the interpretation of lithophyle element geochemical pecularities and deviation of natural trends in relation to calculated ones.