THE GRAND CANARY, SAINT-HELENA, AND TRIST AN-DA-CUNHA OCEANIC ISLANDS- VARIATIONS OF TRACE-ELEMENT PARTITION-COEFFICIENTS IN PYROXENE-MELTEQUILIBRIA DURING ALKALINE MAGMA EVOLUTION

Pyroxene phenocrysts in volcanic series from the Grand Canary, Saint H elena, and Tristan da Cunha oceanic islands were studied, and data on the distribution of REE, Rb, Pb, Ba, Th, U, Nb, Sr, Zr, Hf, Sc, and Sn in monomineral separates of the volcanic matrix and pyroxenes were ob tained. Variations of trace element partition coefficients (K) in pyro xene-melt equilibria were studied over the whole range of rocks presen t in the alkaline basalt series of these islands. Two groups of K valu es-one for basalt-ankaramite of the initial stage and the other for ph onolite-trachyte of the final stage-were discriminated. An abrupt decr ease in the K absolute values and a sharp rise in the K-Eu values (K v alue ratios between LREE and HREE remaining unchanged) were found in t he phonolite-trachyte group. It must be stressed that no gradual trans ition exists between the two groups since there are no K values interm ediate between those found in the primary magmas and in the products o f their final stages of differentiation. Such a discreteness shows tha t the abrupt change in the values is caused by a change in the structu re of the melt and not by temperature factors. It is shown that, contr ary to such high-silica melts as dacite, rhyolite, rhyodacite, etc., a decline in the K values in the examined alkaline magmas took place at the final stages of their evolution. This caused a sharp increase in trace element concentrations in the last portions of the melts. Our st ates demonstrate that the partition of elements in the volcanics testi fies to an equilibrium succession in the phenocryst formation during t he initial and intermediate stages of the volcanic system evolution.