The skaergaard layered series. Part VI. Excluded trace elements

Abstract

In contrast to the smooth trends of major elements and mineral compositions, the excluded trace elements in the Skaergaard Layered Series have an irregular distribution that does not conform to the normal trends of Rayleigh-type fractionation. Their concentrations are about constant or even decline through the Lower and Middle Zones before increasing sharply to reach maximum concentrations 100-200 m above the Sandwich Horizon. As in the case of included elements, the relative concetrations of excluded elements in coexisting phases deviate widely from those predicted by experimentally determined partition coefficients under presumed magmatic conditions. This is seen most clearly in the immiscible melanogranophyres and conjugate ferrogabbros. Although the major elements conform to the experimentally determined relations for immiscible liquids, the trace elements do not; they follow a totally independent trend. The abrupt increase in the concentrations of excluded elements in the upper part of the intrusion could plausibly be attributed to an addition of new magma or to a density inversion that resulted in upward migration of a late liquid or fluid, but these possibilities are inconsistent with the compositional and spatial relations of the upper parts of the intrusion. Although a late residual liquid certainly migrated upward, the most likely explanation for the observed distribution of excluded elements is that the partition coefficients were altered by volatile components, which gradually increased during the early stages of crystallization then began to exsolve near the top of the Middle Zone.