Geochemical studies on cumulus phases of some layered igneous rocks

Abstract

This thesis presents the results of an investigation into the non-equilibrium distribution of trace constituents as a function of crystal growth. The ultrabasic igneous complex of Rhum (Scotland) with its relative uniformity of mineral composition, but variety of mineral habits was used in the investigation. For this, the distributions of elements of contrasting behaviour have been studied, and especially uranium which strongly partitions into the melt, and strontium, which partitions into the feldspar on fractional crystallisation of a basaltic magma. The technique of fission track analysis for uranium is fully described, and further developments have been introduced in order to accurately map the inter- and intra-mineral uranium distributions in polished rock sections. Results of uranium, strontium, nickel and calcium concentrations in the cumulus phases olivine, clinopyroxene and plagioclase feldspar are given. There is a good correlation between the partition coefficients of uranium and the morphologies of the major cumulus phases. Calculations of crystal growth rates for different olivine morphologies have been determined. On these observations, a model is proposed relating both the mode and variation in nucleation rates of the individual morphologies to the degree of supercooling of the melt. Data on other elements are presented to substantiate these arguments. It is shown that olivines of spinifex (harristic) type have grown at a relatively fast rate and not slowly as suggested by some workers. The mode of crystallisation of certain morphologies, as to whether by diffusion-controlled growth, or by interface-controlled growth is discussed. The distributions of uranium and other geochemically compatible elements in the intercumulus minerals are described, and the results of reaction of the cumulus with the intercumulus material studied. Zirconolite (or zirkelite) is recorded in a terrestrial basic intrusion for the first time.