Review of the liquid line of descent in iron rich basalts

Abstract

This research project aims to highlight and identify the main assumptions made, and the methods implemented during the construction of the liquid line of descent (LLD) for iron-rich basalts. The project investigates the potential influence of these assumptions on the resultant LLD. The most important factors involved in the construction of a LLD, namely oxygen fugacity (ƒO2) and H2O are examined and presented here in some detail. Alternative methods for analysing the evolution of these iron-rich rocks are discussed and critically assessed. In order to investigate the assumptions and methods for the construction of the LLD, two case studies were performed on iron-rich intrusions, namely: the Birds River Complex in the Eastern Cape, South Africa; and the Skaergaard layered intrusion, Greenland. For Skaergaard, four of the main LLDs proposed for its liquid evolution were chosen and critically assessed. The Birds River Complex is a small and relatively unknown intrusion, therefore it has received significantly less attention, but presents a good example of a local iron-rich intrusion. These two intrusions were used as testing grounds for the application of the thermodynamic modelling software AlphaMELTS to investigate the potential influence of ƒO2 and H2O on crystallising melts. It was found that H2O had the most significant influence on the maximum iron-enrichment possible for a melt, and that maximum iron-enrichment is attained under anhydrous conditions for all the starting compositions investigated. The effect of ƒO2 was significant, however, its influence varied between starting compositions.