The Petrogenesis of Cr-Poor and Cr-Rich "Megacrysts" in Mantle-Derived, Silica-Undersaturated Magmas

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The purpose of this research is to undertake a detailed geochemical examination of megacrysts suites from the Malaitan alnoite (Solomon Islands), and the Sloan-Nix kimberlite (Colorado). Samples will be gathered from existing collections and initially characterized by EMP analysis. Fractionation trends thus defined will allow critical samples to be selected for IMP analysis. Traverses will be carried out on samples forming end-member compositions to the apparent fractionation trends defined by EMP analyses. This approach will then pin-point samples for isotopic analysis (Sr, Nd, Pb, O), using micro-sampling techniques to investigate potential isotopic zonation within and between megacrysts. This integrated approach is required to: 1) determine the nature of the parental magma; 2) evaluate the mode of crystallization within the mantle regime; 3) determine whether the proposed igneous fractionation occurred in an open- or closed-system environment; 4) establish any link between megacryst crystallization and metasomatic processes; and 5) examine the relationship (if any) between Cr-rich and Cr- poor megacrysts.