In situ U–Pb and Nd–Hf–(Sr) isotopic investigations of zirconolite and calzirtite

Abstract

Although the calcium- and titanium-bearing zirconsilicates, zirconolite and calzirtite, are common minerals in carbonatites little is known regarding their trace element or isotopic characteristics. Available data indicate that they can have significant contents of U, Pb, Nb, Ta, Zr, Hf, rare earth elements (REEs), minor Sr and low Rb/Sr and Lu/Hf ratios. Their compositions indicate that they have potential for U–Pb age determination together with Nd–Hf and possibly Sr isotopic analyses. In this study, zirconolite and calzirtite from carbonatites occurring at Phalaborwa (South Africa), Prairie Lake (Canada), Afrikanda and Kovdor (Kola, Russia), Guli (Siberia, Russia) and Jacupiranga (Brazil) were selected for an isotopic analysis by in situ techniques. Ion microprobe analyses using a CAMECA 1280 ion microprobe show that reliable Pb–Pb ages can be obtained from these minerals as they have extremely high U concentrations, coupled with negligible common Pb. Although these minerals have low Rb/Sr ratios, their low Sr concentrations, in most cases, render in situ Sr isotopic analyses difficult. In contrast, their high Sm, Nd and Hf concentrations, with generally low Lu/Hf ratios, permit precise in situ isotopic analysis of Nd and Hf.