Zr/Hf IN CARBONATITES AND ALKALINE ROCKS: NEW DATA AND A RE-EVALUATION

Abstract

New analyses for Zr and Hf in carbonatites and alkaline silicate rocks are presented together with corresponding data compiled from the literature. Zr/Hf ratios in these rocks cover a range of about one order of magnitude, whereas Zr concentrations vary over four orders of magnitude. Most samples display chondritic to superchondritic Zr/ Hf values. Subchondritic Zr/Hf seems to be analytical artifacts because particularly Hf and sometimes also Zr contents are definitely below detection limits of the applied analytical methods. The uncoupled trends of Zr/Hf vs. both Zr contents and Y/Ho ratios and the fact that carbonatites and associated alkaline silicate rocks do not show markedly different Zr/Hf ratios suggest that carbonatites inherited this signature from their mantle source. The present paper aims to discuss and re-evaluate the common belief that carbonatites have typically superchondritic Zr/Hf, and that this is a clue to interpret some phenomena of mantle metasomatism.