Zirconium isotope considerations of carbonatite genesis and implications for carbonatite metasomatism of the Earth’s mantle

Abstract

We present the first comprehensive survey of stable zirconium (Zr) isotope systematics in carbonatites of various ages from distinctive tectonic settings and with different mineralogy, chemistry, and the presence or absence of significant post-emplacement overprint. All these parameters are plausibly linked with extreme variability in abundances of Zr in carbonatites spanning over five orders of magnitude although Zr typically is considered as an immobile element. No temporal variation in δ^94/90Zr values (per mil deviation from IPGP-Zr reference solution) is observed, indicating a global scale homogeneity of Zr isotope composition of the ultimate mantle sources of carbonatites over time, in accord with homogeneous δ^94/90Zr (per mil deviation from IPGP-Zr solution) value of mantle and crust. Carbonatites devoid of significant post-magmatic mineral assemblages (hydrothermal baryte, goethite, chalcedony and fluorite) define a δ^94/90Zr of +0.31 ± 0.08 ‰, which is ∼0.3 ‰ above the currently accepted mantle value. This difference likely is owing to Zr isotope fractionation during low-degree partial melting at mantle conditions and fractionation of carbonatite melts, although exact isotope fractionation factors between carbonate melt and silicate mantle have not yet been experimentally constrained, and alternative scenarios (diffusion, liquid immiscibility, different coordination of Zr in carbonate and silicate melts) should also be considered. This value also is ∼0.2 ‰ above mean crustal δ^94/90Zr value and contamination by common crustal materials can thus be considered negligible. Most of the δ^94/90Zr variation, in total reaching ∼0.5 ‰, is restricted to carbonatites with clear evidence of post-emplacement processes, resulting in ⁹⁴Zr-depleted signature with δ^94/90Zr as low as −0.10 ‰. This decrease is paralleled by broadly colinear increases in Zr/Hf and Nb/Ta. These observations collectively underscore the enhanced mobility of expectedly immobile Zr and Nb during overprint by F- and OH-rich liquids, paralleled by notable Zr isotope fractionation. Mantle lithologies metasomatized by carbonate-rich liquids may thus potentially acquire a wide range of Zr/Hf and Nb/Ta ratios, and δ^94/90Zr values.