Stable zirconium isotope constraints on the petrogenesis of carbonatites

Refractory as well as insoluble nature of zirconium (Zr) dictates that its behavior mostly is driven by magmatic fractionation whereby Zr-rich phases, such as zircon or baddeleyite, and to a lesser amount pyroxene, can control stable Zr isotope systematics of silicate systems. In carbonatites, HFSE often are carried by pyrochlore, garnet and/or pyroxene, and scavenging of these phases during magmatic evolution of carbonatite liquids may result in significant depletions, particularly apparent for Ti, Zr and Hf. Therefore, fractional crystallization of HFSE-bearing phases may bear on the understanding of the role of carbonatites for HFSE distribution in the mantle.

We have analyzed Zr stable isotope systematics (δ^94/90Zr_IPGP-Zr) of several carbonatites from various geotectonic positions to further constrain their petrogenesis. The preliminary data shows ~0.4‰ variation which is not easily related to major element chemistry of carbonatites, nor emplacement age. Samples of unmodified carbonatites from continental rifts and hot-spots plot above the mantle value (δ^94/90Zr = 0.04 ± 0.04‰), with δ^94/90Zr of up to ~0.35‰ whereas carbonatites from shear zones display resolvedly lower δ^94/90Zr. Carbonatites overprinted by F-rich fluids carry distinctly low δ^94/90Zr, associated with high Nb/Ta, suggesting high mobility of HFSE in F-rich fluids. In contrast, δ^94/90Zr of carbonatites carrying sulfide mineralization does not deviate from that of unmodified carbonatites. These cumulative observations indicate stable Zr isotope fractionation between silicate and carbonate melts in the mantle. Besides, they also indicate strong mineralogical control of HFSE-bearing phases on the stable Zr isotope systematics of carbonatites.