The North Atlantic craton in West Greenland and northern Labrador has been subjected to deep volatile-rich melting events between ca. 610 and 550 Ma that produced compositionally diverse diamond-bearing kimberlite and aillikite magmas. Whereas kimberlite dyke intrusions appear to be restricted to the Maniitsoq area in the craton interior between 568 and 553 Ma, aillikite/carbonatite intrusives preferentially occur at Paleoproterozoic mobile belts such as in the Sarfartoq area (605–550 Ma) of West Greenland.
Although there is an overlap between the major and trace element compositions of the exceptionally fresh Maniitsoq kimberlites and Sarfartoq aillikites, the latter typically show higher TiO2, Al2O3, and K2O, as well as higher Zr, Hf, Cs and Rb contents. Furthermore, the Sarfartoq aillikites are displaced toward lower εHf by ~ 3 epsilon units at similar εNd compared with the isotopically depleted Maniitsoq kimberlites and their garnet and ilmenite megacrysts. The generally lower εHf of aillikites corresponds to lower CO2/K2O and points to the involvement of a K-rich melt component in aillikite genesis, most likely derived from a cratonic metasome. In contrast, the Maniitsoq kimberlite compositions, in particular the high CO2 as well as low Al2O3 and K2O contents, resemble published carbonate-rich melt compositions that were produced experimentally from carbonated peridotite in excess of 6 GPa, i.e., under sublithospheric conditions. By utilizing published high-pressure carbonated peridotite/melt trace element partition coefficients, we demonstrate that many of the hallmark geochemical features of kimberlites, such as relative Zr–Hf depletions, can be produced by low-degree partial melting of carbonated fertile peridotite within the asthenosphere.
For the Greenland-Labrador Diamond Province, we propose that a common asthenosphere-derived carbonated silicate melt component must have been present throughout the North Atlantic craton base at 610-to-550 Ma. This widespread carbonate-rich melt component variably interacted with old phlogopite-bearing cratonic metasomes, giving rise to diverse suites of aillikites, i.e., hybrid carbonated potassic-silicate magmas, that locally separated out carbonate fractions to form intrusive carbonatites at crustal levels. The kimberlites, however, appear to be mixtures of this asthenosphere-derived carbonate-rich melt component and entrainment of materials from the refractory cratonic mantle lithosphere, with little or no involvement of readily fusible phlogopite-rich metasomes. The model developed herein for West Greenland highlights the importance of cratonic mantle lithosphere in exerting a major control on worldwide kimberlitic magma compositions. Moreover, the ability to examine kimberlite magma compositional variability in time and space clearly shows that decoupled Nd–Hf isotope systematics cannot be taken unconditionally as a reliable fingerprint of ultra-deep mantle processes, because this type of signal can also be imparted to kimberlitic melts by interaction with cratonic metasomes.
- Programområde 4: Mineralske råstoffer