Primary melting sequence of a deep (> 250 km) lithospheric mantle as recorded in the geochemistry of kimberlite–carbonatite assemblages, Snap Lake dyke system, Canada

Abstract

Geochemistry of kimberlites and associated carbonatites from the Snap Lake dyke system, Slave craton, Canada, shows that kimberlites are similar, but not identical to Group I kimberlites from South Africa and Siberia. Snap Lake kimberlites are enriched in most incompatible elements such as U, Th, Nb and LREE (up to 1400 times chondritic abundances), and are weakly differentiated relative to primitive mantle in their Sr and Nd isotope compositions. The Snap Lake carbonatites and kimberlites are enriched or depleted in certain trace elements relative to each other. Carbonatites are depleted in Cs, Rb, K, Ta and Ti but enriched in U, Sr, P, Zr, Hf, middle and heavy REEs relative to kimberlites. The observed element distributions are not consistent with experimental data on distribution of trace elements between immiscible carbonate and silicate liquids. However, the chemistry of these rocks shows a continuous systematic change from carbonatites to kimberlites, suggesting their genetic relationship. The range of major element concentrations from low SiO 2 (3 wt.%) magmas to typical kimberlites are in excellent agreement with experimental data for melting of carbonated lherzolite at high pressure. Carbonatites have significantly superchondritic Nb/Ta (22–81) and Zr/Hf (57–128) ratios, which systematically decrease with increase of SiO 2. Modeling of partial melting requires that the source for Snap Lake dyke rocks was enriched in incompatible elements but depleted in HREE relative to the primitive mantle. The most appropriate source rocks are metasomatically enriched peridotites of the deep lithospheric mantle roots. In the melting sequence of this source, the incipient magma was carbonatitic in composition but subsequently became kimberlitic as the degree of partial melting increased to 1%. Rocks of the Snap Lake dyke represent a natural example of primary melting process within a deep (> 250 km) lithospheric mantle.