Deciphering kimberlite-field structure using ilmenite composition: example of Daldyn field (Yakutia)

摘要

The spatial distribution patterns of Mg-bearing ilmenite (Ilm) composition were studied on 54 kimberlite bodies of the Daldyn field in the Yakutian kimberlite province. The representativity of the ilmenites sampled in this study is ensured by analysing ca. 100 grains from each kimberlite body. The major conclusions are as follows: (1) ilmenites from neighbouring pipes within the same linear cluster have similar average compositions and compositional fields on the MgO-Cr2O3 plots; (2) ilmenites from different clusters of pipes show different average compositions and compositional fields on the MgO-Cr2O3 plots. (3) regardless of belonging to different clusters, low-Mg Ilm across the whole Daldyn field is characterized by a direct correlation between Al2O3 and MgO; (4) significant changes of MgO content are observed in high-Mg Ilm, while Al2O3 content remains at the same level. The similarity of Ilm compositions across the kimberlite field, as shown by the MgO-Al2O3 plots, is due to a common asthenospheric source. The similar Ilm compositions in different bodies within cluster of pipes is accounted for by a single supply of magma via a lithospheric mantle channel for all pipes of the cluster. The composition of the kimberlite melts can be altered owing to the incorporation and assimilation of lithospheric mantle rocks rich in Mg and Cr. These changes of the melt cause corresponding changes in the Ilm macrocryst composition, both during and after crystallization of Ilm. Thus, the Ilm macrocryst composition follows a trend from low-Mg/low-Cr for Ilm crystallizing in the asthenosphere, to high-Mg/high-Cr at higher levels in the lithosphere. The key conclusion of this study is that Ilm can be used to decipher the structure of kimberlite fields. This can provide a reliable geological criterion for grouping an association of pipes together in clusters, which were previously identified only through subjective considerations of the spatial proximity of kimberlite bodies.