Thickness of diamond-bearing metasomatic aureoles in the cratonic SCLM

The distribution of subcalcic garnets in the SCLM of the Siberian craton suggests that: a) subcalcic garnets and diamonds are metasomatic phases in deep cratonic SCLM, b) the distribution of both phases in the lithosphere is laterally heterogeneous on relatively small scales and related to ancient structural controls [1]. Thus, the grade of a kimberlite will depend on its intersecting these previously active fluid conduits. However, the most obvious question is – what is the thickness of the metasomatic aureoles related to such ancient mantle conduits/veins? This information has an important application to diamond exploration. Here we assume that the diamond-bearing metasomatic aureole probably consists of harzburgites and dunites and a central part represented by pyroxenites or eclogites, and lherzolites refertilized from dunite/harzburgite by melt-related metasomatism [1; Fig. 5D]. Garnet and chromite xenocrysts from kimberlites have been used to map the vertical distribution of rock types and processes in the Yakutian SCLM [2]. Knowing the depth distribution of different types of peridotite beneath the kimberlite pipes and using our original method we calculated the thickness of the metasomatic aureoles related to ancient mantle conduits/veins in the Yakutian SCLM. Using set with several parameters we obtain a thickness of diamond-bearing metasomatic aureoles varying from ~3 cm to 1.4 meters. Calculated values of the thickness of the diamond-bearing metasomatic aureoles in the deep cratonic SCLM are similar to the thickness of the pyroxenite dikes and adjacent metasomatic aureoles in orogenic peridotitic massifs [3; references therein]. The small calculated thickness of the metasomatic aureoles also is consistent with the close association (often <1km) of both diamond-bearing and barren kimberlite pipes, indicating how easy it is for a kimberlite to miss intersecting the diamond bearing aureoles. [1] Malkovets et al. (2007) Geology 35, 339–342. [2] Griffin et al. (1999) Tectonophysics 310, 1–35. [3] Beyer et al. (2006) Jour. of Petrol. 47, 1611–1636.