High-Mg carbonatitic HDFs, kimberlites and the SCLM

Trace-element patterns of high-Mg carbonatitic high- density fluids (HDFs) trapped in Siberian fibrous diamonds are similar to those of Group I kimberlites, but are slightly more fractionated. The patterns of both are comparable in shape to the average pattern of peridotite xenoliths from the sub-continental lithospheric mantle (SCLM) [1]. Possible scenarios for explaining these similarities include mixing, fractionation and melting: Adding 2.5% of kimberlitic magma or 0.7% of the Siberian high-Mg HDFs to a highly depleted peridotite closely reproduces the SCLM pattern. The formation of the high-Mg HDFs through fractionation of kimberlitic magma calls for 70% crystallization of olivine, pyroxene garnet and carbonate. However, the alkalis and Ba of the calculated fluid are too low and the middle to heavy REE, Zr, Hf, Ti and Y are too high compared to the Siberian high-Mg HDFs. Simple batch melting of 0.5% of a source with average SCLM modal abundance and trace-element composition closely reproduces the trace-element pattern of the Siberian high-Mg HDFs. Higher degrees of melting ( approximately 2%) of the same source yield patterns similar to those of Group I kimberlite. High-Mg HDFs in diamonds from Kankan, Guinea have major-element compositions comparable to that of the Siberian high-Mg carbonatitic HDFs. However, they are depleted in K, Rb, Cs, Nb and Ta and enriched in Ba, Th, U and LREE relative to the Siberian ones. These differences closely correspond to those between the patterns of Group II and Group I kimberlites, respectively. Extending the melting scenario to the Kankan HDFs and Group II kimberlites, the two can be produced by 0.2 and 1% melting of SCLM that carries phlogopite (0.3% and 0.1%, respectively) and a trace of rutile. Whether it is mixing, melting or combination of both, the new constraints indicate a very close genetic relation between high-Mg carbonatitic HDFs, kimberlites and the average SCLM. [1] McDonough (1990) EPSL 101, 1-18.