High-Mg carbonatitic microinclusions in some Yakutian diamonds-a new type of diamond-forming fluid

摘要

The composition of microinclusions in 26 fibrous cubic and coated diamonds from the Daldyn-Alakit kimberlite field, Yakutia were studied using EPMA and FOR and the carbon isotopic composition of 4 of the diamonds was studied using SIMS. Fifteen diamonds carry carbonatitic high-density fluids (HDFs) with high MgO content (17-28 wt.%) whereas the MgO content of fluids in other diamonds does not exceed 14 wt%. We propose that the two groups are distinct and evolved separately. The low-Mg suite is similar to previously reported carbonatitic to silicic HDFs from Africa and Brazil. The high-Mg suite is carbonatitic, but its composition is distinct from previously defined end-members. As the MgO content decreases from 28 to 17 wt%, CaO, Na2O, K2O and Cl also decrease while the silica, Al2O3, TiO2 and P2O5 contents remain constant. Additionally, the water band in the FTIR spectra of high-Mg HDFs is wider relative to the water band in the spectra of low-Mg carbonatitic HDFs. By combining EPMA and FTIR data, we have constrained the major element composition of the high-Mg carbonatitic end-member, which comprises 78 wt.% carbonates, 9% silicates, 6% water, 5% apatite and 2% halides. The composition of the high-Mg suite is closer to that of near-solidus melts of fertile carbonate peridotites and harzburgites. Thus, it should be possible to produce the Mg-rich HDF either by incipient melting, or by cooling and crystallization of a proto-kimberlitic melt at depth. However, the peridotitic system alone cannot explain the high alkali and Cl content of the fluids. We suggest that the elevated alkali and Cl content of the high-Mg carbonatites are related to interaction of carbonate peridotite with saline fluids, or of peridotite with chloride-carbonate melts. The major element composition of the high-Mg carbonatitic HDF is dominated by the low-fraction melts of the peridotite while the content of potassium and other incompatible elements is influenced by the contribution of the saline or carbonatitic fluids.