High-density fluids in diamonds from No. 50 kimberlite pipe, Wafangdian, China: the connection to kimberlites

Abstract We report the major and trace element composition of high-density fluids (HDFs) trapped in microinclusions in 14 diamonds from Wafangdian, Liaoning, China. This is the first detailed report of the major and trace elements of HDFs from the North China Craton. The trapped fluids are similar to those known from other localities, except for one with a unique Cl-K-Ba-Sr-rich composition that is also extremely enriched in Th and the light rare earth elements (REEs). One diamond contains a saline HDF of typical composition. The other twelve diamonds exhibit high-Mg carbonatitic HDFs with trace element patterns that resemble those of the Wafangdian kimberlites, but with higher concentrations. The nitrogen aggregation level of all the diamonds is similar, with 6–37% of the nitrogen residing in B centers, suggesting mantle residence temperatures of 1100–1200 ºC. Comparing the composition of the carbonatitic HDFs to available experimental data on melting of carbonated peridotite, we show that they can be produced by a very low degree of partial melting (< 0.1%) of mantle peridotite. In an oxidized environment, such melts are stable in most of the lithospheric mantle and the main barrier for their existence is the reduced nature of the lithosphere. With increasing temperature, the experimental melts evolve towards kimberlites. The similar trace element patterns of the HDFs and their host kimberlite also suggest derivation from similar sources. Still, the difference in the formation temperatures means that the HDFs were trapped in the lithosphere, whereas kimberlite formation requires asthenospheric conditions. Crystallization of kimberlite at lithospheric levels face difficulties, but forming high-Mg carbonatitic melt at the top of the asthenosphere and its trapping in diamond after minimal crystallization in the lithosphere may solve them. More complex scenarios may be needed to explain the formation of both HDFs and kimberlites.