Geophysical Journal | 2005 volume 27 1

Kimberlites sourse is the mantle of ordinary composition

© Khazan Ya.M., Fialko Yu.A.

The observations imply that the maximum abundances of rare earth elements (REE) in kimberlites from different provinces of the world do not differ practically. The model developed in the paper explains the high kimberlite REE contents by a melt enrichment in course of its percolation through a solid skeleton. In the limit of full saturation the enrichment spectrum coincides with the REE abundances in the infinitesimal melt and does not depend on the melt fraction. This reconciles extremely low melting degrees (0,1%) that are commonly assumed to explain enormous kimberlite enrichment in incompatible elements and much higher melting degrees (>1%) necessary for the melt segregation. Besides, under the condition of equilibrium distribution of trace elements between modal minerals that seems to be justified by observations, the saturated abundances can depend only weakly, if at all, on the host rock modal composition. Taken together with the independence of the melt fraction this explains the close similarity of the REE spectra in kimberlites from different provinces. While percolating, the melt saturates with heavy REE much faster than with light REE. Therefore the sum of the REE should correlate with the average spectrum slope La/Yb. The existence of this evolutionary trend is supported by the data from the kimberlite provinces of South Africa, Yakutia and India, and from the viewpoint of the present model indicates an undersaturation of many kimberlites with REE. Calculated saturated REE spectra are in reasonable agreement with the observed maximum REE abundances with no additional assumptions of the compositional pecularity of the kimberlite source regions, and undersaturated spectra explain well the observed variability of the light REE abundances. The variations of the heavy REE contents should be due to mantle inhomogeneities.

<<back |