Interaction of mantle convection with lithosphere and kimberlite origin
© Aryasova O.V., Khazan Ya.M.
Thermobarometry of xenolites/xenocrysts from 39 kimberlites of Eurasia, Africa, and North America demonstrates that the
deepest xenolites, as well as all the other, are exhumed from the depleted lithospahere implying in agreement with petrologic and
geochemical data that the kimberlite sources are situated within the limits of the depleted lithosphere. It follows from the thermobarometric
data that the deepest xenolite depth decreases with increasing reference heat flow (RHF) characterizing a lithospheric geotherm. Conditions
controlling formation of a rheological sublayer at the lithosphere-astenosphere boundary have been studied. In agreement with observations,
a geodynamic model implies that the mantle convection reaches the base of depleted lithosphere of a "cratonic" thickness (≥130-150 km)
during the post-Archean. The rheological sublayer partially penetrates the lithosphere. As a result, a layer of strongly sheared rocks form,
wherefrom deepest and severely deformed xenoliths with fluidal microstructure similar to that of crustal mylonites seem to be entrained
by kimberlites. The relation between the depleted lithosphere thickness and the reference heat flow predicted by the model agrees well
with the thermobarometry data. In whole, the present results support the model of the kimberlite genesis assuming that the kimberlite
magmatism originates from an interaction of the plume "head" with lithosphere. While this "head" propagates laterally, it follows the relief
of the lithosphere base, metasomatizes lithosphere, and initiates melting with the melt composition depending on pressure at the lithosphere
base. Particularly, under the conditions of the cratonic lithosphere base the melts have kimberlitic composition.