MINERAL INCLUSIONS IN DIAMONDS FROM THE RIVER RANCH KIMBERLITE, ZIMBABWE

More than 99% of mineral inclusions in diamonds from the River Ranch p ipe in the Late Archean Limpopo Mobile Belt (Zimbabwe), are phases of harzburgitic paragenesis, namely olivine (Fo(92-93)), orthopyroxene (M g# = 93), G10 garnets and chromites. The diamond inclusion (DI) chemis try demonstrates a limited overlap with River Ranch kimberlite macrocr ysts: the DI garnets are more Ca-undersaturated, and DI spinel and gar net are more Mg-rich. Most River Ranch diamond inclusions were equilib rated at T = 1080-1320 degrees C, P = 47-61 kbar, and f(O2) between IW and WM buffers. The P/T profile beneath the Limpopo Mobile Belt (LMB) is consistent with a paleo-heat flow of 41-42 mW/m(2), similar to cal culations for Roberts Victor. but hotter than for the Finsch, Kimberle y, Koffiefontein and Premier Mines. This is ascribed to the younger te ctonothermal age of the LMB and its proximity to Late Archean oceans. Like diamond inclusions from all other kimberlites studied, the River Ranch DI have a lithospheric affinity and therefore indicate that an a ncient, chemically depleted, thick (at least 200 km) mantle root exist ed beneath the Limpopo Mobile Belt 530-540 Ma ago. The mantle root mig ht have developed beneath the continental Central Zone of the LMB as e arly as the Archean, and could be alien to the overthrust allochthonou s sheet of the Limpopo Belt. Oxygen fugacity estimates for diamond inc lusions at River Ranch are similar to other diamondiferous harzburgite s beneath the Kaapvaal craton, indicating that the Kaapvaal mantle as a whale was well buffered and homogeneous with respect to f(O2) at the time of peridotitic diamond crystallization.