The Generation of Kimberlites, Lamproites, and their Source Rocks

Measurements of rare earth element (REE) concentrations in SouthAfrican kimberlites and in the Argyle lamproite from WesternAustralia constrain the composition of the source rocks fromwhich these melts originate. To account for the concentrationsof Tm, Yb, and Lu in these magmas, their sources must firsthave been strongly depleted by 20% melting in the garnet stabilityfield, and then enriched by a metasomatic melt rich in lightREE and other incompatible elements. The calculated source compositionsclosely resemble those of coarse, low-temperature, depletedperidotite nodules that are the commonest nodules in kimberlites.Rarer nodule types have undergone either more or less depletionthan have the source regions of kimberlites and lamproites.The REE composition of the metasomatic melt calculated fromthe diopsides and garnets in the sheared nodules, from the diopsidemegacrysts, and from majorite garnet inclusions in diamonds,is in excellent agreement with that expected for a melt producedby melting 0.5% of the source region of ocean ridge basalts.The initial depletion event requires the extraction of 20% meltfrom a region in which garnet and chrome-spinel were stable.The melt distribution obtained from inversion of komatiite compositionssatisfies both these conditions. Kimberlite source rocks areshallower than the layer from which fertile nodules originate.Such nodules must therefore be transported by entrainment ofthe lower boundary of the layer that becomes unstable. Thisproposal can account for their strong fabric.