Peridotitic diamonds from Namibia:: constraints on the composition and evolution of their mantle source

About half the diamonds studied from the Cenozoic placer deposits along the Namibian coast belong to the peridotitic suite. The peridotitic mantle source is heterogeneous ranging from lherzolitic to strongly Ca depleted (down to 0.24 wt.% CaO in garnet) and shows large variations in Cr/Al ratio, illustrated by very low to very high Cr2O3 contents in garnet (2.6-17.3 wt.%). The Cr-rich end of this range includes exceptionally high Cr2O3 contents in Mg-chromite (70.7 wt.%) and clinopyroxene (3.6 wt.%). Garnet-olivine thermometry appears to indicate two groups, one that equilibrated at temperatures between 1200 and 1220 degreesC and a second between 960 and 1100 degreesC. Combined estimates of pressure and temperature based on garnet-orthopyroxene pairs indicate a large variance in geothermal gradients, corresponding to 38-42 mW/m(2) surface heat flow. The trace-element composition of peridotitic garnet inclusions (determined by SIMS) also indicates large diversity. Two principal groups, corresponding to different styles of metasomatic source enrichment, are recognized. The first group ranges from extremely LREEN-depleted patterns, through trough-shaped REEN to sinusoidal patterns with the position of the first peak gradually moving from the LREEN to the MREEN. This series of REE patterns is interpreted to reflect a range of metasomatic agents with decreasing LREE/HREE. Only in the case of the two garnets with REEN peaking at Sm-Eu is this process connected with enrichment in Zr, without significant introduction of Y and Ti. The metasomatism responsible is interpreted as reflecting percolation of CHO-fluids through harzburgite under sub-solidus conditions. A second group of garnets shows an increase from LREEN-MREEN and almost flat (lherzolitic garnet) to moderately declining MREEN-HREEN at super-chondritic levels. This second style of metasomatism is caused by an agent carrying HFSE and showing only moderate enrichment in LREE over HREE, which points towards silicate melts. (C) 2004 Elsevier B.V. All rights reserved.