Diamondiferous subcontinental lithospheric mantle of the northeastern Siberian Craton: Evidence from mineral inclusions in alluvial diamonds

Headless placer diamond deposits occur in Cenozoic alluvium within the Paleoproterozoic Khapchan fold belt of the Olenek province in the northeastern part of the Siberian Platform, yet the known kimberlite pipes from this region are low-grade or non-diamondiferous. To characterise the subcontinental diamondiferous lithospheric mantle of this region, the mineral inclusions in 265 diamonds were exposed and analysed. Inclusions of the eclogite suite are predominant (>72%): garnet, omphacitic clinopyroxene, coesite, K-feldspar, rutile and corundum. The garnet and Cpx inclusions are within the range of eclogitic inclusions worldwide. Diamonds of the peridotitic contain olivine, Cr-pyrope garnet, ortopyroxene and chromite. The olivines have Fo contents between 89.7 and 93.8 mol. % (average - 92.4). Majoritic garnets of both peridotitic and eclogitic parageneses were identified in four diamonds. Most eclogitic diamonds display positive Eu anomalies. High-Ca garnets are LREE-depleted, show strong positive Eu (up to 4.25) and Sr anomalies and have HREE contents lower than those of the lowand intermediate-Ca garnets. The presence of majorite inclusions indicates that some of the diamonds may have sublithospheric origins. Although diamonds with peridotitic-suite garnets constitute a minor percentage of the volume, the composition of these diamonds indicates a depleted composition for the peridotitic mantle. Diamonds of harzburgitic paragenesis compose 57% of the peridotitic suite, which is close to the world average. For an assumed pressure of 5 GPa, eclogitic garnet and clinopyroxene gave temperatures in the range of 1028-1290 degrees C. The composition of mineral inclusions of the peridotitic suite and the equilibrium temperatures of the eclogitic suite suggest that the lithospheric mantle of the Olenek province had a composition and thermal regime that was similar to those of the Archean cratonic mantle at the time the diamonds were brought to the surface. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.