DIAMONDIFEROUS ECLOGITES FROM SIBERIA: REMNANTS OF ARCHEAN OCEANIC CRUST

Abstract

We have investigated eight diamond-bearing bimineralic eclogite xenoliths from the Udachnaya Mine, Yakutia, Siberia, in terms of major elements, , and oxygen isotopic ratios. The β18O-values, measured with the new laser-fluorination technique, are different from mantle values and range between 5.19 and 7.38%. with an average error of 0.08%.. Strontium and neodymium initial isotopic ratios for cpx are between 0.70226 and 0.70699 and 0.51170 and 0.51257, respectively. Chemically and petrographically, the Siberian eclogites are very similar to the South African eclogite suite from Roberts Victor or Bellsbank, the most important similarities being the late Archean age (2.76 Ga) and the δ18O values that deviate from mantle values. However, differences exist in detail, as no samples with δ18O values lower than mantle values have yet been reported from Siberia and the cesium concentrations of the Siberian eclogites are generally lower than those of the Roberts Victor eclogite suite.The data obtained from the studied sample suite are best explained by a model proposing an origin from Archean oceanic crust that was intensely altered prior to subduction to mantle depths. Using oxygen isotopic values, the effects of seawater alteration can be shown and the composition of the unaltered protolith qualitatively estimated. We propose that mantle eclogites from kimberlites were generated by a globally operating subduction process during the late Archean and that differences between samples from different cratons are small compared to their similarities.