Nature of diamonds in Yakutian eclogites: views from eclogite tomography and mineral inclusions in diamonds   总被引：1，自引：0，他引：1  

Mahesh Anand  Lawrence A. Taylor  Kula C. Misra  William D. Carlson  Nikolai V. Sobolev 《Lithos》2004,77(1-4):333-348

We have performed dissections of two diamondiferous eclogites (UX-1 and U33/1) from the Udachnaya kimberlite, Yakutia in order to understand the nature of diamond formation and the relationship between the diamonds, their mineral inclusions, and host eclogite minerals. Diamonds were carefully recovered from each xenolith, based upon high-resolution X-ray tomography images and three-dimensional models. The nature and physical properties of minerals, in direct contact with diamonds, were investigated at the time of diamond extraction. Polished sections of the eclogites were made, containing the mould areas of the diamonds, to further investigate the chemical compositions of the host minerals and the phases that were in contact with diamonds. Major- and minor-element compositions of silicate and sulfide mineral inclusions in diamonds show variations among each other, and from those in the host eclogites. Oxygen isotope compositions of one garnet and five clinopyroxene inclusions in diamonds from another Udachnaya eclogite (U51) span the entire range recorded for eclogite xenoliths from Udachnaya. In addition, the reported compositions of almost all clinopyroxene inclusions in U51 diamonds exhibit positive Eu anomaly. This feature, together with the oxygen isotopic characteristics, is consistent with the well-established hypothesis of subduction origin for Udachnaya eclogite xenoliths. It is intuitive to expect that all eclogite xenoliths in a particular kimberlite should have common heritage, at least with respect to their included diamonds. However, the variation in the composition of multiple inclusions within diamonds, and among diamonds, from the same eclogite indicates the involvement of complex processes in diamond genesis, at least in the eclogite xenoliths from Yakutia that we have studied.  相似文献   

Evidence for Archean ocean crust with low high field strength element signature from diamondiferous eclogite xenoliths   总被引：6，自引：0，他引：6  

Dorrit E. Jacob  Stephen F. Foley 《Lithos》1999,48(1-4):317-336

Late Archean (2.57 Ga) diamond-bearing eclogite xenoliths from Udachnaya, Siberia, exhibit geochemical characteristics including variation in oxygen isotope values, and correlations of δ¹⁸O with major elements and radiogenic isotopes which can be explained by an origin as subducted oceanic crust. Trace element analyses of constituent garnet and clinopyroxene by Laser-ICPMS are used to reconstruct whole-rock trace element compositions, which indicate that the eclogites have very low high field strength element (HFSE) concentrations and Zr/Hf and Nb/Ta ratios most similar to modern island arcs or ultradepleted mantle. Although hydrothermal alteration on the Archean sea floor had enough geochemical effect to allow the recognition of its effects in the eclogites and thus diagnose them as former oceanic crust, it was not severe enough to erase many other geochemical features of the original igneous rocks, particularly the relatively immobile HFSEs. Correlations of the trace element patterns with oxygen isotopes show that some, generally Mg-richer, eclogites originated as lavas, whereas others have lower δ¹⁸O and higher Sr and Eu contents indicating an origin as plagioclase-bearing intrusive rocks formed in magma chambers within the ocean crust. Major and trace element correlations demonstrate that the eclogites are residues after partial melting during the subduction process, and that their present compositions were enriched in MgO by this process. The original lava compositions were picritic, but not komatiitic, whereas the intrusives had lower, basaltic MgO contents. The HFSE signature of the eclogites may indicate that ocean floor basalts of the time were relatively close to island arcs and recycled material, which would be consistent with a larger number of smaller oceanic plates. Their composition appears to indicate that komatiitic ocean crust compositions were restricted to the early Archean which is not known to be represented among the eclogite xenolith population.  相似文献