Diamondiferous eclogite xenoliths from the Mir Kimberlite, Yakutia, Siberia : geochemistry and petrogenesis

Mantle eclogites from kimberlites often retain the geochemical signatures of their protoliths, including those ultimately derived from recycled oceanic crust. The numerous kimberlites of the Yakutian Kimberlite Province, Siberia provide the opportunity to study eclogites from various depths and geographic locations of the Siberian Platform, and several unusual diamondiferous eclogite xenoliths have been investigated from the Mir Kimberlite, located relatively near the edge of the craton. This study includes detailed petrography, mineral chemistry, whole-rock analyses, and oxygen isotope analyses. Trace-element analyses by INAA and secondary ion mass spectrometer (SIMS), coupled with major-element mineral chemistry, provide the basis for comparisons with xenoliths from the Udachnaya kimberlite, Siberia.

The Mir eclogites are coarse-grained, bimineralic assemblages of garnet and omphacitic clinopyroxene, some exceeding 1 cm in the longest dimension. Modal garnet:clinopyroxene ratios vary from 64:36 to 26:74. Primary kyanite occurs in one rare sample, the only kyanite eclogite reported from the Mir kimberlite, while common from other Siberian kimberlites. Accessory minerals include amphibole, rutile, serpentine, various sulfides, chlorite, and calcite, with phlogopite occurring as kelyphitic rims. EMP analyses of garnet indicate end-member compositions that vary from Py₆₃Alm₂₄Gr₁₅ to Py₃₂Alm₅₃Gr₁₅ to Py₃₀Alm₃₃Gr₃₇.

Clinopyroxene compositions are approximately Wo₂₄En₃₀Fs₆Jd₄₀, but jadeite reaches 66% in one sample. Whole-rock analyses indicate bulk compositions similar to that of oceanic basalt, though some samples have an unusually high Na&sub2;O content (commonly 3%) due to high-Na clinopyroxenes and some secondary amphiboles. Similar mineral compositions are observed in other Yakutian eclogite xenoliths, particularly from Udachnaya.Clinopyroxene REE analyses display convex-upward LREE patterns. Such patterns are common in Yakutian eclogitic xenoliths, especially those of the Udachnaya kimberlite pipe, but are virtually absent in xenoliths from South African kimberlites. Garnet REE patterns for two samples "cross-over" in the MREE, leaving a less enriched HREE pattern. Whole-rock REE patterns have been LREE-enriched, probably the result of metasomatism. Oxygen isotope compositions of five samples fall within the accepted range of mantle values (5.7 ± 0.7%). Two samples fall outside this range (+3.11 and +7.13%), which may be the result of high and low-temperature, hydrothermal alteration, respectively, of an oceanic precursor.

Comparisons made with eclogite xenoliths from other Yakutian pipes, specifically from Udachnaya, reveal similar major- and trace-element compositions, but the oxygen isotope values of two Mir samples are unusual. A crustal origin for these two samples may be indicated by the oxygen isotope values; however, this does not rule out a crustal component to all these eclogites. In reality, a mantle origin is suspected by a general lack of evidence to the contrary. All of these eclogites may have crustal progenitors.