Origin of Graphite-Diamond-Bearing Eclogites from Udachnaya Kimberlite Pipe

Kimberlite-borne mantle eclogites represent an important diamond source rock. Although the origin and stability of diamond, as opposed to its low-pressure polymorph graphite, have been studied for decades, their relationship in rare natural samples where both polymorphs coexist remains poorly constrained. To shed new light on this issue, seven graphite-diamond-bearing eclogites from the kimberlite pipe Udachnaya, Siberian craton were comprehensively investigated with respect to their petrography, mineral chemical composition and omphacite Sr-87/Sr-86, acquired in situ by laser ablation multicollector inductively coupled plasma mass spectrometry. The calculated P-T conditions for basaltic group eclogites (Eu/Eu* < 1) correspond to a pressure range of 4.8-6.5 GPa and temperatures of 1060-1130 degrees C, whereas gabbroic eclogites with positive Eu- and Sr-anomalies have a smaller pressure variation (4.8-5.8 GPa), but a larger range in temperature (990-1260 degrees C). Reconstructed bulk compositions for gabbroic eclogites indicate an oceanic crustal origin for their protoliths, with accumulation of plagioclase and olivine +/- clinopyroxene (gabbronorite or olivine gabbro). The protoliths of basaltic eclogites probably formed from the complementary residual melt. The presence of coesite and low Mg# in basaltic eclogites suggest that their light rare earth element depletion was the result of <10% partial melting during subsequent subduction and emplacement into the cratonic lithosphere. Extremely unradiogenic Sr-87/Sr-86 (0.70091-0.70186 for six of seven samples) not only provides new evidence for the Archean age (2.5-2.9 Gyr) of Yakutian graphite-diamond-bearing eclogites and for formation of their protoliths in a depleted mantle source, but also suggests that they were not significantly metasomatically overprinted after their formation, despite their extended residence in the cratonic mantle lithosphere. The mineralogical and petrographic features indicate that the primary mineral association includes garnet, omphacite, +/- coesite, +/- kyanite, +/- rutile, graphite, and diamond. Graphite occurs in the samples in the form of idiomorphic crystals (the longest dimensions being 0.4-1 mm) in garnet and kyanite and extends beyond their grain boundaries. Diamonds occur as octahedral cubic transparent, slightly colored or bright yellow crystals as large as 0.1-2 mm. Furthermore, idiomorphic and highly ordered graphite occurs as inclusions in diamond in four samples. The carbon isotope composition for diamond and graphite has a narrow range (-4 to -6.6 parts per thousand) for both groups (gabbroic and basaltic), indicating a mantle source and limiting the role of subducted isotopically light biogenic carbon or reduction of isotopically heavy carbonate in diamond crystallization. Importantly, the presence of graphite and diamond inclusions in garnet, omphacite, and kyanite in three samples indicates a co-formation close in time to eclogitization. Combined, the petrographic and geochemical evidence suggests that both polymorphic carbon modifications can form in the diamond stability field, as also suggested by experiments and some natural examples, although the exact mechanism remains unresolved. Furthermore, this study provides natural evidence that graphite can be preserved (metastably) deep within the diamond stability field, without recrystallizing into diamond, for a long time, >= 2.5 Gyr.