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

Abstract 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 from kimberlite pipe Udachnaya, Siberian craton were comprehensively investigated with respect petrography, mineral chemical composition omphacite 87Sr/86Sr, acquired situ by laser ablation multicollector inductively coupled plasma mass spectrometry. The calculated P–T conditions basaltic group (Eu/Eu* < 1) correspond a pressure range 4·8–6·5 GPa temperatures 1060–1130 °C, whereas gabbroic positive Eu- Sr-anomalies smaller variation (4·8–5·8 GPa), but larger temperature (990–1260 °C). Reconstructed bulk compositions indicate oceanic crustal protoliths, accumulation plagioclase olivine ± clinopyroxene (gabbronorite or gabbro). protoliths probably formed complementary residual melt. presence coesite low Mg# suggest that earth element depletion was result <10 % partial melting during subsequent subduction emplacement into cratonic lithosphere. Extremely unradiogenic 87Sr/86Sr (0·70091–0·70186 six samples) not only provides evidence Archean age (2·5–2·9 Gyr) Yakutian formation depleted source, also suggests they significantly metasomatically overprinted after formation, despite extended residence mineralogical petrographic features primary association includes garnet, omphacite, coesite, kyanite, rutile, diamond. Graphite occurs form idiomorphic crystals (the longest dimensions being 0·4–1 mm) garnet kyanite extends beyond grain boundaries. Diamonds occur octahedral cubic transparent, slightly colored bright yellow large 0·1–2 mm. Furthermore, highly ordered graphite inclusions four samples. carbon isotope has narrow (−4 −6·6 ‰) groups (gabbroic basaltic), indicating limiting role subducted isotopically biogenic reduction heavy carbonate crystallization. Importantly, three indicates co-formation close time eclogitization. Combined, geochemical polymorphic modifications can field, suggested experiments some examples, although exact mechanism unresolved. study be preserved (metastably) deep within without recrystallizing long time, ≥2·5 Gyr.