[S-IT25] Deep Carbon: Diamond formation and carbon speciation in Earth and planetary processes

[SIT25-04] Origin of carbonado, an enigmatic polycrystalline diamond deduced from its microtexture and inclusions

★Invited Papers

*大藤 弘明¹、淺野 奈津子^1,2、鍵 裕之³ (1.愛媛大学地球深部ダイナミクス研究センター、2.日本電子株式会社、3.東京大学大学院理学系研究科付属地殻化学実験施設)

キーワード：カーボナード、ダイヤモンド、包有物

Carbonado is a type of polycrystalline diamond, which shows a grayish to black color and a massive and irregular morphology with a porous internal texture. It is distinct from ordinary mantle-derived diamonds in the following respects: extremely low carbon isotope composition (-25~-30 ‰), absence of mantle-derived primary inclusions, high concentration of radiogenic noble gases, etc (Kagi et al., 1994; Heaney et al., 2005; Kagi et al., 2007). Therefore, the origin of carbonado has long been controversial. A recent study (Ishibashi et al., 2010) found several lines of evidence that H₂O-rich fluid is present within constituent diamond grains of carbonado, suggesting its formation in close association with C-H-O fluid in the Earth’s mantle. However, the detail of the formation process and condition of carbonado is still unclear.

Here, we found several types of primary mineral inclusions (majoritic garnets, phengite, rutile, apatite, etc.) in nano-sized negative crystals within diamond grains by detailed FE-SEM and TEM observations. Those precipitates usually occur as an assemblage of a few to several mineral phases that are mostly in euhedral forms in the negative crystals. They are most likely quenched products from silicic fluid that were trapped during the crystal growth of diamonds that comprise carbonado. The presence of these mineral phases in negative crystals suggests that the formation of carbonado occurred in fluid-saturated environments to which crustal materials (e.g. basalt) are supplied potentially by the subduction of oceanic plates or extensive collision of continental plates to form a thick mantle keel.