キンバーライト:地球深部の化学的環境を探るための鍵

抄録

Kimberlites have a lot of quite unique characteristics in their chemical and isotope compositions. Since they carry diamonds to the surface, their magma sources are regarded to be located from a depth of more than 150 km at least. Their radiogenic solid element isotopes and noble gas isotopes suggest that their magma sources are different from those of MORBs (Mid-oceanic ridge basalts) and similar to those of OIBs (Oceanic island basalts). Based on such isotope signatures and some silicate inclusions in diamonds which show a sign of the uppermost part of the lower mantle origin (e.g., Harte and Harris, 1994), it is inferred that kimberlite magmas might have an origin in the mantle. Furthermore, compared to radiogenic isotope signatures of solid elements of OIBs, if considered as initial ratios, those of kimberlites are more tightly clustered close to the value of Bulk Earth in space and time. Noble gas isotope signatures of kimberlites (Sumino et al., 2006) make it difficult to explain such a distribution of radiogenic isotopes of solid elements by a simple mixture between depleted and enriched sources. If relatively scattered isotope signatures of solid elements for OIBs from different localities reflect different degrees of mixing of primary OIB source materials with recycled and/or crustal materials, kimberlites might reflect more genuine characteristics of primary mantle materials than OIBs. This implies that source materials of kimberlites would be less fractionated from the primary (Bulk Earth-like) materials than those of the other igneous rocks and the lower mantle might still keep a signature of less fractionated character with regard to isotope compositions. Such a character is compatible with an inference that at least deeper parts of the lower mantle might be less degassed than the upper mantle (e.g., Kaneoka, 2008). Furthermore, chemical characteristics of kimberlites including the redox state of magmas are different from those of OIBs. Some of them might be attributed to different degree of effects of shallow materials together with recycled ones during the ascent of mantle plumes to a surface. Although kimberlites have often been found as altered ones, fresh kimberlites and/or constituent minerals such as perovskites are regarded to keep more primary signatures of the chemical state of the deep mantle. Thus, in spite of rarity in their occurrence, kimberlites would play an important role as a key to reveal the chemical state of the Earth's deep mantle directly based on terrestrial materials.