第201回
日時	平成11年7月7日（水）１７：００より
場所	東京工業大学理学部地球惑星科学教室会議室 石川台２号館３１５号室
講 演 者：	Ben Harte（エジンバラ大学地球科学科教授）
講演題目：	Deep Mantle Mineral Inclusions in Diamonds from Sao Luiz, Brazil.
	An exceptional set of mineral inclusions in diamonds from the Sao Luiz alluvial diamond deposit, suggest derivation from both the lower mantle and the relatively deep (sub-lithosphere) upper mantle. The upper mantle suite is eclogitic in composition, and includes both normal-Si and high-Si (majoritic) garnets together with clinopyroxene. The majoritic garnets suggest depths of origin of ca.250-400 kms, but may have been formed initially at greater depths, because they show complex reaction (exhumation?) textures. The lower mantle suite of inclusions consists of: (Mg,Fe)O, (Mg,Fe)SiO3, CaSiO3, (Mg,Fe)3(Al,Fe)2Si3O12, and SiO2. The (Mg,Fe)SiO3 may be both low and high in Al2O3, and the (Mg,Fe)3(Al,Fe)2Si3O12, is a tetra zonal phase with composition similar to pyrope-almandine garnet and referred to as TAPP (tetragonal almandine-pyrope phase). The associations of minerals in one diamond and the mineral compositions suggest the following mineral assemblages: ferropericlase + MgSi-perovskite(low-Al); ferropericlase + MgSi-perovskite(low-Al) + TAPP; ferropericlase + SiO2; ferropericlase + MgSi-perovskite(high-Al). It is presumed that CaSiO3, as the principal Ca-bearing phase may accompany all of these mineral assemblages, and that the assemblages represent a range of expected peridotitic-eclogitic bulk compositions. Comparison with experimental data, suggests that the mineral assemblages have formed in the uppermost part of the lower mantle at depths of ca. 660-800 kms, and that it was here that they became trapped in diamonds. Two distinctive features of composition are also seen in the Sao Luiz lower mantle suite: (a) some high-Fe (Fe/(Fe+Mg)>0.3) ferropericlase-magnesiowustite inclusions; (b) the occurrence of positive Eu anomalies in the CaSiO3 inclusions. These features may suggest materials (protoliths) derived respectively from D” and the crust, which have been convected and subducted to the uppermost lower mantle, where encapsulation in diamond occurred. Such features suggest the occurrence of layered mantle convection. PAPER IN PRESS: Harte B, Harris, J.W., Hutchison, M.T., Watt, G.R. and Wilding, M.C. Lower, mantle mineral associations in diamonds from Sao Luiz, Brazil. In: Field Observations and High Pressure Experimentation: a Tribute to Francis R.(Joe) Boyd. (Fei, Y., Bertka, C.M. and Mysen, B.O., eds.) Geochemical Society Spec. Publ. No. 6 (1999).