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Forearc diamond from Japan

¹ Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan
² Geochemical Laboratory, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan

View larger version (33K): [in this window] [in a new window]   Figure 1. Cenozoic igneous activity in southwest Japan up to 4 Ma (after Kimura et al., 2003; Yamaji and Yoshida, 1998). Location of host lamprophyre in Shikoku Island is marked with star symbol. Distribution of Cretaceous to Tertiary Shimanto accretionary complex is also shown.

View larger version (29K): [in this window] [in a new window]   Figure 2. A: Photomicrograph of clinopyroxene showing diamond-bearing inclusions. Characteristic spindle or lemon-fruit shapes of inclusions are result of expansion of fluid during decompression and splitting of host mineral along cleavage plane. B: Raman spectrum for diamond-bearing inclusion showing wave numbers characteristic of diamond, CO2 gas, a carbonate mineral, a hydrous mineral, and host clinopyroxene (cpx). Diamond is identified both by strong peak at a wave number of 1331 cm–1 and three regions of photolumi-nescence (marked as PL), all characteristic of diamond.

View larger version (49K): [in this window] [in a new window]   Figure 3. Three-dimensional Raman analyses of diamond-bearing inclusion. A: Depth profile across diamond-bearing inclusions. Location of peak corresponding to diamond occurs within clinopyroxene (cpx). Schematic cross section shows location of other identified minerals and CO2 gas (see key sketch in part B). B: Areal mapping showing distribution of various minerals and CO2 gas within an inclusion. Diamond is in contact with carbonate and a hydrous mineral. Scale bars are 2µm.

View larger version (29K): [in this window] [in a new window]   Figure 4. Phase diagram for CaO-MgO-Al2O3-SiO2 (CMAS)-CO2 system and inferred pressure-temperature (P-T) path of diamond-bearing materials. Location of reaction curves is taken from literature (Kennedy and Kennedy, 1976; Luth, 1995; Luth, 2001; Presnall et al., 2002). Pl—plagioclase; Spl—spinel; Grt—garnet; Di—diopside; En—enstatite; Fo—forsterite; Coe—coesite; Mgs—magnesite; Dol—dolomite; Arg—aragonite. Thermodynamic calculations verify that chemical compositions of clinopyroxene and dolomite have minor effects on curve of Dol + Coe = Di + CO2.