Boninite is the only igneous rock in the IUGS recommendations, which was named after a Japanese island ‘Bonin’, a corrupted Japanese of ‘Munin-to’. Homogeneous glass inclusions in chrome spinel of boninite from Ogasawara, Guam and Oman Ophiolite preserve at least three primary magmas followed by discrete fractionation paths designated high-silica, low-silica, and ultralow-silica boninites, in decreasing degree of source mantle depletion.
The T-P conditions and the source depletion of boninites led to a model of subduction initiation along the Izu-Bonin-Mariana (IBM) arc at 52 Ma. The Philippine Sea and Pacific plate boundary was underlain by heterogeneous mantle with depleted harzburgite blocks embedded in depleted mid-ocean ridge basalt (MORB) mantle (DMM). Subsidence of the old and dense Pacific Plate caused upwelling of the heterogeneous mantle, causing adiabatic melting of DMM to MORB, while the refractory harzburgite was uplifted without melting. Fluids from the subducted slab induced flux melting of the MORB residue and harzburgite to generate low- and ultralow-silica boninites and high-silica boninite, respectively. The variations of magma geochemistry in the IBM arc indicate sustained subarc mantle convection, arc magmatism and backarc spreading, which were driven by trench retreat resulted from the foundering dense Pacific Plate.
The Oman Ophiolite formed at a divergent plate boundary in the Neotethys and subsequently experienced arc tholeiite and low-silica boninite magmatism, indicating depletion of the source mantle through time. The arc magmatism was caused by thrusting of a rotating microplate over young and buoyant lithosphere, imposing a compressive stress field on the overriding plate. This prohibited forearc spreading and mantle wedge convection, leading to rapid cooling and termination of the arc magmatism in <3 Myr.
Os isotopic modeling demonstrate that the enigmatic harzburgite for the IBM high-silica boninite source fractionated from the primitive upper mantle at 1.5-1.7 Ga, whereas the IBM (ultra) low-silica boninite source fractionated at 3.6-3.1 Ga. The average DMM and the source of Oman low-silica boninite differentiated at 2.6-2.0 Ga.
Cathodoluminescence (CL) is the emission of photons from a material stimulated by an incident electron beam. CL microscopy and spectroscopy provide useful information on the existence and distribution of lattice defects and trace elements in minerals with a spatial resolution of a few micrometers. The CL properties depend on the nature of defect and impurity centers, their concentrations, chemical composition and crystal structure of materials, which are closely related to pressure and temperature conditions during hydrothermal metasomatic reaction, shock event, and radiation damage from natural nuclides. CL of minerals, therefore, has been used as an important tool in earth and planetary sciences, as follows; (1) CL images of alkali feldspar grains in syenite (Andes, Chile) show blue, violet, pink, red, and brown colors with variable brightness. Their CL spectra exhibit a emission band assigned to Ti4＋ impurity and/or Al-O−-Al defect centers in the blue region and one to Fe3+ impurity centers in T1 and T2 sites in the red-infrared region, of which the wavelengths and intensities change depending on a kind of the microtextures in the grains. (2) CL spectra of unimplanted and He＋-ion-implanted (corresponding to natural α particles) albite (Minas Gerais, Brazil) consist of emission bands in the ultraviolet to infrared regions. The red emission intensities correlate positively with the radiation dose, implying the possibility for a geodosimetry. (3) CL images of unshocked and experimentally shocked sanidine show a red-violet color below 20.0 GPa and a blue one above 20.0 GPa. CL spectra of these shocked sanidine have ultraviolet to blue emission bands, of which intensities increase with the shock pressure. This correlation gives quantitative values of the shock pressures that alkali feldspar grains in martian meteorites have experienced. Therefore, the CL features of feldspar has a potential for a universal shock barometer with high spatial resolution.