2019 年 48 巻 2 号 p. 63-75
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.
