Thermal and chemical evolution of the subarc mantle revealed by spinel-hosted melt inclusions in boninite from the Ogasawara (Bonin) Archipelago, Japan

Abstract

10Primitive melt inclusions in chrome spinel from the Ogasawara Archipelago comprise 11 two discrete groups of high-SiO2, MgO (high-Si) and low-SiO2, MgO (low-Si) boninitic suites 12 with ultra-depleted dish-and V-shaped, and less depleted flat rare earth element (REE) patterns. 13The most magnesian melt inclusions of each geochemical type were used to estimate the T-P 14 conditions for primary boninites, that range from 1345°C-0.56 GPa to 1421°C-0.85 GPa for the 15 48-46 Ma high-Si and low-Si boninites, and 1381°C-0.85 GPa for the 45 Ma low-Si boninite. 16

Did boninite originate from the heterogeneous mantle with recycled ancient slab?

“…Bulk boninites and boninitic melt inclusions from the Izu–Ogasawara–Mariana forearc region are plotted in Figure , together with the associated PABs and arc tholeiitic rocks (Figure ). Although not all boninitic rocks and melt inclusions as shown in Figure have MgO > 8 wt% and meet the definition of boninite by the IUGS recommendation (Le Maitre, ), they plot mainly on the fractional crystallization paths of olivine ± spinel, followed by orthopyroxene or clinopyroxene, as shown by Umino et al (), and are hence classified as “boninitic suites”, hereafter. Following Umino et al, the 48–46 Ma boninitic melt inclusions from the Ogasawara Archipelago are divided into high‐SiO 2 (high‐Si) and low‐SiO 2 (low‐Si) type (Figure ), both of which have less than 9 wt% CaO at 8 wt% MgO (low‐Ca type).…”

“…See the text for the detail. Green, red and light blue curves are fractional crystallization paths for the primary magmas of high‐ S i‐ D , low‐ S i‐ F and ultralow‐ S i boninites at 0.2 GPa by using A lpha MELTS (Antoshechkina & Asimow, ) run on MELTS mode (Ghiorso & Sack, ) after Umino et al (). The overall variations of boninitic melt inclusions are reproduced by the simulated paths of melts saturated with water, which fractionated olivine (ol) and subsequently orthopyroxene (opx) for the high‐ S i boninite or clinopyroxene (cpx) for the low‐ S i boninites, with a small amount of spinel (sp).…”

“…Some inclusions are widespread and can be explained by earlier crystallization of orthopyroxene either at higher pressures or due to degassing during the ascent, combined with magma mixing. Refer to Umino et al () for the detail…”

“…Umino et al () assumed the most Mg‐rich boninitic melt inclusions in spinel among individual geochemical types to be the primary boninite magmas which were in equilibrium with harzburgitic residues and estimated the temperatures and pressures for these melt compositions to be multiply saturated with olivine and orthopyroxene by using the method of Putirka (). The melt inclusion compositions selected as primary magmas by Umino et al and revised estimates on the T–P conditions for the Ogasawara boninites are listed in Table .…”

“…Kanayama et al () divided boninites into high‐MgO and high‐SiO 2 type (high‐Si boninite), which is severely depleted in fluid‐immobile incompatible elements with U‐shaped chondrite‐normalized rare earth element (REE) patterns, and low‐MgO and low‐SiO 2 type (low‐Si boninite), which is less depleted and has a flat to U‐shaped REE pattern. By analyzing primitive melt inclusions hosted in chrome spinels, Umino et al () demonstrated the coexistence of the high‐Si and low‐Si boninitic magmas. The estimated T–P genetic conditions for their primary boninites range from 1 420 °C at 0.85 GPa for the high‐Si boninite to 1 345–1 380 °C at 0.56–0.85 GPa for the low‐Si boninite.…”