Boninite

Boninite is a high-magnesium, silica-rich volcanic rock typically formed in forearc regions during the early stages of subduction. Its distinctive chemistry reflects melting of mantle wedge material depleted by earlier melting and enriched by fluids from the subducting slab.

Subduction initiation: The onset of one tectonic plate descending beneath another. Boninite commonly forms as a new subduction zone begins and its mantle wedge undergoes intense melting.
Igneous rock: Rock formed when molten material cools and solidifies. Boninite is a volcanic member of this broad rock family.
Tholeiitic basalt: A basaltic volcanic rock series characterized by iron enrichment during magma differentiation. Boninite generally has more silica and magnesium and a different trace-element signature.
Bonin Islands: A Japanese island chain in the western Pacific, also known as the Ogasawara Islands. The islands gave boninite its name and host classic examples of the rock.
Forearc: The region between a subduction-zone trench and its volcanic arc. Boninite erupts in this part of the overriding plate, close to the trench.
Silica: Silicon dioxide, a major chemical component of many rocks and minerals. Boninite is unusually silica-rich compared with many mantle-derived volcanic rocks.
Island arc basalt: Basalt erupted in a volcanic island arc above a subduction zone. Both reflect slab influence, but boninite typically forms earlier and in more depleted mantle.
Izu–Bonin–Mariana arc: A subduction-related volcanic arc system in the western Pacific. Its forearc rocks preserve boninites associated with subduction initiation.
Mantle wedge: Mantle material between a subducting plate and the overriding plate. Its depleted rocks are the source that melts to produce many boninites.
Magnesium number: A ratio comparing magnesium with iron in a rock or mineral composition. Its high value is central to identifying boninite and estimating its primitive character.

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Flux meltingBroader topic: Boninites can form when unusually water-rich fluids promote melting of depleted mantle.