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Boninite is a mafic extrusive rock high in both magnesium and silica, thought to be usually formed in fore-arc environments, typically during the early stages of subduction. The rock is named for its occurrence in the Izu-Bonin arc south of Japan. It is characterized by extreme depletion in incompatible trace elements that are not fluid mobile (e.g., the heavy rare earth elements plus Nb, Ta, Hf) but variable enrichment in the fluid mobile elements (e.g., Rb, Ba, K). They are found almost exclusively in the fore-arc of primitive island arcs (that is, closer to the ocean trench) and in ophiolite complexes thought to represent former fore-arc settings or at least formed above a subduction zone.
Boninite is defined by
- high magnesium content (MgO = 8-15%)
- low titanium (TiO2 < 0.5%)
- silica content is 57 - 60%
- high Mg/(Mg + Fe) (0.55-0.83)
- Mantle-normal compatible elements Ni = 70-450 parts per million, Cr = 200-1800 ppm
- Ba, Sr, LREE enrichments compared to tholeiite
- Characteristic Ti/Zr ratios (23-63) and La/Yb ratios (0.6-4.7)
Most boninite magma is formed by second stage melting in forearcs via hydration of previously depleted mantle within the mantle wedge above a subducted slab, causing further melting of the already depleted peridotite. Although a forearc environment is ideal for boninite genesis, other tectonic environments, such as backarcs, might be capable of forming boninite as well. The extremely low content of titanium, which is an incompatible element within melting of peridotite is the result of previous melting events that removed most of the incompatible elements from the residual mantle source. The first stage melting typically forms island arc basalt. The second melting event is partly made possible by the addition of hydrous fluids to the shallow, hot, depleted mantle, leading the enrichment in large ion lithophile elements in the boninite.
Boninite attains its high magnesium and very low titanium content via high degrees of partial melting within the convecting mantle wedge. The high degrees of partial melting are caused by the high water content of the mantle. With the addition of slab-derived volatiles, and incompatible elements derived from the release of low-volume partial melts of the subducted slab, the depleted mantle in the mantle wedge undergoes melting.
Evidence for variable enrichment or depletion of incompatible elements suggests that boninites are derived from refractory peridotite which has been metasomatically enriched in LREE, Sr, Ba and alkalis. Enrichment in Ba, Sr and alkalis may result from a component derived from subducted oceanic crust. This is envisaged as contamination from the underlying subducted slab, either as a sedimentary source or as melts derived from the dehydrating slab.
Boninites can be derived from the peridotite residue of earlier arc tholeiite generation which is metasomatically enriched in LREE before boninite volcanism, or arc tholeiites and boninites can be derived from a variably depleted peridotite source which has been variably metasomatised in LREE.
Areas of fertile peridotite would yield tholeiites while refractory areas would yield boninites.
|Bonin Islands||Pacific Ocean||Eocene||mostly volcanic breccias and pillow lava flows|
|Cape Vogel||Papua New Guinea||Paleocene|
|Troodos||Cyprus||Cretaceous||upper pillow lavas of ophiolite complex|
|Guam||Pacific Ocean||Paleogene||late Eocene to early Oligocene|
|Setouchi||Japan||Miocene||sanukitoids, 13 million years old|
|Baja California||Mexico||Miocene||14 to 12 million years old, includes bajaite|
|New Caledonia||Pacific Ocean||Mesozoic||Permian-Triassic and Cretaceous age|
|Mariana Trench||Pacific Ocean||Eocene|
|North-east Lau Basin||Pacific Ocean||Modern||Eruption of boninite lava was observed in 2009 at West Mata volcano in the Lau Basin by scientists using a remotely-operated submersible. Previously, boninite had been found only near extinct volcanoes more than one million years old.|
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- Le Maitre, R. W. and others (Editors), 2002, Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, Cambridge University Press, 2nd, ISBN 0-521-66215-X
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- Resing, J. A., K.H. Rubin, R. Embley, J. Lupton, E. Baker, R. Dziak, T. Baumberger, M. Lilley, J. Huber, T.M. Shank, D. Butterfield, D. Clague, N. Keller, S. Merle, N.J. Buck, P. Michael, A. Soule, D. Caress, S. Walker, R. Davis, J. Cowen, A-L. Reysenbach, and H. Thomas, (2011): Active Submarine Eruption of Boninite at West Mata Volcano in the Extensional NE Lau Basin, Nature Geosciences, 10.1038/ngeo1275.