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Boninite is anextrusiverock high in bothmagnesium andsilica, thought to be usually formed infore-arc environments, typically during the early stages ofsubduction. The rock is named for its occurrence in theIzu-Boninarc south ofJapan. It is characterized by extreme depletion in incompatible trace elements that are not fluid mobile (e.g., the heavyrare-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 ofprimitiveisland arcs (that is, closer to theocean trench) and inophiolite complexes thought to represent former fore-arc settings or at least formed above a subduction zone.
Boninite is considered to be aprimitiveandesite derived from melting ofmetasomatisedmantle.
SimilarArcheanintrusive rocks, calledsanukitoids, have been reported in the rocks of several earlycratons. Archean boninite lavas are also reported.
Boninite typically consists ofphenocrysts ofpyroxenes andolivine in a crystallite-rich glassymatrix.
Boninite is defined by
Most boninitemagma is formed by second stage melting inforearcs viahydration of previously depletedmantle within the mantle wedge above asubducted slab, causing further melting of the already depletedperidotite. A forearc environment is ideal for boninite genesis, but other tectonic environments, such asbackarcs, might be able to form boninite.[1] The content of titanium (an incompatible element within melting of peridotite) is extremely low because previous melting events had removed most of the incompatible elements from the residual mantle source. The first stage melting typically forms island arcbasalt. The second melting event is partly made possible by hydrous fluids being added 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 theconvecting 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 inLREE, strontium, barium, andalkalis. 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 boninitevolcanism, 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, and refractory areas would yield boninites.
| Name | Location | Age | Comments |
|---|---|---|---|
| Bonin Islands | Pacific Ocean | Eocene | mostly volcanicbreccias andpillow lava flows[2] |
| Zambales ophiolite | westernLuzon | Eocene | upper volcanic unit: high silica boninite, low silica boninite, boninitic basalt. lower volcanic unit: low silica boninite series volcanics[3] |
| Cape Vogel | Papua New Guinea | Paleocene | [2] |
| Troodos | Cyprus | Cretaceous | upper pillow lavas ofophiolite complex[2] |
| Guam | Pacific Ocean | Paleogene | late Eocene to earlyOligocene[2] |
| Setouchi | Japan | Miocene | sanukitoids, 13 million years old[2] |
| Baja California | Mexico | Miocene | 14 to 12 million years old, includesbajaite[2] |
| New Caledonia | Pacific Ocean | Mesozoic | Permian-Triassic and Cretaceous age[2] |
| Mariana Trench | Pacific Ocean | Eocene | [2] |
| North-eastLau Basin | Pacific Ocean | Modern | Eruption of boninite lava was observed in 2009 atWest Mata volcano in the Lau Basin by scientists using a remotely-operatedsubmersible. Previously, boninite had been found only near extinct volcanoes more than one million years old.[4] |
Petrology
