Boninite is an extrusive

rock Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wales ...

high in both magnesium and silica, thought to be usually formed in

fore-arc Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vol ...

environments, typically during the early stages of

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...

. 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 The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...

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 Oceanic trenches are prominent long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic tren ...

) and in ophiolite complexes thought to represent former fore-arc settings or at least formed above a subduction zone. Boninite is considered to be a ''primitive'' andesite derived from melting of metasomatised

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

. Similar

Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth during the Archea ...

intrusive rocks, called

sanukitoid {{No footnotes, date=December 2021 Sanukitoids are a variety of high-Mg granitoid found in convergent margin settings. The term "sanukitoid" was originally used to define a variety of Archean plutonic rock, but now also includes younger rocks wit ...

s, have been reported in the rocks of several early

craton A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and ...

s. Archean boninite lavas are also reported.

Petrology

Boninite typically consists of phenocrysts of pyroxenes and olivine in a crystallite-rich glassy

matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** '' The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...

Geochemistry

Boninite is defined by * high magnesium content (

MgO Magnesium oxide ( Mg O), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ion ...

= >8%) * low titanium ( TiO₂ < 0.5%) * silica content is 52–63% * high Mg/(Mg + Fe) (0.55–0.83) * Mantle-normal

compatible Compatibility may refer to: Computing * Backward compatibility, in which newer devices can understand data generated by older devices * Compatibility card, an expansion card for hardware emulation of another device * Compatibility layer, compon ...

elements Ni = 70–450 parts per million, Cr = 200–1800 ppm * Ba, Sr, L REE enrichments compared to

tholeiite The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma ...

* Characteristic Ti/ Zr ratios (23–63) and La/ Yb ratios (0.6–4.7)

Genesis

Most boninite magma is formed by second stage melting in

forearc Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the v ...

s via

hydration Hydration may refer to: * Hydrate, a substance that contains water * Hydration enthalpy, energy released through hydrating a substance * Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound * ...

of previously depleted

within the mantle wedge above a subducted slab, causing further melting of the already depleted peridotite. A forearc environment is ideal for boninite genesis, but other tectonic environments, such as

backarc The back-arc region is the area behind a volcanic arc. In island volcanic arcs, it consists of back-arc basins of oceanic crust with abyssal depths, which may be separated by remnant arcs, similar to island arcs. In continental arcs, the back-a ...

s, might be able to form boninite. 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 arc

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

. 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 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, strontium, barium, and

alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of ...

s. 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, and refractory areas would yield boninites.

Examples

References

* Anthony J. Crawford and W. E. Cameron, 1985. '' Petrology and geochemistry of Cambrian boninites and low-Ti andesites from Heathcote, Victoria'' Contributions to Mineralogy and Petrology, vol 91 no. 1
Abstract
* Dobson, P.F., Blank, J.G., Maruyama, S., and Liou, J.G. (2006) ''Petrology and geochemistry of boninite series volcanic rocks, Chichi-jima, Bonin Islands, Japan''. International Geology Review 48, 669–701 (LBNL #57671) * Dobson, P.F., Skogby, H, and Rossman, G.R. (1995) ''Water in boninite glass and coexisting orthopyroxene: concentration and partitioning''. Contrib. Mineral. Petrol. 118,414-419. * 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, *Blatt, Harvey and Robert Tracy, 1995, ''Petrology, Second Edition: Igneous, Sedimentary, and Metamorphic'', W. H. Freeman, 2nd, p. 176 {{ISBN, 0-7167-2438-3 * Hickey, Rosemary L.; Frey, Frederick A. (1982) ''Geochemical characteristics of boninite series volcanics: implications for their source.'' Geochimica et Cosmochimica Acta, vol. 46, Issue 11, pp. 2099–2115 * 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. Volcanic rocks Intermediate rocks Igneous petrology