Boninite is an

extrusive Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contra ...

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 Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

and

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

, 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 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 arc Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...

s (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 An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found ...

complexes thought to represent former fore-arc settings or at least formed above a subduction zone. Boninite is considered to be a ''primitive''

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...

derived from melting of metasomatised mantle. 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 Arc ...

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 w ...

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 an ...

s. Archean boninite lavas are also reported.

Petrology

Boninite typically consists of

phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

s of pyroxenes and

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

in a crystallite-rich glassy matrix.

Geochemistry

Boninite is defined by * high magnesium content ( MgO = >8%) * low

titanium Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...

( TiO₂ < 0.5%) *

content is 52–63% * high Mg/(Mg + Fe) (0.55–0.83) * Mantle-normal compatible elements Ni = 70–450 parts per million, Cr = 200–1800 ppm * Ba, Sr, L REE enrichments compared to tholeiite * Characteristic Ti/ Zr ratios (23–63) and La/ Yb ratios (0.6–4.7)

Genesis

Most boninite

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

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 vo ...

s via hydration of previously depleted mantle 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- ...

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 ...

. 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 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 Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

, 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