Boninite is a high magnesium mafic extrusive rock formed in back-arc environments, typically in seafloor spreading centres but also in terrestrial back-arc spreading centres. The rock is named for its occurrence in the Izu-Bonin arc south of Japan. General Name, Symbol, Number magnesium, Mg, 12 Chemical series alkaline earth metals Group, Period, Block 2, 3, s Appearance silvery white Atomic mass 24. ... In geology, mafic minerals and rocks are silicate minerals, magmas, and volcanic and intrusive igneous rocks that have relatively high concentrations of the heavier elements. ... Extrusive refers to a mode of igneous rock formation, in which hot magma from inside the Earth flows out (extrudes) onto the surface. ... The Ogasawara Islands (小笠原諸島) are an Japan. ... An island arc is a type of archipelago formed by plate tectonics as one oceanic tectonic plate subducts under another and produces magma. ...

Boninite is considered to be a primitive andesite derived from melting of metasomatized mantle. A sample of andesite (dark groundmass) with amygdaloidal vesicules filled with zeolite. ... Metasomatism is a geologic process where metamorphism causes an alteration in a mineral or rock mass that involves a chemical change of the substance with the addition of material, as when chrysolite (olivine) is converted to serpentine basically by the addition of water. ... Earth cutaway from core to exosphere. ...

Similar Archean intrusives called sanukitoids have been reported in the rocks of several early cratons. The Archean is a geologic eon; it is a somewhat antiquated term for the time span between 2500 million years before the present and 3800 million years before the present. ... World geologic provinces. ...

Petrology

Boninite typically consists of phenocrysts of pyroxenes and olivine in a crystallite rich glassy matrix. Example of phenocrysts in rhomb porphyry from the Oslo rift area in Norway A phenocryst is a relatively large and usually conspicuous crystal formed in the mass of a porphyritic igneous rock. ... Figure 1:Mantle-peridotite xenolith with green peridot olivine and black pyroxene crystals from San Carlos Indian Reservation, Gila Co. ... Olivine basalt The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. ...

Geochemistry

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 compaible elements Ni = 70-450 ppm, 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)

General Name, Symbol, Number titanium, Ti, 22 Template:Elementbox s kewlio really eries Group, Period, Block 4, 4, d Appearance silvery metallic Atomic mass 47. ...

Genesis

Boninite magma is formed in back arcs via hydration of previously depleted mantle within the mantle wedge above a subducted slab. Titanium, which is an incompatible element within melting of peridotite is retained in the residual mantle material after generation of basalt. The Juan de Fuca plate sinks below the North America plate at the Cascadia subduction zone. ... Peridotite Peridotite is a dense, coarse-grained rock, consisting mostly of the minerals olivine and pyroxene. ...

Boninite attains its high magnesium and high 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, Zr, 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.

References

• 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
• 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. Geochemical characteristics of boninite series volcanics: implications for their source. Geochimica et Cosmochimica Acta, vol. 46, Issue 11, pp.2099-2115