Boninite

Boninite is a mafic extrusive rock high in both magnesium and silica, 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 compatible 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.

Boninite is considered to be a primitive andesite derived from melting of metasomatised mantle.

Similar Archean intrusive rocks, called sanukitoids, have been reported in the rocks of several early cratons.

Petrology

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

Geochemistry

Boninite is defined by

Genesis

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.[1] 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.

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.

Examples

Examples of Boninite[2]
NameLocationAgeComments
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.[3]

References

  1. ↑ Encarnación, John; Mukasa, Samuel B; Evans, Cynthia A (1999-04-01). "Subduction components and the generation of arc-like melts in the Zambales ophiolite, Philippines: Pb, Sr and Nd isotopic constraints". Chemical Geology. 156 (1–4): 343–357. doi:10.1016/S0009-2541(98)00190-9.
  2. ↑ A. J. Crawford, (1989) Boninites, London, Unwin Hyman, ISBN 0-04-445003-6
  3. ↑ http://www.nsf.gov/news/news_summ.jsp?cntn_id=116098 National Science Foundation Press Release 09-243 Marine Scientists Discover Deepest Undersea Erupting Volcano.
This article is issued from Wikipedia - version of the 11/20/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.