Boninite

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Boninite is a mafic volcanic rock that is considered a primitive andesite . It occurs as extrusive rock with high magnesium and silicon content in forearc basins , typically in the early stages of subduction processes . It is named after the occurrence on the Bonin Islands ( Ogasawara-guntō ), which form an arch of the islands in the south of Japan .

The rock is characterized by an extreme depletion of incompatible trace elements that cannot be transported by solutions that migrate through the rock (the HREE as well as niobium , tantalum and hafnium ). In contrast, transportable elements such as rubidium , barium or potassium occur in varying amounts. Such rocks have so far been found almost exclusively on the deep-sea trench side of young island arches or in ophiolite complexes , which are attributed to former such environments. The characteristic composition is attributed to the melting of a material changed by metasomatosis from the earth's mantle .

Intrusions of Archean which have a similar chemical composition are Sanukitoid called, they come together with the rock several old longlifety before.

Petrology and Geochemistry

Boninites are characterized petrologically by crystals of pyroxene and olivine normally visible to the naked eye in a crystal- rich, glassy matrix . Many boninites contain clinoenstatite , which is polysynthetically twinned according to the (100) face. It is believed that these clinoenstatites were formed from protoenstatite during cooling .

The geochemical composition of boninites has the following characteristics:

Emergence

Boninite magma is formed by the melting of previously melted material in the vicinity of a forearc by hydration of a previously impoverished mantle in a mantle wedge over subducted crust, which causes the melting of already depleted peridotite . The extremely low content of titanium, which is an incompatible element with regard to the melting of peridotite, is due to the melting of a previously depleted source from the earth's mantle. The first, preceding stage of the melt normally leads to the formation of tholeiitic arch basalts .

Boninites acquire their high magnesium and low titanium contents through a high proportion of partial melt in the mantle wedge affected by mantle convection over a subduction zone . The high proportion of partial melt is caused by the high water content of the mantle, as occurs over subduction zones. The supply of volatile components and incompatible elements from the partial melt of the subducted plate leads to the onset of melting processes in the wedge of jacket material located above. Evidence of different enrichment or depletion of incompatible elements indicates that boninites arise from peridotitic material that has been enriched with LREE, strontium, barium and alkalis through metasomatosis . This enrichment could be due to an admixture of material from the subducted crust fragment, either from sedimentary rocks or from melted, drained crust.

Boninites can be derived from the peridotitic residue of a tholeiitic magma formation, which was enriched in LREE by metasomatosis before the boninite volcanism . A second possibility for the formation of island arch tholeiites and boninites is the presence of a differently impoverished peritidotitic parent rock that has been enriched to various degrees by metasomatosis. Areas of enriched peridotites in this model would produce tholeiitic magmas, while impoverished peridotites would produce boninites.

Examples

Examples of Boninit occurrences
place region Age Remarks
Bonin Islands Pacific Ocean Eocene especially volcanic breccia and Pillowlava -Ergüsse
Cape bird Papua New Guinea Paleocene
Troodos Cyprus chalk upper pillow lava of the Troodos ophiolite complex
Guam Pacific Ocean Paleogene late Eocene to early Oligocene
Setouchi Japan Miocene Sanukitoids (granitic rocks with a high MG content), 13 million years old
Baja California Mexico Miocene 12 to 14 million years old, contain Bajaite
New Caledonia Pacific Ocean Mesozoic Permo - Triassic and Cretaceous
Mariana Trench Pacific Ocean Eocene

literature

  • Anthony J. Crawford and WE Cameron: Petrology and geochemistry of Cambrian boninites and low-Ti andesites from Heathcote, Victoria . In: Contributions to Mineralogy and Petrology . tape 91 , no. 1 , 1985, pp. 93-104 , doi : 10.1007 / BF00429431 .
  • PF Dobson, JG Blank, S. Maruyama and JG Liou: Petrology and geochemistry of boninite series volcanic rocks, Chichi-jima, Bonin Islands, Japan . In: International Geology Review . tape 48 , 2006, p. 669-701 .
  • PF Dobson, H. Skogby and GR Rossman: Water in boninite glass and coexisting orthopyroxene: concentration and partitioning . In: Contributions to Mineralogy and Petrology . tape 118 , 1995, pp. 414-419 .
  • RW Le Maitre et al. (Ed.): Igneous Rocks. A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks . 2nd Edition. Cambridge University Press, Cambridge et al. 2002, ISBN 0-521-66215-X .
  • Harvey Blatt and Robert Tracy: Petrology. Igneous, Sedimentary, and Metamorphic . 2nd Edition. WH Freeman, New York NY 1995, ISBN 0-7167-2438-3 , pp. 176 .
  • Rosemary L. Hickey and Frederick A. Frey: Geochemical characteristics of boninite series volcanics: implications for their source . In: Geochimica et Cosmochimica Acta . tape 46 , no. 11 , 1982, pp. 2099-2115 .

Individual evidence

  1. ^ WB Dallwitz, DH Green JE Thompson: Clinoenstatite in a Volcanic Rock from the Cape Vogel Area, Papua . In: Journal of Petrology . tape 7 , 1966, pp. 375-403 ( cloudfront.net [PDF; 32.9 MB ; accessed on May 19, 2019]).
  2. ↑ Keiichi Shiraki, Naoshi Kuroda, Hayaomi Urano & Shigenori Manuyama: Clinoenstatite in boninites from the Bonin Islands, Japan . In: Nature . tape 285 , 1980, pp. 31-32 , doi : 10.1038 / 285031a0 .
  3. ↑ T. Sameshima, J.-P. Paris, Philippa M. Black and RF Herring: Clinoenstatite-bearing lava from Nepoui, New Caledonia . In: American Mineralogist . tape 68 , 1983, pp. 1076-1082 ( minsocam.org [PDF; 733 kB ; accessed on May 19, 2019]).
  4. ^ AJ Crawford: Boninites . Unwin Hyman, London 1989, ISBN 0-04-445003-6 .