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KOMATIITES

KOMATIITES. Jingao Liu March 3 rd ,2008. Origin of its name. Komatiites are ultramafic mantle-derived volcanic rocks Named from their type locality along the Komati River in South Africa. http://www.univie.ac.at/earth-sciences/image/lehramt_808031/lade_1/03.jpg. www.komatigorge.co.za/.

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KOMATIITES

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  1. KOMATIITES Jingao Liu March 3rd,2008

  2. Origin of its name • Komatiites are ultramafic mantle-derived volcanic rocks • Named from their type locality along the Komati River in South Africa. http://www.univie.ac.at/earth-sciences/image/lehramt_808031/lade_1/03.jpg www.komatigorge.co.za/

  3. Chemistry • SiO2: typically 40 - 45% • MgO: greater than 18% • Low K2O (<0.5%) • Low CaO and Na2O (combined < 2%) • Low Ba, Cs, Rb (incompatible element) enrichment; ΣLILE <1,000ppm • High Ni(>400ppm), Cr(>800ppm), Co (>150ppm) ultramafic low incompatible elements http://en.wikipedia.org/wiki/Komatiite

  4. www.geofuel.lviv.net/PETGF/MAGMA/komati10.htm

  5. Komatiites:Barb. (Barberton, South Africa, 3.5 Ga,), C (Commondale, South Africa, 3.3 Ga), T (Tisdale, Canada, 2.7 Ga), B (Ball, Canada, 2.9 Ga), Munro (Munro, Canada, 2.7 Ga) and Gorg. (Gorgona, South America, 0.088 Ga). • Modern magmas:OIB (ocean island basalt, GeoRoc online database), boninites (GeoRoc online database). http://www.mantleplumes.org/Komatiites.html Komatiites (fields with solid boundaries), basaltic komatiites (filled squares) modern mafic magmas (fields with dashed boundaries)

  6. Mineralogy • Main minerals: • olivine (Fo90 and upwards) • pyroxene (calcic and often chromian) • anorthite (An85 an upwards) • chromite • Minor minerals: • amphibole (with >20%MgO), • phlogopite, baddeleyite, ilmenite and pyrope garnet http://en.wikipedia.org/wiki/Komatiite

  7. Texture: stratigraphic • olivine cumulates • bladed olivine spinifex • pyroxene spinifex • olivine chill zone button top This feature reflects the cooling processes of ultramafic magma

  8. Texture: spinifex http://scienceblogs.com/highlyallochthonous/2007/12/a_deskcropfull_of_komatiite.php

  9. Thin Section: spinifex

  10. Rare Earth Element Hanski et al, 2001 JP

  11. Hanski et al, 2001 JP

  12. With a signature of depletion of LREE to HREE http://www.geokem.com/images/scans/Gorgona_Komatiites-EMORB.gif

  13. The PGE contrast between komatiites and basalts is consistent with the relationship between PGE partition coefficients and melting degrees of mantle sources. Puchtel and Humayun, Ninth Annual V. M. Goldschmidt Conference

  14. Occurrence • mostly restricted in distribution to the Archean shield areas • rarely few found in ages of Proterozoic and Phanerozoic The distribution may reveal the conditions to generate komatiites are gone. But the questions are what the conditions were and what this could tell us about the mantle evolution of the Earth.

  15. Signatures • Extremely high melting point (>1600˚C), whereas basaltic lave about 1200 ˚C • High degrees of mantle partial melting, usually greater than 50%, resulting in high MgO with low K2O and other incompatible elements. LREE depleted relative to HREE • Melts: low dynamic viscosities, behaving like water • Mostly restricted in distribution to the Archean shield areas

  16. Formation Modeling • Subduction Model: The addition of water from downgoing slabs into the mantle source may lower the melting temperature, which however is still very high. This model might explain the komatiites with high silicon contents and hydrated.

  17. Formation Modeling • Plume Model: Numerical model showing two isothermal surfaces (green & blue) which show the effects of dragging a lithospheric plate over an up-welling mantle plume. http://www.mantleplumes.org/Komatiites.html http://faculty.gg.uwyo.edu/cheadle/

  18. Implications • Elemental and isotopic analyses of komatiites would test the models. • The study of komatiites would be helpful to address the particular mantle evolution of the early Earth as well as kimberlites..

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