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Alkali basalt

From Wikipedia, the free encyclopedia
Type of volcanic rock
Alkali basalt is one of the rocks comprising thePali-Aike volcanic field, inArgentina.[1]

Alkali basalt oralkali olivine basalt is a dark-colored,porphyriticvolcanic rock[2] usually found in oceanic and continental areas associated withvolcanic activity, such asoceanic islands,continental rifts andvolcanic fields.[3] Alkali basalt is characterized by relatively highalkali (Na2O and K2O) content relative to otherbasalts and by the presence ofolivine and titanium-richaugite in itsgroundmass andphenocrysts, andnepheline in itsCIPW norm.[4][5]

Geochemical characterization

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Depiction of the total alkali-silica diagram. Alkali basalts are generally located in the upper left corner of the basalt region (region B).[6]

Alkali basalt is chemically classified as a rock in region B (basalt) of thetotal alkali versus silica (TAS) diagram that contains nepheline in its CIPW norm. Basalts that do not contain normative nepheline are characterized assub-alkali basalts, which includetholeiitic basalts andcalc-alkaline basalts.[6]

Petrography

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The groundmass of alkali basalt is mainly composed of olivine, titanium-rich augite andplagioclasefeldspar and may havealkali feldspar orfeldspathoid interstitially, but is poor insilica minerals, such ashypersthene andquartz.[4]

Phenocrysts are ubiquitous in alkali basalt and, similarly to the groundmass, are usually made up of olivine and titanium-rich augite but can also have plagioclase andiron oxides with lower frequency.[3][4]

Geologic context

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Alkali basalt can be found in areas associated with volcanic activity, such as oceanic islands (Hawaii,Madeira,[7]Saint Helena,[8]Ascension, etc.), continental rifts and volcanic fields.[3] Continental alkali basalt can be found in every continent, with prominent examples being theRio Grande Rift (USA), theEast African Rift and thePali-Aike volcanic field.[9]

The results from thegamma ray spectrometer onVenera 8 onVenus suggest it landed on alkali basalt.[10]

References

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  1. ^Skewes, Milka Alexandra; Stern, Charles R. (1979). "Petrology and geochemistry of alkali basalts and ultramafic inclusions from the palei-aike volcanic field in Southern Chile and the origin of the patagonian plateau lavas".Journal of Volcanology and Geothermal Research.6 (1–2):3–25.Bibcode:1979JVGR....6....3S.doi:10.1016/0377-0273(79)90044-1.
  2. ^"Basalt | Definition, Properties, & Facts".Encyclopedia Britannica.
  3. ^abcHaldar, S. K. (2017).Platinum-Nickel-Chromium Deposits. Elsevier. p. 45.ISBN 9780128020418.
  4. ^abcWinter, John DuNann (2001).An Introduction to Igneous and Metamorphic Petrology. Upper Saddle River, New Jersey: Prentice Hall. pp. 148, 182.ISBN 0132403420.
  5. ^Irvine, T. N.; Baragar, W. R. A. (1971). "A Guide to the Chemical Classification of the Common Volcanic Rocks".Canadian Journal of Earth Sciences.8 (5):523–548.Bibcode:1971CaJES...8..523I.doi:10.1139/e71-055.
  6. ^abLe Maitre, R. W.; Streckeisen, A.; Zanettin, B.; Le Bas, M. J.; Bonin, B.; Bateman, P., eds. (2002).Igneous Rocks: A Classification and Glossary of Terms. Cambridge University Press. pp. 36–37.ISBN 9780521662154.
  7. ^Hughes, D. J.; Brown, G. C. (1972). "Basalts from Madeira: A petrochemical contribution to the genesis of oceanic alkali rock series".Contributions to Mineralogy and Petrology.37 (2):91–109.Bibcode:1972CoMP...37...91H.doi:10.1007/BF00371069.ISSN 1432-0967.S2CID 140549581.
  8. ^Baker, Ian (1969). "Petrology of the Volcanic Rocks of Saint Helena Island, South Atlantic".GSA Bulletin.80 (7):1283–1310.Bibcode:1969GSAB...80.1283B.doi:10.1130/0016-7606(1969)80[1283:POTVRO]2.0.CO;2.ISSN 0016-7606.
  9. ^Farmer, G.L. (2005). Rudnick, R.L. (ed.).Treatise on Geochemistry: The crust. Gulf Professional Publishing. p. 97.ISBN 9780080448473.
  10. ^Ulivi, Paolo; Harland, David M (2007).Robotic Exploration of the Solar System Part I: The Golden Age 1957-1982. Springer. pp. 159–160.ISBN 9780387493268.

External links

[edit]
Types ofbasalts
Basalts by tectonic setting
Basalts by form and flow
Basalts by chemistry
Important minerals
Types of rocks
Igneous rock
Sedimentary rock
Metamorphic rock
Specific varieties
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