Alkali basalt

Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock[2] usually found in oceanic and continental areas associated with volcanic activity, such as oceanic islands, continental rifts and volcanic fields.[3] Alkali basalt is characterized by relatively high alkali (Na2O and K2O) content relative to other basalts and by the presence of olivine and titanium-rich augite in its groundmass and phenocrysts, and nepheline in its CIPW norm.[4][5]

Alkali basalt is one of the rocks comprising the Pali-Aike volcanic field, in Argentina.[1]

Geochemical characterization
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 the total alkali versus silica (TAS) diagram that contains nepheline in its CIPW norm. Basalts that do not contain normative nepheline are characterized as sub-alkali basalts, which include tholeiitic basalts and calc-alkaline basalts.[6]

Petrography

The groundmass of alkali basalt is mainly composed of olivine, titanium-rich augite and plagioclase feldspar and may have alkali feldspar or feldspathoid interstitially, but is poor in silica minerals, such as hypersthene and quartz.[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 and iron oxides with lower frequency.[3][4]

Geologic context

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 the Rio Grande Rift (USA), the East African Rift and the Pali-Aike volcanic field.[9]

The results from the gamma ray spectrometer on Venera 8 on Venus suggest it landed on alkali basalt.[10]

References
  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. Haldar, S. K. (2017). Platinum-Nickel-Chromium Deposits. Elsevier. p. 45. ISBN 9780128020418.
  4. Winter, 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. Le 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.
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