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]

Geochemical characterization

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

^ 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.
^ "Basalt | Definition, Properties, & Facts". Encyclopedia Britannica.
^ ^a ^b ^c Haldar, S. K. (2017). Platinum-Nickel-Chromium Deposits. Elsevier. p. 45. ISBN 9780128020418.
^ ^a ^b ^c Winter, John DuNann (2001). An Introduction to Igneous and Metamorphic Petrology. Upper Saddle River, New Jersey: Prentice Hall. pp. 148, 182. ISBN 0132403420.
^ 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.
^ ^a ^b 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.
^ 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.
^ 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.
^ Farmer, G.L. (2005). Rudnick, R.L. (ed.). Treatise on Geochemistry: The crust. Gulf Professional Publishing. p. 97. ISBN 9780080448473.
^ 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

Alkali basalt on Mindat.org

[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.

[:2-3] Haldar, S. K. (2017). Platinum-Nickel-Chromium Deposits. Elsevier. p. 45. ISBN 9780128020418.

[:0-4] 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.

[:3-6] 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.

[Ulivi159-10] 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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