Carbonatite

Carbonatite
Igneous rock
Carbonatite from Jacupiranga, Brazil. This rock is a mixture of calcite, magnetite, and olivine.
Composition
Carbonate minerals (>50%)
Carbonatite lava at Ol Doinyo Lengai volcano, Tanzania

Carbonatite (/kɑːrˈbɒnəˌtaɪt/) is a type of intrusive or extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals.[1] Carbonatites may be confused with marble and may require geochemical verification.

Carbonatites usually occur as small plugs within zoned alkalic intrusive complexes, or as dikes, sills, breccias, and veins.[2] They are almost exclusively associated with continental rift-related tectonic settings. It seems that there has been a steady increase in the carbonatitic igneous activity through the Earth's history, from the Archean eon to the present.

Nearly all carbonatite occurrences are intrusives or subvolcanic intrusives. This is because carbonatite lava flows, being composed largely of soluble carbonates, are easily weathered and are therefore unlikely to be preserved in the geologic record. Carbonatite eruptions as lava may therefore not be as uncommon as thought, but they have been poorly preserved throughout the Earth's history.[3]

Carbonatite liquid compositions are significantly more alkaline than what is preserved in the fossil carbonatite rock record as composition of the melt inclusions shows.[4]

Only one carbonatite volcano is known to have erupted in historical time, the active Ol Doinyo Lengai volcano in Tanzania. It erupts with the coolest lava in the world, at 500–600 °C (932–1,112 °F). The lava is natrocarbonatite dominated by nyerereite and gregoryite.

  1. ^ Bell, Keith, ed. (1989). Carbonatites: Genesis and Evolution. London: Unwin Hyman.
  2. ^ Andersson, Magnus; Malehmir, Alireza; Troll, Valentin R.; Dehghannejad, Mahdieh; Juhlin, Christopher; Ask, Maria (2013-04-17). "Carbonatite ring-complexes explained by caldera-style volcanism". Scientific Reports. 3 (1): 1677. Bibcode:2013NatSR...3E1677A. doi:10.1038/srep01677. ISSN 2045-2322. PMC 3628075. PMID 23591904.
  3. ^ Stoppa, Francesco; Jones, Adrian P.; Sharygin, Victor V. (2009). "Nyerereite from carbonatite rocks at Vulture volcano: implications for mantle metasomatism and petrogenesis of alkali carbonate melts". Central European Journal of Geosciences. 1 (2): 131–51. doi:10.2478/v10085-009-0012-9. S2CID 130277058.
  4. ^ Guzmics, Tibor; Mitchell, Roger H.; Szabó, Csaba; Berkesi, Márta; Milke, Ralf; Ratter, Kitti (2012). "Liquid immiscibility between silicate, carbonate and sulfide melts in melt inclusions hosted in co-precipitated minerals from Kerimasi volcano (Tanzania): evolution of carbonated nephelinitic magma". Contributions to Mineralogy and Petrology. 164 (1): 101–22. Bibcode:2012CoMP..164..101G. doi:10.1007/s00410-012-0728-6. S2CID 129035814.