What Is Carbonatite?

  • Definition: Carbonatite is an intrusive or extrusive igneous rock characterized by its mineralogical composition, which consists of more than 50% carbonate minerals1.
  • Distinctive Features:
    • Carbonate Minerals: These rocks contain a significant proportion of carbonate minerals, such as calcite and dolomite.
    • Geochemical Verification: Due to their similarity to marble, carbonatites may require geochemical analysis for confirmation.
    • Occurrence: Carbonatites are typically found as small plugs within alkalic intrusive complexes, dikes, sills, breccias, and veins. They are closely associated with continental rift-related tectonic settings.
    • Historical Rarity: Although carbonatite lava flows are rare due to weathering (as they consist largely of soluble carbonates), one notable exception is the active Ol Doinyo Lengai volcano in Tanzania, which erupts with the world’s coolest lava at 500–600 °C (932–1,112 °F).

Genesis and Formation Models:

  1. Magmatic Origin:
    • Swedish geologist Harry von Eckermann argued for the magmatic origin of carbonatite based on his study of the Alnö Complex.
    • The eruption of Ol Doinyo Lengai in Tanzania in 1960 confirmed that carbonatite is indeed derived from magma.
  2. Formation Models:
    • Low-Degree Partial Melts: Some carbonatites form directly from very low-degree partial melts in the mantle.
    • Liquid Immiscibility: Others result from immiscibility between a carbonate melt and a silicate melt.
    • Extreme Crystal Fractionation: Peculiar crystal fractionation processes also contribute to their formation.

Global Occurrences:

  • Carbonatites span Earth’s history, with ages ranging from Archean to present.
  • Notable examples include Tupertalik in Greenland (3007 Ma old) and the ongoing activity at Ol Doinyo Lengai in Tanzania.

In summary, carbonatites are enigmatic rocks formed by unusual processes and sourced from unexpected materials. Their composition challenges traditional notions of igneous rock formation, making them a fascinating subject of study in geology123.