carbonatite

Exotic Rocks: Carbonatites: 

Exotic Rocks: Carbonatites Rachel Potter March 3, 2008 Geol789P Short Presentation http://www.geology.sdsu.edu/how_volcanoes_work/Unusual%20lava.html

Carbonatite Definition and Chemistry: 

Carbonatite Definition and Chemistry Definition: >50 modal % carbonate minerals Unusual chemistry: Very low SiO 2 (<10%) High REE High volatile content (CO 2 , F, Cl, S) High Ba, Sr, P, Nb These chemical compositions indicate igneous formation from carbonatite magma sources

Carbonatite Mineralogy: 

Carbonatite Mineralogy Calcite Calcite-carbonatite (s ö vite) Dolomite Dolomite-carbonatite Ankerite: Ca(Fe, Mg, Mn)(CO 3 ) 2 Ferrocarbonatite Natrocarbonatite Na-K-Ca Oldoinyo Lengai Associations Clinopyroxene Alkali Amphibole Biotite Magnetite Apatite 20% CaO FeO + Fe 2 O 3 + MnO MgO 50% F D C Woolley and Kempe, 1989

Chemistry continued: 

Chemistry continued Low Sr ratios indicate a mantle source East African Carbonatite Line: Mixing of 2 long lived reservoirs EMI and HIMU Addition of H 2 O and CO 2 can produce dolomitic or carbonated phlogopite-peridotites at >70 km depth Taken from Winter, 2001

Producing carbonatite melt: 

Producing carbonatite melt Partial melting of hydrous-carbonated lherzolite Fractional crystallization of alkaline silicate melt Liquid immiscibility between alkaline silicate melts and carbonatite melts

Occurrences (~350 known): 

Occurrences (~350 known) Intrusive: Form as individual intrusions in crustal domes or grouped intrusions in a crustal swell (1000km) Temps of 500° to >1000°C Extrusive: Only known active volcano is Oldoinyo Lengai Temps measured at 576-593°C (Pinkerton et al., 1995)

Occurrences: 

Occurrences Intraplate settings (over half are in Africa) Oceanic: Cape Verde Islands, Canary Islands Continental: associated with rifting? Small intrusive complexes Occur with alkaline silicate rocks Less common with mafic rocks Associated with pyroxenites, peridotites, Group 1 kimberlites, and lamprophyres

Typical Sequence for Alkaline complexes: 

Typical Sequence for Alkaline complexes Shallow early alkaline intrusion Carbonatite intrusion cuts alkaline rocks Dike or cone-sheets coming off carbonatite intrusion, Fe-enriched calcite + mag, ankerite, or siderite Pipe-like bodies, Fe and REE enriched brecciated, replacement textures, hydrothermal Fenitization : metasomatic alteration of wallrocks to fenites addition of alkalis and desilification of wallrock quartzo-feldspathic rocks  aegerine, alkali amphibole, etc.

Hand Sample: 

Hand Sample www.dvminerals.com/img2006/Ro-122.jpg Schellingen, Kaiserstuhl, Germany Pyrochlore (Koppite) (Na,Ca) 2 Nb 2 O 6 (OH,F) www.umanitoba.ca/.../arc/pictures/carb-kvd.jpg Quebec Magnetite (Fe 3 O 4 ) ijolite: nepheline and augite

Thin Section: 

Thin Section www.unich.it/~stoppa/images/polino1.gif Umbria, Italy Extrusive Carbonatite www.umanitoba.ca/.../arc/pictures/carb-kvd.jpg Kovdor, Russia calcite (Cal) phlogopite (Phl) resorbed crystals of forsterite (Fo) fluorapatite (FAp).

Oldoinyo Lengai: 

Oldoinyo Lengai East African Rift: Northern Tanzania http://web.mst.edu/~rhagni/OLphotos.html

Oldoinyo Lengai: 

http://www.geology.sdsu.edu/how_volcanoes_work/Unusual%20lava.html Erupted 1958-1966, and 1983-present T = 576-593°C, low density, low viscosity Phenocrysts of nyerereite (Na,K) 2 Ca(CO 3 ) 2 gregoryite (Na,K) 2 CaCO 3 Matrix of nyerereite, gregoryite, sylvite, fluorite Most of volcano is actually nephelinite and phonolite Oldoinyo Lengai Hornitas

Zaitsev and Keller, 2006: 

Zaitsev and Keller, 2006 thermonatrite nahcolite trona pirssonite

Zaitsev and Keller, 2006: 

Zaitsev and Keller, 2006 Nyerereite (Nr) replaced by pirssonite Field of view is 1.37 mm Shortite (Sh), nahcolite (Nh) and calcite– thermonatrite–trona–fluorite groundmass (Gr) Field of view is 1.37 mm. Nahcolite vein with comb structure Field of view is 3.45 mm