Processes of crustal carbonatite formation by liquid immiscibility and differentiation, elucidated by model systems

Citation
Wj. Lee et Pj. Wyllie, Processes of crustal carbonatite formation by liquid immiscibility and differentiation, elucidated by model systems, J PETROLOGY, 39(11-12), 1998, pp. 2005-2013
Citations number
43
Categorie Soggetti
Earth Sciences
Journal title
JOURNAL OF PETROLOGY
ISSN journal
00223530 → ACNP
Volume
39
Issue
11-12
Year of publication
1998
Pages
2005 - 2013
Database
ISI
SICI code
0022-3530(199811/12)39:11-12<2005:POCCFB>2.0.ZU;2-
Abstract
Experimental studies on several silicate-carbonate joins provide a framewor k in the system CaO-Na2O-(MgO + FeO)-(SiO2 + Al2O3) (+ CO2) which illustrat es possible processes for the formation of carbonatites. The two key featur es are the silicates-carbonate liquidus surface, and the miscibility gap li quidus surface. Crystallizing parental carbonated silicate melts may reach a silicate-CO2 eutectic, a silicate-carbonate field boundary, or a miscibil ity gap. Some hydrous carbonated silicate melts may bypass the high-tempera ture miscibility gap and reach the silicate-carbonate field boundary. Immis cible carbonate-rich liquids in model systems simulating magmatic condition s tend to be concentrated near calciocarbonatite compositions (< similar to 80% CaCO3; e.g. nepheline sovite), but may be more alkalic from silicate p arents with higher Na/Ca values. An immiscible carbonate-rich liquid separa ting from the high-temperature parent silicate liquid will cool with the pr ecipitation of silicates only, until it reaches the silicate-carbonate fiel d boundary, where it is capable of precipitating carbonate minerals, which can form carbonate cumulates. Some parents may reach this boundary by direc t crystallization, but most probably traverse the miscibility gap. Along th is field boundary, the coprecipitation of calcite drives the liquid toward residual alkali-rich compositions. The carbonate liquidus (>85% CaCO3) is a 'forbidden volume' for magmas. Vapor loss from carbonatite magma can intro duce alkalis into country rocks, but this does not cause alkali depletion o f magma; calcite precipitates to maintain the magma composition. Hydrous ma gnesiocarbonatite magmas can precipitate cumulate sovites.