The role of speciation in alkaline igneous fluids during fenite metasomatism

Fenites associated with alkaline igneous rocks show a progression from a high temperature assemblage consisting of sodium-rich alkali feldspar + a sodium-ironrich mafic mineral, to an extreme end member assemblage consisting of pure potassium feldspar + iron oxide. The latter assemblage is only found in association with low temperature carbonatites. Segments of this distribution trend can be found in the contact aureole of single intrusive centers.