The role of speciation in alkaline igneous fluids during fenite metasomatism

Abstract

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.

In the east African Kisingiri nephelinite volcano, ijolite intruded a granodioritic basement, producing a fenitized contact aureole. During metasomatism of granodiorite, according to the mass balance model of Rubie (1982), feldspar only participated in an alkali exchange reaction, while quartz was replaced by sodic pyroxene. Outward from the intrusive contact, with decreasing temperature, feldspar became progressively K-enriched, while pyroxene was enriched in the acmite component. It is predicted that alkaliexchange reactions were controlled by NaCl⁰-KCl⁰ aqueous complexes close to the intrusive contact, while further out in the aureole Na⁺-K⁺ ions dominated at the lower temperatures and enhanced the level of potassium metasomatism of feldspar. With decreasing temperature in the aureole, the K/Na ratio of the fluid decreased and consequently the activity of acmite increased.

Around carbonatites, where the level of CO₂ in the escaping fluid can be expected to be high, Na₂CO ⁰₃ -K₂CO ⁰₃ complexes may dominate. Alkali exchange between feldspar and these aqueous species enhances, even further, the stabilization of pure potassium feldspar. Boiling may also play an important role in potassium metasomatism as carbonatites are frequently associated with pyroclastic rocks. Formation of hematite instead of sodic pyroxene may be attributed to low$a_{SiO_2 }$, high$a_{Fe^{3 + } }$ and a CO₂-rich fluid.

Important variables which determine the products of alkali metasomatism are shown to be temperature, pressure and CO₂ content of the fluid, as well as the K/Na ratio of the fluid.