The Grønnedal-Ika complex is dominated by layered nepheline syenites which were intruded by a xenolithic syenite and a central plug of calcite to calcite–siderite carbonatite. Aegirine–augite, alkali feldspar and nepheline are the major mineral phases in the syenites, along with rare calcite. Temperatures of 680–910°C and silica activities of 0·28–0·43 were determined for the crystallization of the syenites on the basis of mineral equilibria. Oxygen fugacities, estimated using titanomagnetite compositions, were between 2 and 5 log units above the fayalite–magnetite–quartz buffer during the magmatic stage. Chondrite-normalized REE patterns of magmatic calcite in both carbonatites and syenites are characterized by REE enrichment (La_CN–Yb_CN = 10–70). Calcite from the carbonatites has higher Ba (∼5490 ppm) and lower HREE concentrations than calcite from the syenites (54–106 ppm Ba). This is consistent with the behavior of these elements during separation of immiscible silicate–carbonate liquid pairs. ε_Nd(T = 1·30 Ga) values of clinopyroxenes from the syenites vary between +1·8 and +2·8, and ε_Nd(T) values of whole-rock carbonatites range from +2·4 to +2·8. Calcite from the carbonatites has δ¹⁸O values of 7·8 to 8·6‰ and δ¹³C values of −3·9 to −4·6‰. δ¹⁸O values of clinopyroxene separates from the nepheline syenites range between 4·2 and 4·9‰. The average oxygen isotopic composition of the nepheline syenitic melt was calculated based on known rock–water and mineral–water isotope fractionation to be 5·7 ± 0·4‰. Nd and C–O isotope compositions are typical for mantle-derived rocks and do not indicate significant crustal assimilation for either syenite or carbonatite magmas. The difference in δ¹⁸O between calculated syenitic melts and carbonatites, and the overlap in ε_Nd values between carbonatites and syenites, are consistent with derivation of the carbonatites from the syenites via liquid immiscibility.

Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth’s internal dynamics. Element exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Here we show in high-pressure/low temperature rocks that element transfer during mineral dissolution and reprecipitation can occur in an alkali-Al–Si-rich amorphous material that forms directly by depolymerization of the crystal lattice and is thermodynamically decoupled from aqueous solutions. Depolymerization starts along grain boundaries and crystal lattice defects that serve as element exchange pathways and are sites of porosity formation. The resulting amorphous material occupies large volumes in an interconnected porosity network. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface. This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth.