Fluid-rock reactions in ferrocarbonatites

Earth's mantle and crust are made almost entirely of silica-based rocks. Carbonatites are an exception – a family of unusual, carbon-based igneous rocks that may form by partial melting of the mantle or through igneous and hydrothermal processes in the crust. They are stock-full of stuff that doesn't usually like to be in rocks, such as volatiles (CO₂, H₂O, F, Cl) and rare earth elements (REEs) – actually in such high concentrations that these elements crowd together to form their own exotic minerals like burbankite, (Na,Ca)₃(Sr,Ba,REE)₃(CO₃)₅. Because of their unusual chemistry, carbonatites are in a state of severe thermodynamic disequilibrium with their environment. This imposes heavy secondary changes to their primary mineralogy and texture. In other words, carbonatites don't make it easy for us to understand them, which is why generations of igneous petrologists, including myself, become obsessed.

The aim of this project is to make sense of a rare iron-rich subgroup of carbonatites called ferrocarbonatites, found in the 1.3 Ga Grønnedal-Íka complex in southwest Greenland. The area is famous for a close-by cryolite deposit and a stunning submarine ikaite column garden, both unique natural wonders that exist because of the unique geochemical framework in the locality. The ikaite columns themselves are being studied by other members of the group for their low-temperature carbon sequestration potential.

Our approach to studying the history of the Grønnedal-Íka carbonatites was to combine microtextural analysis with the trace element chemistry of the main minerals (siderite, calcite, ankerite-dolomite-kutnohorite, magnetite), determined in situ by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Trace element patterns of minerals are dependent on the environment in which the minerals form and may be used to tease out details of fossil igneous and hydrothermal events. Connecting these two independent sources of information comes with an advantage compared to conventional bulk chemical analytical methods, which are devoid of textural context and therefore risk mixing together primary and secondary chemical signals.

The main outcome of this study was to establish the trace element systematics for common minerals in ferrocarbonatites. Most importantly, we identify chemical signals that make it possible to separate between secondary and primary mineral assemblages and provide the first evidence for the existence of primary igneous siderite.