Calcite-saturated natrocarbonatites: composition, crystal morphology, and weathering

Interpretation of calcite-dominated fossil carbonatite volcanoes is complicated by the instability of many igneous carbonatite minerals on Earth’s surface. One hypothesis suggests that they originate by eruption of alkali-free calcic carbonatite lavas. However, liquid calcite is not thermodynamically stable at atmospheric pressure. A second hypothesis suggests that calcite is secondary and formed after magmatic alkali carbonate minerals, primarily nyerereite, lost their Na or K to surface water. Here, we experimentally test a combined hypothesis in which solid calcite phenocrysts are suspended in natrocarbonatite lava that solidifies primarily to nyerereite, and determine calcite solubilities in sodic carbonate liquids. Then, we dissolve alkalis in water over several months to show formation of secondary calcite after nyerereite. Textural and geochemical observations from our experiments are consistent with many natural volcanic carbonatites. Magmatic calcite occurs as rounded and elongated sub-hexagonal prisms, whereas secondary calcite exhibits a range of fine-grained morphologies. Magmatic calcite only contains Sr as a significant minor element, whereas secondary calcite is chemically diverse with Na, K, P, and Ba as important minor elements. Strontium is also present in secondary calcite, but in lower concentrations than primary magmatic calcite. Calcite types with both primary and secondary characteristics occur in many natural carbonatites, indicating that alteration of initially calcite–nyerereite-bearing natrocarbonatites to calcite carbonatites was more common in Earth’s geological past than previously recognised.