Natural carbonation in alkali basalts: Geochemical evolution of Ca–Mg–Fe carbonates at Sverrefjellet, Svalbard

This study investigates hydrothermal carbonate cements in Quaternary alkali basalts from the Sverrefjellet volcano (Svalbard), offering insights into in-situ natural mineral carbonation. XRD and SEM-BSE-EDS analyses identify two main morphologies, nodular and banded, composed of solid-solution series between magnesite, calcite, and siderite, with distinct compositional zonation. Nodular cements usually show concentric zoning from Mg-rich cores (Ca_0.05Mg_0.95CO₃) to Ca-enriched rims (Ca_0.40Mg_0.60CO₃), reflecting evolving fluid chemistry. Fe-rich nodules (Ca_0.10Mg_0.50Fe_0.40CO₃) are found near pyrite and display dissolution textures linked to localized redox reactions. Banded cements initiate at the basalt interface as Ca-rich proto-dolomite (Ca_0.65–0.58Mg_0.35–0.42CO₃), transitioning outward to magnesite (Ca_0.10Mg_0.90CO₃) and ferroan magnesite (Ca_0.10Mg_0.50Fe_0.40CO₃). Ca/Mg ratios decrease with distance from the interface (1.81 to 0.13), while Fe/Mg exceeds 13.5 locally due to Fe-rich coatings and inclusions. Four sequential crystallization stages were identified: (1) irregularly laminated Ca-Mg carbonates, (2) oscillatory-zoned dolomite-magnesite, (3) radiaxial-fibrous Ca-bearing magnesite, and (4) Fe-oxide-rich nanocrystalline rinds. Basaltic silicate and glass dissolution (forsterite, enstatite, anorthite) supplied divalent cations. Redox shifts promoted Fe incorporation. Early Ca²⁺ depletion altered fluid chemistry toward Mg²⁺ and Fe²⁺, while oscillatory zoning reflects episodic fluid compositional variations. Pyrite and siderite dissolution imply late-stage oxidation and secondary porosity development. These carbonates are hydrothermal in origin, supported by high-temperature phases, fan-like growth textures, and Ca-to-Mg/Fe transitions, consistent with fluid-rock interaction at 60–220 °C and pH 5.2–6.5. The absence of hydrated carbonates and presence of alteration phases also supports hydrothermal precipitation. Comparisons with engineered systems (e.g., CarbFix) underscore the role of temperature in overcoming kinetic barriers to magnesite formation, though metastable proto-dolomite and Mg sequestration in clays reveal limits to carbonation efficiency. These findings constrain predictive models for CO₂ mineralization in basaltic reservoirs, highlighting the interplay of hydrothermal conditions, fluid evolution, and reaction kinetics.