MAGNECRETES: FORMATION AND ALTERATION OF MAGNESIUM CARBONATES IN GROUNDWATER AND REGOLITH IN CENTRAL QUEENSLAND, AUSTRALIA, AND PROCESSES RELEVANT TO MARTIAN CARBONATE DEPOSITS

Mg carbonate minerals like magnesite (MgCO₃) are direct evidence of active water and carbon cycles on the surface of ancient Mars. As Mg is a dominant element in the Martian crust, and CO₂ was a major component of the Martian atmosphere, the recipe for Martian magnesite is apparent. However, the comparative scarcity of magnesite deposits on Earth contributes to limited process-based models for their formation. We report the distribution, textures, and elemental and stable isotopic compositions of widespread magnesite deposits in central Queensland, Australia, where the deep weathering of magnesite-bearing serpentinites creates Mg-rich groundwater that, in turn, forms magnesite nodules and indurated bodies by replacing alluvial sands. By analogy to groundwater calcretes, these “magnecretes” form diagenetically from groundwater ascending through the variably saturated and permeable capillary zone above the water table. This geochemically and hydrologically dynamic process leads to distinctive deposit geometries, mineralogies, microtextures, and compositions that preserve catchment-scale information on water-rock-biota interactions. Simultaneously, descending soil solutions cyclically alter both tectonic magnesite veins in the serpentinite bedrock and the diagenetic magnecretes in downstream sedimentary units. Although this process obscures some primary features of magnesite geochemical composition, it recrystallizes the magnesite and preserves additional surface environmental information, palimpsest textures and pseudomorphic minerals. In both the ultramafic and alluvial settings, magnesite formation, alteration, and destruction are closely linked to the silica cycle: magnesite replaces primary minerals, forms in association with diverse authigenic phyllosilicate minerals, and is replaced by chert and hydrous/amorphous silica phases. The close association of Mg-rich carbonates with phyllosilicate minerals and metal oxides in central Queensland is similar to meteorite, rover, and orbiter observations of Martian Mg carbonates. Frameworks for interpreting Martian carbonates should include the dynamic and complex processes moving magnesium, silica, carbon, and water from their sources to sinks.