Trace element compositions and redox shifts preserved in magnesites, sediments and soils from the Kunwarara magnesite mine

Magnesium carbonates in surficial environments act as CO₂ sinks and can record aspects of the palaeohydrological cycles on Earth and Mars. In natural environments, magnesium carbonates can be intimately intermixed at the micrometer scale with complex assemblages of other non-carbonate minerals. To better determine magnesium carbonate composition in complex samples and minimize contamination from secondary Fe/Mn-oxides/hydroxides, we developed and assessed methods for sample cleaning, selective digestion, and quadrupole inductively coupled plasma mass spectrometry to measure the trace and minor elemental composition. By pre-cleaning and selectively digesting carbonate, we identified previously unrecognized geochemical trends in magnesite ± dolomite nodules and their host fluvial sediments collected along a depth profile at the Kunwarara magnesite mine, Queensland, Australia. In particular, Ce anomalies in magnesite diminishes with depth coincident with decreasing abundances of authigenic Fe/Mn-oxides/hydroxide minerals in the host sediments. These results reveal how the magnesium carbonates capture interactions between the ascending groundwaters and descending surface waters. We further demonstrate the value of magnesium carbonate-specific trace element data with reanalysis of previously published ion microprobe data from Martian meteorite ALH84001, which also shows Ce fractionation. Accurate mineral-specific trace and minor element measurements in Earth and Martian magnesium carbonate samples improve our understanding of the timing and identities of carbonate mineral-forming reactions that occurred on both planets.