Formation and alteration of magnesite nodules from Kunwarara, Queensland, Australia, as an analogue to Mg-carbonate formation on Mars

Emily Lewis Cardarelli, Theodore M Present, Paulo Vasconcelos, Linda Kah, Carl Swindle, Surjyendu Bhattacharjee, Kenneth Farley

Magnesite (MgCO3) is a magnesium carbonate mineral that records the aqueous environmental conditions of its formation. On Earth, magnesite forms in metamorphic, diagenetic or pedogenic environments, and distinguishing between these environments is critical for understanding the nature of fluid chemistry during magnesite precipitation. Magnesite has been identified across the Nili Fossae region on Mars and in Jezero crater by orbital spectroscopic observations and in-situ instrument observations acquired with the Perseverance rover. Core samples with magnesite may provide constraints on the chemical conditions of the ancient aqueous environments of Jezero crater, and may also be an important phase to target for the preservation of potential biosignatures. This work explores pedogenic magnesite phases found in Vertisols of the Kunwarara Mine, Australia, as a potential analog environment for magnesite identified within Jezero crater, Mars. We document the principal microtextures and investigate the processes involved in the formation and diagenesis of magnesite nodules and magnecretes. Kunwarara magnesite nodules show complex textural relationships at the outcrop scale, and these relationships extend to the nanoscale in samples that were collected along a depth profile. By characterizing textural and chemical variations in magnesite at different scales, this work reveals a continuum between diagenetic and pedogenic magnesites. It illustrates that diagenetic reactions produce magnesite from ascending Mg²⁺-rich groundwater interacting with detrital phases; groundwater interaction with descending meteoric solutions result in the conversion of magnesite into authigenic dolomite. Overall, this work shows how the superposition of textures and elemental compositions permits reconstruction of pedogenic processes leading to magnesite authigenesis.