FORMATION MECHANISM OF MODERN DOLOMITE AND CA-BEARING MAGNESITE IN LAKE BEEAC, AUSTRALIA

Lake Beeac is a playa located in the southeast province of Victoria, Australia that has been studied for the last 30 years due to the large amount of sedimentary dolomite, magnesite, and Mg-rich smectite present (De Deckker and Last, 1989, Bristow et al. 2020). Lake Beeac lies in an area with Miocene to Holocene age olivine tholeiite and olivine basalt which is a good aquifer for artesian springs and lacks effective drainage pathways due to the high elevation surrounding the lake (De Deckker and Last, 1989, De Deckker, 2019). This aquifer likely supplies dissolved Mg, Ca, and Si into the playa. The average sedimentation rate for Lake Beeac can be calculated to be approximately 0.07 to 0.08mm of sediment per year based on measured ¹⁴C ages (De Deckker and Last, 1989, Bristow et al. 2020). XRD results show that ~ 52% of the volume of the average sediment comes from the carbonate phases. By using this percentage, the sedimentation rate becomes 0.036 to 0.042mm per year of dolomite and magnesite collectively. Lake Beeac magnesite has a d₁₀₄-spacing of 2.77Å which signifies 10 mol. % of CaCO₃ in the structure while the partially ordered dolomite has d₁₀₄-spacing ranging between 2.89-2.87Å, relating to a Mg percentage fluctuating between 50 and 55 respectively (Fang and Xu 2019, Hobbs and Xu 2021). TEM images show that both dolomite and magnesite range from ~ 20 nm to several hundred nm with a mosaic-like texture. The dolomite crystals show compositional variation and strain contrast. These observations support the idea that Lake Beeac is precipitating primary dolomite through the input of groundwater that is enriched in Ca, Mg, and Si rather than through dolomitization. The crystals are formed through surface-induced nucleation in presence of dissolved silica as a catalyst. This is a good proxy for paleo sea water. These past oceans were likely enriched in Si from weathered continental material that acted as a catalyst for dolomite precipitation by inhibiting the solvation shell surrounding the Mg²⁺ cation (Fang and Xu 2022). Dolomite cannot precipitate in modern seawater due to a relatively low concentration of Si.