DIRECT DOLOMITE PRECIPITATION: INVESTIGATION OF LACUSTRINE DOLOMITE AND MAGNESITE IN LAKE BEEAC, VICTORIA, AUSTRALIA

Abstract

Lake Beeac is a playa lake located in the southeast province of Victoria, Australia that has been studied for the last 40 years due to the large amount of sedimentary dolomite, magnesite, and Mg-rich smectite present (De Deckker and Last, 1989; De Deckker, 2019; 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 lakes (De Deckker and Last, 1989, De Deckker, 2019). This aquifer likely supplies dissolved Mg, Ca, Si and dissolved bicarbonate into the playa. The average sedimentation rate for Lake Beeac can be approximated 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 0.042 mm per year of dolomite and magnesite collectively. Lake Beeac magnesite has a d104-spacing of 2.77 Å which signifies ~10 mol. % of CaCO3 in the structure while the partially ordered dolomite has d104-spacing ranging between 2.89-2.87 Å, 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 mosaic-like texture. The dolomite crystals show compositional variation and strain contrast in TEM images. Additionally, the presence of authigenic dioctahedral Mg-bearing illite and trioctahedral saponite support the presence of Lake Beeac is a playa lake located in the southeast province of Victoria, Australia that has been studied for the last 40 years due to the large amount of sedimentary dolomite, magnesite, and Mg-rich smectite present (De Deckker and Last, 1989; De Deckker, 2019; 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 lakes (De Deckker and Last, 1989, De Deckker, 2019). This aquifer likely supplies dissolved Mg, Ca, Si and dissolved bicarbonate into the playa. The average sedimentation rate for Lake Beeac can be approximated 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 0.042 mm per year of dolomite and magnesite collectively. Lake Beeac magnesite has a d104-spacing of 2.77 Å which signifies ~10 mol. % of CaCO3 in the structure while the partially ordered dolomite has d104-spacing ranging between 2.89-2.87 Å, 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 mosaic-like texture. The dolomite crystals show compositional variation and strain contrast in TEM images. Additionally, the presence of authigenic dioctahedral Mg-bearing illite and trioctahedral saponite support the presence of dissolved silica within the groundwater at concentrations at or exceeding 1mM. 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. Paleozoic seawater was likely enriched in dissolved silica from weathered continental materials that acted as a catalyst for dolomite precipitation by weakening water molecules surrounding the surface Mg2+ (Fang and Xu 2022). Dolomite cannot precipitate in modern seawater due to a relatively low concentration of dissolved silica Si.