[S-CG45] Petrology, Mineralogy & Resource Geology

[SCG45-P07] Geochemical and morphological features of pedogenic dolomite in South Australia

*Aki Sakuma¹, Akihiro Kano¹, Chizuru Takashima², Akira Murata¹, Asuka Yamaguchi³, Taiga Okumura¹, Kana Nagashima⁴ (1.Department of Earth and Planetary Science, School of Science, the University of Tokyo, 2.Faculty of Education, Saga University, 3.Atmosphere and Ocean Research Institute, The University of Toky, 4.Earth Surface System Research Center, Japan Agency for Marine-Earth Science and Technology)

Keywords:Pedogenic Carbonate, Dolomite, South Australia

Pedogenic carbonates are carbonate minerals precipitated from pore water in soils in arid areas with strong evaporation, and found as nodules or calcrete layers in the soils of various ages and regions. While pedogenic carbonate mostly consists of high-magnesium calcite, dolomite is also occasionally precipitated in soils (ex, Kearsey et al., 2012). To elucidate the precipitation mechanism, it is better to observe the modern pedogenic carbonates rather than those of geologic ages which might be affected by diagenesis. However, precipitation of dolomite in modern soils is rarely reported (Capo et al., 2000). In addition, previous studies of Quaternary pedogenic dolomite are only limited to soils developed on mafic volcanic rocks and the precipitation of dolomite in soils developed on other rocks such as sedimentary rocks and metamorphic rocks have not been reported yet. Although dolomite is a thermodynamically stable mineral, it is difficult to simulate the natural conditions for precipitation of dolomite crystals in laboratory, and synthesis has required special conditions such as microorganisms or high temperatures. Recently, it was pointed out that repetition of evaporation and dissolution caused the precipitation of dolomite crystals in playa lakes and coastal environments under a high contrast in the climate seasonality (Kim et al., 2023). Pedogenic dolomite precipitation also occurs with seasonal precipitation changes, which may reveal the precipitation mechanism of dolomite in natural environments.
In this study, we observed modern pedogenic carbonate samples collected from three soil profiles developed on different types of host rocks in South Australia with polarized microscope and scanning electron microscope, and conducted elemental analysis, identification of minerals, and carbon and oxygen isotope analysis. We found that the pedogenic carbonate samples are mainly composed of calcite, quartz, dolomite, and feldspar based on bulk XRD pattern. In the pedogenic carbonate samples precipitated in soils developed on sedimentary rocks, dolomite crystals are distributed around the particles and in laminated structures based on µXRF and µXRD analyses of polished slab samples. Whereas in soils developed on metamorphic rocks, dolomite crystals are distributed relatively homogenously between clastic grains. In some samples, euhedral rhombohedral dolomite crystals are observed by SEM observation. Carbon and oxygen isotope shows typical values of pedogenic carbonate, which supports the precipitation of carbonate in soils rather than the capture of carbonate-rich clasts finely crushed by physical destruction during pedogenesis.