Textural and compositional evidence for the evolution of pedogenic calcite and dolomite in a weathering profile on the Kohala Peninsula, Hawai’i

Pedogenic dolomite and high magnesium calcite (HMC) occur as micritic matrix and void filling microspar in soils developed on semi-arid 350 Ka basalt surfaces on Hawai’i Island. The void filling carbonate presents an unusual appearance of alternating concentric < 5 μm to ≈ 30 μm layers of HMC and dolomite at shallow (100 cm) depths and similar layers that are exclusively dolomite between depths of 170 cm and 350 cm. Elevated Mg/Ca ratios in soil water, caused by rapid basalt weathering, likely provided geochemical conditions that permitted evaporative precipitation of magnesian carbonates. Variations in the amount of soil water, soil water chemistry, and/or crystallization rate probably controlled the precipitation of HMC vs. dolomite at 100 cm, whereas increased residence time of soil water and more stable soil water chemistry below the 170 cm depth led to the exclusive formation of dolomite.