Pedogenic carbonate minerals have progressively developed on basalt in a Quaternary weathering chronosequence (5-350 ka) on the island of Hawaii. The sites were isolated from direct contact with seawater or the water table, and were minimally influenced by eolian input throughout their history. The carbonates consist chiefly of high-magnesium calcite (HMC) and well-ordered stoichiometric dolomite. Pedogenic development of such minerals is rare, and the conditions of their formation bear on interpretations of paleosols and the development of sedimentary magnesian carbonates.
87Sr/86Sr ratios of chronosequence carbonates range from 0.7038 to 0.7048. The data indicate that > 75% of the strontium was derived from the weathering of volcanic parent material, even in the oldest soils, where few primary minerals are available to provide Ca and Mg. This is in contrast to continental calcretes, which are commonly dominated by atmospheric inputs and are generally composed of low-Mg calcite (LMC). Stable-isotope data are consistent with carbonate precipitation from soil waters in an arid environment.
The magnesium content of the carbonate increases with substrate age, and textural and mineralogical changes suggest direct precipitation of dolomite as well as dolomitization of early-formed LMC and HMC. Opal was detected in young (5-10 ka) samples, and gibbsite is present in older samples, indicating a shift from silica-dominated to aluminum-dominated soil solution. High concentrations of dissolved silica and aluminum during early weathering likely precluded formation of Mg-rich secondary clay minerals such as smectite or palygorskite. Weathering of basalt with a molar Mg/Ca ratio of 0.8-1.5, and the absence of secondary magnesian silicates, can account for elevated soil-water Mg/Ca ratios (0.7-1.0 in soils > 70 ka) that led to magnesian carbonate formation. Over time, the formation of a clay-rich soil profile prevented rapid evaporation of soil water, possibly facilitating crystallization of stoichiometric dolomite.