TEXTURAL AND COMPOSITIONAL EVIDENCE FOR THE EVOLUTION OF PEDOGENIC CALCITE AND DOLOMITE IN A WEATHERING PROFILE ON THE KOHALA PENINSULA, HAWAI'I

Magnesian calcite and well-ordered stoichiometric dolomite occur in a soil chronosequence developed on 5 ka to 350 ka basalt flows on the semi-arid southwestern Kohala Peninsula on the Island of Hawai’i. Capo et al. (2000; Geol., v. 28, no. 3) and Whipkey et al. (2002; J. Sed. Res., v. 72, no. 1) noted a progressive increase in Mg content in the soil carbonates with increasing flow age and, using Sr isotopes, demonstrated that the source of carbonate Ca²⁺ and Mg²⁺ ions is primarily the subaerial weathering of basalt. New microtextural and compositional evidence provides further details regarding the genesis of calcite and dolomite in these soils.

In a weathering profile developed on a 350 ka Pololu Volcanics lava flow, pedogenic carbonate occurs below 1 m depth as micritic calcareous coatings and fracture fill on relatively unweathered basalt. Abundant mm-scale voids, probably related to root activity, in this micritic matrix are generally lined with isopachous or blocky carbonate microspar. Stained thin sections (Alizarin S Red) and SEM-EDS analysis indicate progressive depth-related changes in matrix and void-filling carbonate mineralogy. At 100 cm depth, magnesian calcite is the dominant matrix carbonate, and voids are commonly lined with several roughly concentric layers (each typically < 5 µm to ~30 µm thick) of calcite and dolomite microspar. Similar concentric zonation is observed in voids at 170 cm; however, at this depth there is an increased proportion of dolomite relative to calcite in the void lining, and the matrix is predominantly dolomite. Below 220 cm to the maximum sampled depth of 350 cm, soil matrix and voids are essentially completely dolomitic; zonation of void carbonates is not distinguished in stained thin sections (SEM and EMPA work is in progress). Other current work attempts to discern whether void-filling dolomite layers in the upper profile (100 cm and 170 cm) reflect partial dolomitization of early-formed calcite, or alternating calcite/dolomite deposition, possibly due to variations in pore water chemistry.