CHEMICAL LEACHING OF OCEAN ISLAND BASALT: MULTIDIMENSIONAL REGRESSION OF PH, RAINFALL, VEGETATION COVER, AND TIME COMPARED TO LEACHING IN FIVE LAVA FLOW UNITS IN HAWAII

Chemical analyses of weathered basalt reveal that mobile elements are leached or enriched in varying degrees, as represented by the calculation of tau values where each element is normalized to its concentration in unaltered rock to minimize the effects of parental lithology. This study sought to determine the correlation of rainfall, pH, vegetation cover and time to tau. Rainfall, pH, and vegetation cover are highly correlated and reflect soil water conditions and oxidation state. Tau values were calculated from XRF analysis on soils from five flow units: Puna Basalt (~ 0 Ma), Hawi Volcanics (~ 0.17 Ma), East Molokai Volcanics (~ 1.40 Ma), Koolau Basalt (~ 2.4 Ma) and Makaweli Member (~3.7 Ma). The increasing ages allow for the observation of changes in these correlations with time. This work was enabled by several data sets collected over 15 years, in several studies of weathering in Hawaii.

The tau values of individual elements were compared to rainfall, pH, and vegetation cover in least squares regressions to estimate the correlation between them. Rainfall, pH, and vegetation cover explained from 0 to 70% of the variability in tau. The strongest of these correlations were found for tau of MnO, SiO2, CaO, and MgO. When compared across flows of various ages, the different tau values had the most significant correlations in Hawi volcanics and the least in Puna basalts with intermediate values in the other flow units.These patterns suggest that the dissolution of basalt is rapid initially and then proceeds at different rates. By 0.17 Ma, patterns of leaching are well established, but begin thereafter to exhibit varying rates of weathering, perhaps due to other processes. These may include original/secondary porosity and permeability regimes that affect water/rock ratios over time.

Basalt dissolution is a major source of soils and possible sink for atmospheric CO2. Understanding basalt dissolution is therefore a valuable area of research with diverse environmental implications. The data presented here indicates the need for further research into the processes of basalt dissolution, to understand the factors affecting tau variability not related to rainfall, pH, vegetation cover, or time.