INVESTIGATING SMECTITE AS AN IMPORTANT INTERMEDIATE PRODUCT BETWEEN IGNEOUS MINEROLOGY AND A MATURE ASSEMBLAGE OF KAOLINITE-GROUP CLAYS, GIBBSITE, AND FE-OXIDES

The formation of soils and weathered rock is important for long term global climate studies as the weathering process of basalts effectively removes carbon dioxide from the atmosphere. Locally, a basic understanding of the nature and depth of the weathering profile is important for agriculture and engineering. Previous research regarding the weathering of ~2 Ma basalts on Oahu determined that weathering produces Fe-oxides and kaolinite group clays. However, there are localized areas of smectite, particularly in drier regions of Oahu. This clay may indicate an intermediate stage as basalt ultimately weathers to kaolinite, indicating our understanding of the processes of basalt weathering is incomplete.

To attempt to determine if smectite is a pervasive intermediate phase of basalt weathering, this research focusses on weathering of 0.4 Ma Pololu and 0.2 Ma Hawi lavas from the Kohala district of the Big Island of Hawaii, effectively reducing the length of in situweathering time by factors of 5 to 10. A preliminary study of soils and saprolites from various climates in this region of the Big Island indicates varying amounts (0% - 65%) of halloysite, kaolinite, gibbsite without smectite.

It is possible that smectite is present, but below the surface confined to narrow transition zones between weathered and fresh lava. To test this, we have sampled a wave cut cliff in the Kohala region that exposes complete weathering profiles where intermediate smectites might be preserved. Samples from the top of the cliff face are highly weathered and gradationally change to unaltered basalt at the base of the cliff, as well as gradually inward toward fresh core stones in certain thick horizons, likely the remnants of aa flows. Three vertical profiles were sampled by rappelling traverses. We anticipate that this will provide the best opportunity to capture intermediate swelling clays as an intermediate weathering product or rule them out as an important intermediary between igneous mineralogy and a mature assemblage of kaolinite-group clays, gibbsite, and Fe-oxides.