Biogeochemistry of carbonate-silicate deposits associated with microbial mats in basaltic caves, Kauai, Hawaii

Secondary mineral deposits in basaltic sea caves on the island of Kauai, Hawaii were investigated to determine their nature and origin. The deposits are extremely variable in macroscopic character, ranging from millimeter-thin, powdery coatings to hard crusts several centimeters thick. Complex assemblages of aragonite, calcite, monohydrocalcite, dolomite, magnesite, hydromagnesite and talc-like kerolite were identified by X-ray diffraction and electron probe microanalysis. The clay mineral component, dominated by kerolite, was further elucidated by a combination of detailed X-ray diffraction analysis, electron microscopy and thermal analysis. Minor amounts of gypsum, kaolinite and poorly crystalline serpentine also occur in the deposits. Major cation and strontium isotope ratios in the cave waters and minerals indicate that the secondary minerals have crystallized from fresh water seeping from the basaltic host rock, and the mineralization process is ongoing at some locations. Many deposits are associated with actively forming microbial mats. Scanning and transmission electron microscopy indicate that these mats are dominated by bacteria and cyanobacteria, and contain abundant extracellular polymers, which concentrate ions from solution and serve as nucleation sites for mineral crystallization. Microstromatolitic structures are common in the thicker deposits suggesting that they are essentially lithified microbial mats. Stable isotopes of carbon and oxygen, geochemical modelling using PHREEQC, and petrographic evidence suggest that the formation of the mineral deposits is the result of a combination of direct precipitation from solution via evaporation and CO₂ degassing, alteration of precursor minerals, microbial processes (e.g., photosynthesis) and physicochemical conditions within the mats. This is believed to be the first known detailed description of such an occurrence of minerals from basaltic caves and the first known occurrence of kerolite in Hawaii. These deposits provide important insights into the mineralogy and geochemistry of low-temperature alteration and carbonation of terrestrial basalts, and the potential role of microbes in such processes. Such information is useful for understanding the feasibility and limitations of subsurface geological disposal of CO ₂, especially in basaltic rocks.