SEM, PETROGRAPHIC STUDIES OF STALAGMITE CAPS AND TRAPPING OF FLUID INCLUSIONS

Most carbonate speleothems contain fluid inclusions of different sizes, which are believed to be trapped cave drip waters from which the speleothem precipitated. When formed under isotopic equilibrium conditions, these fluid inclusions can be used to obtain direct information about paleoprecipitation and to calculate absolute paleotemperatures by analyzing their hydrogen isotopic signals. However, the genesis of fluid inclusions is poorly understood. Six actively growing stalagmites were collected from three caves in British Columbia and Indiana (one from Marengo Cave, Indiana is made of aragonite). About 2cm thick growth caps were taken from each stalagmite to study growth surfaces under SEM, and then thin sections were made for petrographic analysis. It has been found that these stalagmites have voids of varying sizes to trap fluid inclusions, usually between 50-300 microns in width, but macro cavities of up to several millimeters and nanometer size pores as small as 200-500nm have also been observed. Stalagmites fed by drips with relatively short drop distances, high EC and drip rates, usually display perfect calcite rhombohedrons on cap surfaces and generally have very few loci along the c-axis for hosting fluid inclusions. However, during periods of more rapid growth (which might correspond to wetter climates) fluid inclusions may form along growth surfaces between, and elongated parallel to, the c-axis of calcite crystals. Surfaces of stalagmites formed by dripwater with long drop distances, lower electrical conductivity (EC) and drip rates, are dominated by irregular calcite rhombs with well developed micro voids and fractures: elongated inter-crystal fluid inclusions can be formed subsequently by partial coalescence of length-fast calcite, leading to a concentration of inclusions along the growth axis. On the surface of the aragonite stalagmite, micrometer-sized acicular crystals project, between which the voids occur. In thin section, voids are common between large botryoids or even between single crystal needles. In all stalagmites, hiatuses or weathering surfaces are enriched in inclusions, which may preserve a distinctive record of climate history at critical points in speleothem growth.