Origin of Fabrics in Speleothems Composed of Columnar Calcite Crystals

Abstract

ABSTRACT Most calcite in speleothems is composed of columnar crystals (palisade calcite) and exhibits fabrics similar to those in some porefilling calcites interpreted to be replacive after acicular carbonate cements. The columnar crystals do not interfere with each other's growth (suggesting that they are secondary features) and this, together with the occurrence of layers of acicular calcite in some speleothems, leads to a conclusion that columnar crystals have replaced acicular carbonate. The evidence, however, is misleading. The same crystal fabrics can be explained by normal, but somewhat complex, growth processes. Inclusions (and patterns made by them) constitute the most important clues to the origin of the columnar crystals. Most inclusions are fluid-filled cavities and six types of growth layering are distinguished on the basis of variations in inclusion abundance, size and pattern. Growth layers defined by parallel, linear inclusions are interpreted to have formed during the incomplete lateral coalescence of numerous syntaxial overgrowth crystallites which grow upon the speleothem surface. The linear inclusions represent remnants of the former inter-crystallite spaces. Complete crystallite coalescence generates inclusion-free calcite, whereas inhibition of the lateral coalescence of the overgrowth crystallites generates layers of acicular calcite. During episodes of cave-flooding, however, he crystallites merge and overgrow each other and precipitation eventually occurs upon large, planar crystal faces. It is believed that the distinctive fabrics of palisade calcite are formed because precipitation usually occurs from thin water films that flow over the growing speleothem surfaces. Large crystal terminations do not form on the speleothem surface because they form projections that disturb the water flow away from the projections which, as a consequence, are gradually eliminated. Small crystal terminations (crystallites), on the other hand, do not disturb the water-flow and thus come to dominate the growth surfaces. Petrographic distinction columnar calcite crystals in speleothems (and other vadose calcites with similar fabrics) and mosaics of columnar crystals that have replaced earlier, acicular-carbonate cements is commonly difficult. Such distinctions are attempts to distinguish between calcite crystals that have grown penecontemporaneously from numerous syntaxial overgrowths (calcites in speleothems) and other calcites in which replacement occurs at a much later date, possibly accompanied by replacement of a metastable phase (replacement of acicular cements).