Botryoidal aragonite and its diagenesis

Botryoidal aragonite is a spectacular growth-form occurring as mamelons, up to 100 mm in diameter. Three examples of this particular carbonate cement have been discovered in two distinct areas: in New Caledonia, small-scale mamelons have been recognized within Pleistocene reefal terraces at Ouvea, an Island of the Loyalty archipelago, and in the Red Sea, large-scale mamelons of botryoidal aragonite exist within Pleistocene reefal terraces along the Um Gheig region of the Egyptian Coast. In addition, a similar botryoidal cement, partly dolomitized but exhibiting aragonite relics, occurs within a Miocene reef in the same region. Mamelons of botryoidal aragonite are isolated and/or coalescent but grow only on fixed substrates. They occur within cavities of varied origin. Their fabrics are characterized by fans of elongate euhedral crystals of aragonite fibres. Botryoidal aragonite can be preceded or followed by other types of cements or internal sediment. Despite similar mineralogy, petrography and ultrastructure, there are differences between the fabrics of the Pleistocene botryoids from the Red Sea and Ouvea and the Miocene botryoids from the Red Sea. The former are of submarine origin as confirmed by the strontium content (8500–10,500 ppm) and isotopic composition (δ¹⁸O between -0.10 and +0.19% PDB). The latter, related to a Miocene karst, are rich in strontium (average 13,600 ppm), and have an isotopic composition (average δ¹⁸O -10.50% PDB) indicative of non-marine precipitation. Diagenesis of these botryoidal aragonites consists of slight dissolution for Pleistocene botryoids and mineralogical transformation for the Miocene botryoids. The latter exhibit the diagenetic sequence aragonite → calcite → dolomite. The aragonite to calcite transformation is a dissolution-reprecipitation process, the void distribution and size influencing the distribution and the size of the replacement calcite crystals.