The origin of carbonate mud in isolated carbonate platforms of Belize, Central America

Carbonate mud from three small isolated carbonate platforms of Belize (Central America) is largely a product of the breakdown of skeletal grains. The composition of the 63-20- and 20-4-mum fractions of 37 samples was determined by point counting under SEM; composition of the <4-mum fraction was assessed qualitatively under SEM. The 63-4-mum fractions are dominated by fragments of mollusks, the codiacean alga Halimeda, and other skeletal types. About one-third of the particles in the 63-4-mum fractions remained unidentified, probably due to obliteration of diagnostic features by early recrystallization processes such as micritization. Nano-grains (<1 mum) and short (3-5 mum) aragonite needles are most common in the <4-mum fraction. These grains are interpreted to be largely fragments of codiacean algae (Halimeda, Penicillus). The 20-4- and <4-mum sediment fractions are composed of 80 and 75% aragonite on average, respectively. Trace element composition of strontium averages 7,900 ppm in the 20-4-mum fraction and in codiacean algae samples, and 5,600 ppm in the <4-mum fraction. Geochemical (trace element) data also argue against inorganic aragonite precipitation within the water column, and favor a skeletal origin. The lower strontium contents in the <4-mum fraction as compared with the 20-4-mum fraction may also be a consequence of early recrystallization processes.