Carbonate veins in marine manganese oxides from the Blake Plateau contain well-crystallized aragonite in stellate groups and needles up to 3mm long. Oxygen isotope analyses suggest that the aragonite formed at a temperature 4 degrees to 6 degrees C. colder than present sediment temperature of 8 degrees to 9 degrees C. The morphology of the aragonite, its complete enclosure in manganese nodules and the apparent 600 to 800 m water depth that prevailed throughout the growth of the nodule and veins, strongly suggest that the aragonite is of inorganic origin. Calcites with smaller than normal cell dimensions occur in some of the carbonate veins and may be due to the substitution of iron or manganese rather than magnesium ions in the calcite structure.

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