Abstract

The petrography and geochemistry of marine magnesian calcite cements recovered from the sea floor on the lower-thermocline lower slope of Little Bahama Bank address three issues concerning inorganic carbonates: (1) crystal habit of marine magnesian calcite cements, (2) cation composition of marine precipitates, and (3) origins of cathodoluminescence in calcite.

Bladed and blocky limpid spar cements formed contemporaneously within the same intraparticle pore, as shown by banded cathodoluminescence traced without interruption through both cement habits. Bladed and blocky cements exhibit a positively covariant linear relationship for Mg and Sr, having low proximal (early formed) compositions and high distal (late formed) compositions. Linear covariance of Mg and Sr is interpreted to indicate that the concentrations of these cations were controlled by kinetic effects as they formed, with early precipitation occurring more rapidly than later precipitation. Precipitation rates apparently did not affect cement habit.

Limpid, isopachous, bladed spar cements, the distal third of which are brightly cathodoluminescent, have a wide range of Mg and Sr compositions, which is almost completely spanned by both distal cathodoluminescent and proximal noncathodoluminescent parts of cements occurring within the same pore. Mg and Sr do not have a linear relationship. These cements are interpreted to have recrystallized in deep, cool sea water. During recrystallization, magnesium was lost, and in some cases, manganese, the principal cathodoluminescence excitor, was presumably gained.

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