--> ABSTRACT: Cathodoluminescent Bimineralic Ooids from Pleistocene of Florida Continental Shelf, by R. P. Major, R. B. Halley, and K. J. Lukas; #91038 (2010)

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Cathodoluminescent Bimineralic Ooids from Pleistocene of Florida Continental Shelf

R. P. Major, R. B. Halley, K. J. Lukas

A bored and encrusted late Pleistocene ooid grainstone was recovered from the sea floor at a depth of approximately 40 m on the continental shelf of eastern Florida. Ooid cortices are dominantly bimineralic, consisting generally of inner layers of radial magnesian calcite and outer layers of tangential aragonite. The transition from radial to tangential cortex layers occurs at an approximate diameter of 0.30 to 0.35 mm. Ooid nuclei are predominantly rounded cryptocrystalline grains, although quartz grains and a variety of skeletal grains also occur as nuclei. Ooids were partially cemented by blocky calcite, followed by partial infilling of interparticle porosity by micrite and subsequently by sparse acicular cement.

The radial cortex layers and some ooid nuclei are brightly cathodoluminescent, whereas the tangential cortex layers, micrite matrix, acicular cement, and the bulk of blocky calcite cement are nonluminescent. Some blocky calcite cement has a thin outer cathodoluminescent zone. Electron microprobe analyses indicate that radial layers contain approximately 10 mole % MgCO3 and tangential layers generally contain 10,000 to 11,000 ppm strontium. The manganese content of radial layers ranges as high as 250 ppm, whereas the manganese content of tangential layers is generally below the detection limit of approximately 100 ppm. Iron content varies over a wide range. Scanning electron microscope analysis indicates no microdolomites.

We assume manganese is the principal cathodoluminescent activator. Although there is some uncertainty concerning the partition coefficient for manganese into magnesian calcite and the influence of kinetic effects at varying rates of precipitation is poorly understood, we favor the interpretation that these cathodoluminescent magnesian calcite cortex layers have undergone diagenetic alteration in seawater of the Florida continental shelf. During this diagenesis magnesium was lost and manganese was gained. Thus we suggest that cathodoluminescence may result from diagenetic alteration on the sea floor.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.