Secular variation of calcium carbonate mineralogy; an evaluation of ooid and micrite chemistries

Bates, Nicholas R. and Brand, Uwe (1990) Secular variation of calcium carbonate mineralogy; an evaluation of ooid and micrite chemistries. Geologische Rundschau, 79 (1), 27-46. (doi:10.1007/BF01830446).

Abstract

Three ooid types are recognized from the Lower Tournaisian »Kohlenkalk« shelf facies at Velbert, Germany. Ooids from this unit have a predominantly concentric laminae fabric. Radial-concentric and small radial fibrous ooids are minor components to this oolite. The diagenetic response of Kohlenkalk ooid chemistry is significantly different from that observed in contemporaneous crinoid and brachiopod material. Fabric evidence suggests that radial-concentric and radial-fibrous ooids were probably originally aragonite/high-Mg calcite and high-Mg calcite respectively. Fabric and trace elemental chemistries of the concentric fabric ooids suggests that they were originally precipitated as aragonite and subsequently altered to low-Mg calcite.

Recent papers have proposed temporal shifts in the dominant mineralogy of shallow marine non-skeletal carbonates between calcite and aragonite. Changing Phanerozoic atmospheric pCO2 levels and oceanic Mg/Ca ratios may have been factors controlling the dominant mineralogy. The chemistries of the Kohlenkalk ooids in conjunction with other ooid and micrite data spanning the Mid-Paleozoic to Recent are evaluated in context with these temporal shifts between »calcite« and »aragonite seas«. The strontium chemistries of the ooids (¯x = 1010 ppm, range 145–3010 ppm) and micrites (¯x = 841 ppm, range 3–8800 ppm) suggests they had an aragonite precursor mineralogy. No statistical correlation was observed between ooid/micrite chemistries, their mineralogies and the proposed secular trend. Therefore, we suggest that aragonitic ooids and micrites were dominant components of shallow-marine carbonate environments throughout the Phanerozoic. The distribution and abundance of aragonitic and calcitic ooids in the geologic past was probably dependant on local hydraulic, physicochemical, and environmental conditions, areally constrained by global tectonics, eustatic, climatic and atmospheric effects, with significant diagenetic overprinting of the original geochemical and fabric information.

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