Primary Magnesium, Stable Isotopic, and Trace Elemental Composition of Late Devonian Marine Calcite Neptunian Dike Cement from Canning Basin, Australia

Bradley N. Opdyke, Bruce H. Wilkinson, Kyger C. Lohmann, R. Kevin Given

Numerous recent studies have documented secular variations in the isotopic (¹⁸O, ¹³C, ³⁴S, Sr^87/86) composition of Phanerozoic marine carbonate and first-order changes in the position of global sea level. These changes, in combination with suggested relations of sea floor spreading rates and ridge thermal histories with the composition of sea water, strongly implicate changes in other oceanic chemical parameters through the Phanerozoic. In order to evaluate secular trends in the initial trace element composition of inorganic marine carbonate precipitates, magnesium, manganese, iron, and stable isotopic compositions were determined for marine calcite cement from the Upper Devonian reef complex in the anning basin of Western Australia.

These cements comprise alternating layers of inclusion-rich fibrous calcite crystals and clear inclusion-free equant crystals. In comparison to fibrous calcites, inclusion-free cement is significantly enriched in ¹⁸O (-3.7 ^pmil), ¹³C (+2.8 ^pmil), and Mg (6 mole %), and is depleted in Fe and Mn. The isotopic and trace elemental composition of fibrous layers reflects increased alternation of inclusion-rich cement with the subsequent incorporation of manganese- and iron-enriched and oxygen- and carbon-depleted diagenetic intergrowth. Manganese plus iron concentrations vary with depleted ¹³C and ¹⁸O values, indicating a coupling of stable-isotope and trace-element chemistry during diagenesis. Extrapolation of these trends toward the marine end member composition yields values identical with those of inclusion-free cement, indicating that the clear calcite yields primary trace-elemental and stable-isotopic compositions for latest Devonian marine calcite cements.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.