Petrography and geochemistry of Cambrian-Ordovician marine cements: Implications for Early Paleozoic cementation processes and seawater chemistry
The San Saba Member of the Wilberns Formation (Llano Uplift) and the Cow Head Group (Newfoundland) contain a variety of carbonate cements which exhibit petrographic evidence of having formed within Cambrian and Ordovician marine environments. Early marine cements which have not recrystallized provide information on coeval seawater chemistry. Marine radiaxial cements from the Cow Head Group and syntaxial cements from both units display mottled luminescence and Fe, Mn, Sr, Mg, stable isotope and strontium isotope values indicative of recrystallization. In contrast, marine low-magnesium calcite equant cements from the Cow Head Group typically are nonluminescent, suggesting that they have not recrystallized. These cements exhibit a positive covariant relationship between strontium and magnesium which is similar to trends observed in modern and Devonian marine cements but has a much steeper slope. These relationships suggests that the equant cement precipitated from unaltered seawater. The difference in slope implies that some component of Late Cambrian seawater, such as Sr/Ca and Mg/Ca or pCO2, was different from the modern. Early low-magnesium calcite bladed cements occur in both units. Preserved sector and concentric growth zoning in very dull to nonluminescent regions of this cement suggest that they have not recrystallized. Bladed cements from the San Saba contain primary one-phase fluid inclusions with seawater salinities, which further indicates that they preserve primary marine signatures. These cements exhibit stable isotope and strontium isotope values which also are consistent with the preservation of marine signatures. The very dull to nonluminescent areas of these cements exhibit a negative covariant relationship between strontium and magnesium, which contrasts with the positive covariant Sr-Mg trend normally associated with marine precipitates. These cements are also associated with evidence for the early preferential dissolution of presumably aragonitic bioclasts. These relationships suggest that the bladed cements precipitated from pore fluids evolving toward higher Sr/Ca and lower Mg/Ca ratios resulting from the near-seafloor dissolution of aragonite. The prevalence of this evidence of early aragonite dissolution further implicates pCO2 as partly responsible for the observed differences in mineralogy and geochemistry of Late Cambrian marine cements.
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