Late Archean Aragonite Precipitation: Petrography, Facies Associations, and Environmental Significance
Dawn Y. Sumner, John P. Grotzinger, 2000. "Late Archean Aragonite Precipitation: Petrography, Facies Associations, and Environmental Significance", Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, John P. Grotzinger, Noel P. James
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Large crystal pseudomorphs, composed of limestone and dolomite, that radiate upward to form centimeter- to meter-tall fans are known from every well-preserved Late Archean carbonate platform on earth. In many cases these crystal fans are an important facies, constituting as much as 50% of the observed volume of carbonate rock. Texturally, the fans are composed of elongate blades consisting of a mosaic of crystals with randomly oriented optic axes. In some pseudomorphs, trains of inclusions define the fibrous character of the precursor mineral, and the blades exhibit blunt terminations when draped by micrilic sediment. Some of the pseudomorphs contain strontium concentrations of up to 3700 ppm. Associated facies include strongly elongate giant stromatolites, hummocky cross-stratified sandstones, ooid-intraclast packstone to grainstone, small domal stromatolites, and several thinly laminated micritic facies that may display desiccation cracks.
Previously, some of these crystal fans have been interpreted as calcite-and dolomite-replaced pseudomorphs after gypsum, formed under restricted conditions resulting from evaporative concentration of seawater. However, replacement textures and elevated strontium concentrations suggest that the crystal fans are more likely the result of neomorphism of large botryoids of aragonite that formed thick crusts directly on the sea floor. Furthermore, occurrence of the crystal fans in direct association with strongly elongate giant stromatolites and hummocky cross-stratified sediments suggests precipitation of the fans in open marine, wave- and current-swept environments. Although evaporation of seawater may have contributed to the growth of fans in some peritidal environments, most occurrences are not associated with any other indicators of evaporitic conditions such as halite or gypsum pseudomorphs.
The reinterpretation of most reported occurrences of Late Archean gypsum pseudomorphs as aragonite pseudomorphs indicates that calcium sulfate precipitation from Late Archean seawater was rare, and that precipitation of aragonite as thick crusts on the sea floor was significantly more abundant than during any subsequent time in earth history. Rapid aragonite precipitation rates and the paucity of calcium sulfate precipitation can be accounted for in a model for Late Archean seawater featuring, relative to present-day seawater, higher supersaturation with respect to calcium carbonate and high HC03 concentrations.
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Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World - Precambrian carbonates are usually regarded at the simple cousins of the sedimentary realm, composed of stromatolites and dolostones, texturally not challenging and commonly altered beyond recognition by the vagaries of time, diagenesis and metamorphism. However, these carbonates that formed deep in time are commonly exquisitely preserved and contain within them a record of the evolving young earth. SEPM Special Publication 67 explores these aspects. Resulting from a 1997 SEPM/CSPG symposium entitled? Precambrian Carbonates,? these 18 papers demonstrate the importance of understanding these rocks, since within them is contained a record of the early ocean, atmosphere, and biosphere.