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Unformatted text preview: e1 COMMENT Dawn Y. Sumner Geology Department, University of California, Davis, California 95616, USA Hardie (2003) identifi ed intervals of “aragonite” and “calcite seas” by modeling Mg/Ca ratios in Neoarchean to Neoproterozoic seawater using estimates of hydrothermal and weathering fl uxes of key ions. His results agree with most published reports of aragonite pseudomorphs with the exception of the time period between 2.5 and 2.6 Ga. During this in- terval, his model predicts calcite seas, but aragonite pseudomorphs have been reported (Hardie, 2003). Hardie’s preferred interpretation for the difference between model results and the presence of aragonite pseudo- morphs is that the described pseudomorphs were not originally aragonite. I disagree with this interpretation. Evidence for Neoarchean Aragonite Precipitation. Sumner and Grotzinger (2000) documented evidence for aragonite precipitation in four geographically diverse carbonate units spanning 2.9–2.5 Ga. The 2.6–2.5 Ga Campbellrand-Malmani carbonate platform, South Africa, contains abundant pseudomorphs that Hardie (2003) argues probably were gypsum. In this 2-km-thick platform, aragonite pseudomorphs are abundant and consist of fi brous marine cements in addition to radiating bundles of decimeter-long crystals. The fi brous cements contain most of the features typical of calcite replacing aragonite (Loucks and Folk, 1976; Assereto and Folk, 1980; Mazzullo, 1980; Sandberg, 1985; Peryt et al., 1990), including: (1) relict crystal morphology defi ned by inclusions and [Mn] that demonstrates the crystals were fi brous and had blunt to feathery fi ber bundle terminations; (2) replacement by optically unoriented, equant to elongate calcite crystals with unit extinction; and (3) strontium concen- trations of up to 3700 ppm. The fi brous cements show no morphological similarity to any report- ed gypsum textures; they are entirely analogous to Phanerozoic aragonitic cements using the same identifi cation criteria used in carbonate platforms of any age with the exception that relict aragonite inclusions have not been identifi ed (see Sumner and Grotzinger, 2000). Some large botryoidal and prismatic crystal pseudomorphs inter- preted by Sumner and Grotzinger (2000) show similarities to gypsum pseudomorphs with respect to the size and geometrical arrangement of crystals. However, crystal morphology, petrographic characteristics, and geochemical compositions are similar to the fi brous cement pseudomorphs rather than gypsum pseudomorphs. The only similarity in crystal proper- ties to gypsum are pseudohexagonal cross sections in some prismatic pseudomorphs, which is also consistent with an aragonite precursor. Mea- surements of interfacial angles are consistent with either an aragonite or gypsum precursor due to the sensitivity of results to small errors in cross section orientation, the absence of micrite drapes on pseudomorphs, and the originally fi brous character of macroscopically prismatic pseudo-...
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This note was uploaded on 01/31/2011 for the course GEL 1 taught by Professor Barfod during the Spring '08 term at UC Davis.
- Spring '08