Abstract
Owing to its diagenetic instability, aragonite is rare in the geological record and almost entirely absent from pre-carboniferous sedimentary rocks. The former presence of this mineral in older deposits has to be inferred from petrographic, chemical or isotopic proxies. Crystals of aragonite that formed around 4563 million years ago occur in carbonaceous chondrite meteorites, showing that under certain conditions, the orthorhombic polymorph of Ca-carbonate can survive essentially indefinitely. Together with other carbonate minerals, phyllosilicates and sulphides, this aragonite formed by low-temperature water-mediated alteration of anhydrous minerals and glass in the interior of the meteorite’s parent asteroid(s). The survival of aragonite for such a long time can be attributed to the loss of free water by its incorporation into phyllosilicates, and to the very low permeability of the fine-grained and organic-rich rock matrix that prevented the ingress of fresh solutions via intergranular flow. By analogy with these meteorites, terrestrial aragonite is likely to survive where it has been similarly isolated from liquid water, particularly in organic-rich mudrocks, and such deposits may provide important new evidence for deducing the original mineralogy of skeletal and non-skeletal carbonates in deep-time.
Highlights
Finding carbonate minerals in the geological record whose original crystal structure and composition have been preserved is crucial for understanding long-term changes to the Earth system including its climate and ocean chemistry
By analogy with these meteorites, terrestrial aragonite is likely to survive where it has been isolated from liquid water, in organic-rich mudrocks, and such deposits may provide important new evidence for deducing the original mineralogy of skeletal and non-skeletal carbonates in deep-time
Secular variations in ocean chemistry have been identified through fluctuations between aragonite and calcite in the dominant mineralogy of inorganic marine carbonates (Sandberg 1975, 1983), and this record can be traced back 2700 million years (Hardie 1996, 2003; Ries et al 2008)
Summary
Finding carbonate minerals in the geological record whose original crystal structure and composition have been preserved is crucial for understanding long-term changes to the Earth system including its climate and ocean chemistry. Aragonite is scarce in Upper Palaeozoic sedimentary rocks (Wendt 1977), and there has been only one description of the mineral from Lower Palaeozoic deposits (Balthasar et al 2011) and one from the Archaean (Lepot et al 2008) The former presence of aragonite can only be inferred using proxies including the petrographic characteristics of replacive calcite and its chemical and isotopic compositions Despite significant changes to their original mineralogy, the CM carbonaceous chondrites have largely retained their initial chemical compositions (McSween 1979) This apparent contradiction is explained by alteration having taken place in a chemically nearclosed system within which aqueous solutions were essentially static, probably owing to the very low permeability (10-19 to 10-17 m2; 0.1–10 lD) of the fine-grained matrix (Bland et al 2009). This value of permeability is very similar to that of terrestrial mudrocks (Neuzil 1995)
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