Mathematics – Logic
Scientific paper
Sep 1972
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1972gecoa..36..985c&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 36, Issue 9, pp.985-1005
Mathematics
Logic
3
Scientific paper
Mercury has been determined on 406 samples of Archean (>>2.4 b.y.) shale from 153 localities in the Superior Province of the Canadian Shield and on 396 samples of shale of Aphebian (1.6-2.4 b.y.) age from 54 localities. The Archean samples, which are of mainly volcanogenic origin, and which commonly have been metamorphosed to lower greenschist grade, average 129 ppb Hg and have a median content of 86 ppb. The Aphebian shales are of varied lithology: from mature miogeosynclinal sediments to chemically immature shales associated with greywackes. They are variably metamorphosed up to lower greenschist facies and average 513 ppb Hg, with a median value of 408 ppb. The Hg content of the Archean and Aphebian shales has been related to that of twenty other elements previously determined on the samples. For the Archean shales, the geochemistry of Hg was dominated by the derivation of Hg and Zn from volcanic springs and subsequent precipitation of the two elements with the carbonaceous-sulphide fraction of contemporaneous muds. For the Aphebian shales, there is no similar correlation between Zn and Hg. Here the Hg is dominantly associated with the carbonaceous fraction of the rock. Significant loss of Hg during metamorphism of the Archean and Aphebian shales is discounted because the Hg was probably bound in thermally stable metal-organic compounds or as a minor component of sulphide minerals. For comparison, 48 samples of Palaeozoic shale from widely distributed localities across the eastern seaboard of Canada, and representing a wide variety of depositional environments, have been analysed for Hg. These shales average 42 ppb Hg, with a median content of 19 ppb. This average is similar to previous data for Devonian to Cretaceous shales from the Russian Platform. The Aphebian shales have a mean 12 times greater than the Palaeozoic samples and a median 21 times greater. The high levels of Hg in the Aphebian samples are consistent across the wide range of geological ages, lithologies, and geographic distance represented by the samples. It is suggested that this enrichment of Hg in early Proterozoic sediments was related to an increased rate of degassing of Hg from the earth. This increase may have been caused by a change in the sites of magma generation from shallow depths in the oceanic crust and mantle to deeper levels in the mantle--a change that followed upon the development of a thicker and more stable crust at the close of Archean time.
Cameron Eion M.
Jonasson Ian R.
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