Computer Science
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58..313s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 1, pp.313-322
Computer Science
3
Scientific paper
Carbonate allochems from the mid-Pennsylvanian Buckhorn fauna of the Boggy Formation, Oklahoma, USA, consist of primary aragonite preserved in nautiloids and of secondary low-Mg calcite present in crinoids and corals. Unaltered nautiloids contain 11-38 ppb U and 23-422 ppb Pb and have low 238 U / 204 Pb ( ) values from 4.1-14.8. They give a 238 U- 206 Pb isochron age of 311 ± 48 Ma (2 ), which is in agreement with the assigned stratigraphic age (mid-Desmoinsian; ~305 Ma) of the Boggy Formation sediments. Their initial lead isotope ratios are 206 Pb / 204 Pb = 19.11 ± 0.08, 207 Pb / 204 Pb = 15.70 ±0.04 and 208 Pb / 204 Pb ~ 38.7 and reflect an upper crustal source for Pb in mid-Pennsylvanian seawater. These values are similar to those of coals of this age. The contents of both U and Pb in the Buckhorn nautiloids are significantly higher than those found in the modern nautilus Nautilus pompilius . These enrichments are attributed to bitumen which was added to the carbonates during diagenesis. The diagenetic transformation of aragonite to low-Mg calcite in the Buckhorn nautiloids was apparently associated with a net loss of Pb, thereby increasing their values. The secondary carbonates of crinoids contain 265-352 ppb U, 136-262 ppb Pb and their values range from 68-111; consequently, their present-day uranogenic lead isotopic ratios are higher than those of the Buckhorn nautiloids. The U-Pb data for some diagenetic low-Mg components indicate an age of ~280 Ma suggesting an early timing for the post-depositional alteration process. The apparent initial lead isotope ratios of crinoids are variable and some are significantly higher than the coexisting aragonite ( 206 Pb / 204 Pb = 20.50, 207 Pb / 204 Pb = 15.73). This very radiogenic signature is similar to those of Mississippi Valley-type ore deposits and both types may share a similar genesis.
Brand Uwe
Farquhar Ronald M.
Smith Eric P.
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