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U-Pb systematics and alteration trends of Pennsylvanian-aged aragonite and calcite

Geochimica et Cosmochimica Acta
Publication Date
DOI: 10.1016/0016-7037(94)90466-9
  • Earth Science


Abstract 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 238U 204Pb (μ) values from 4.1–14.8. They give a 238U- 206Pb isochron age of 311 ± 48 Ma (2σ), which is in agreement with the assigned stratigraphic age (mid-Desmoinsian; ~305 Ma) of the Boggy Formation sediments. Their initial lead isotope ratios are 206Pb 204Pb = 19.11 ± 0.08 , 207Pb 204Pb = 15.70 ±0.04 and 208Pb 204Pb ~ 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 theirμ values. The secondary carbonates of crinoids contain 265–352 ppb U, 136–262 ppb Pb and their μ 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 ( 206Pb 204Pb = 20.50 , 207Pb 204Pb = 15.73 ). This very radiogenic signature is similar to those of Mississippi Valley-type ore deposits and both types may share a similar genesis.

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