U-PB SYSTEMATICS AND ALTERATION TRENDS OF PENNSYLVANIAN-AGED ARAGONITE AND CALCITE

Carbonate allochems from the mid-Pennsylvanian Buckhorn fauna of the B oggy 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, an d have low U-238/Pb-204 (mu) values from 4.1-14.8. They give a U-238-P b-206 isochron age of 311 +/- 48 Ma (2sigma), which is in agreement wi th the assigned stratigraphic age (mid-Desmoinsian; approximately 305 Ma) of the Boggy Formation sediments. Their initial lead isotope ratio s are Pb-206/Pb-204 = 19.11 +/- 0.08, Pb-207/Pb-204 = 15.70 +/- 0.04, and Pb-208/Pb-204 approximately 38.7, and reflect an upper crustal sou rce 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 Buckh orn nautiloids are significantly higher than those found in the modem nautilus Nautilus pompilius. These enrichments are attributed to bitum en which was added to the carbonates during diagenesis. The diagenetic transformation of aragonite to low-Mg calcite in the Buckhorn nautilo ids was apparently associated with a net loss of Pb, thereby increasin g their mu values. The secondary carbonates of crinoids contain 265-35 2 ppb U, 136-262 ppb Pb, and their mu values range from 68-111; conseq uently, their present-day uranogenic lead isotopic ratios are higher t han those of the Buckhorn nautiloids. The U-Pb data for some diageneti c low-Mg components indicate an age of approximately 280 Ma suggesting an early timing for the post-depositional alteration process. The app arent initial lead isotope ratios of crinoids are variable, and some a re significantly higher than the coexisting aragonite (Pb-206/Pb-204 = 20.50, Pb-207/Pb-204 = 15.73). This very radiogenic signature is simi lar to those of Mississippi Valley-type ore deposits and both types ma y share a similar genesis.