DIRECT RADIOMETRIC AGE-DETERMINATION OF CARBONATE DIAGENESIS USING U-PB IN SECONDARY CALCITE

Carbonate rocks and corals from the Middle Devonian Lucas formation, Ontario have U-238/Pb-204 (mu) values ranging from 132 to 559 and give concordant, apparent Pb-206*/U-238 and Pb-207*/U-235 ages in approximate agreement with the primary age of sedimentation. A carbonate rock from one part of the section gives an anomalously young age and contains patches of secondary calcite spar. This spar is predominantly pale purple under cathodoluminescence (CL) and contains 250-600 ppb U and 15-22 ppb Pb. The mu-values range from 2 x 10(3)-10(4), making the Pb very radiogenic. Apparent Pb-206*/U-238 ages range from 208 to 249 Ma and Th-232/U-238 (kappa) values are from 0.005 to 0.40. The younger ages are correlated with an increase in orange CL and higher apparent kappa-values. The age scatter is attributed to mixing of a subordinate calcite spar which infiltrated the older spar. The young calcite is bright orange under CL, has 7200 ppb U, mu of 6.2 x 10(4), a kappa of 0.23 and is dated at 45 Ma. The age of the older spar has been determined by a sequential dissolution technique. The resulting U-Pb data can be plotted as U-Pb "age spectra" and yield "plateau" dates ranging from 231.5 +/- 1.4 to 244.9 +/- 1.6 Ma and an integrated Pb-206*/U-238 age of 238.2 +/- 1.1 Ma. Also, the sequential dissolution increases the mu-values progressively to almost-equal-to 5 x 10(4) making these ages independent of the initial Pb correction. The occurrence of two young ages of diagenetic calcite in these strata is consistent with the idea of recurrent fluid activity along fracture zones in subsurface sediments in the Michigan Basin. This study shows that it is now possible to date secondary carbonate diagenesis by U-Pb and by inference to determine precisely the timing of activity on ancient fracture zones in carbonate rocks.