Late Diagenetic Redistribution of Uranium and Disturbance of the U-Pb Whole Rock Isotope System in a Black Shale

Abstract

The Middle Ordovician Llandeilo-Caradoc Formation black shales, deposited at about 460 Ma on the southwest margin of the Welsh Basin, show evidence of a late diagenetic uranium redistribution event. Evidence for this redistribution event includes a disturbance in the U-Pb whole-rock isotopic system at about 193 Ma marked by a large range in the whole-rock 238 U/ 204 Pb between 2 and 90. One explanation for this large range is that individual whole rocks have experienced either a gain or a loss of U at the time of the disturbance. This hypothesis is supported by the large range in measured U contents from the Llandeilo-Caradoc Formation black shales (0.16 to 6.6 ppm; upper crust ≈ 2.8 ppm, post-Archean average shale (PAAS) ≈ 3.1 ppm; Taylor and McLennan 1985). Uranium that was likely sequestered during deposition and early diagenesis has been redistributed on a greater than whole-rock scale (centimeters to meters). The U-Pb isotope systematics identified in this study and other Pb isotope data from the Welsh Basin (Evans 1996) constrain a reasonable U concentration of 2.91 ppm for an unaltered Llandeilo-Caradoc shale at the time of deposition (∼ 460 Ma). Assuming 2.91 ppm to be an average U content at the time of deposition, some rocks in the Llandeilo-Caradoc Formation have lost as much as 95% of the U likely present at the time of deposition and early diagenesis whereas others exhibit an almost equal enrichment. When the U-Pb isotopic and elemental data from whole rocks and individual U-bearing phases (i.e., carbonate, apatite, monazite, organic matter) are examined in conjunction with the paragenetic sequence, controls on the distribution of U in the Llandeilo-Caradoc whole rocks and a pattern of open-system conditions become apparent. Despite the presence of potential U-rich diagenetic phases such as apatite and monazite, most of the U in the relatively enriched samples is associated with a non-extractable, unidentified diagenetic component. Uranium mass-balance considerations and whole-rock U-Pb isotope systematics require that the non-extractable component formed at about 193 Ma during a late diagenetic uranium redistribution event. The redistribution of U within the Llandeilo-Caradoc Formation has important implications for the use of U-rich sedimentary minerals as geochronometers and for the use of U as a paleo-redox indicator. Specifically, a trace-element indicator like uranium may in some cases record information about the postdepositional history of a sedimentary rock rather than document the absolute timing and conditions of deposition. Moreover, this unique history can be properly evaluated only through an integrated petrographic-geochemical approach that can discern the distribution and source (i.e., depositional versus diagenetic) of U in an individual whole rock.