DATING SEDIMENTARY SEQUENCES: IN SITU U/TH-PB MICROPROBE DATING OF EARLY DIAGENETIC MONAZITE AND AR-AR DATING OF MARCASITE NODULES: CASE STUDY FROM NEOPROTEROZOIC BLACK SHALES IN THE SOUTHWESTERN U.S

U/Th-Pb in situ dating of monazite offers rich potential for directly dating sedimentary sequences and for understanding their provenance. We have obtained microprobe dates on both authigenic and detrital monazite from the Neoproterozoic Chuar and Uinta Mountain Groups of the western U.S., and substantiate theses results with a U-Pb zircon dated ash and Ar-Ar dated authigenic marcasite nodules. Black shales in both sequences contain monazite that ranges up to 20 microns in size. Larger grains are rounded and exhibit zoning patterns (1.74-1.4 Ga) identical to monazite from the underlying crystalline basement, consistent with derivation of detrital grains from the local, well-dated basement. A second population of smaller grains (<5 um) are interpreted as early diagenetic products based on textural and compositional data. These yield ages of ca. 770 and ca. 760 Ma from the Chuar Group compatible with their stratigraphic order. These dates are also compatible stratigraphically with a 750 Ma Ar-Ar age from sedimentary marcasite and a 742 Ma zircon age from an ash bed, both also from the Chuar Group. An age of 700-800 Ma from sedimentary monazite of the Uinta Mountain Group is consistent with regional correlations of these sequences. The presence of diagenetic monazite indicates the operation of a biogenic phosphorous cycle similar to present processes in anoxic mudstones. These results allow the direct dating of time of deposition of previously undatable sedimentary sequences by a process that is directly related to their deposition. We can quantify sedimentation rate (50 m/m.y. for the 1600 m thick Chuar Group); place direct dates on large magnitude C-isotope shifts (~10 per mil), with negative excursions at 770, 760, and 750 Ma, just predating the first (Sturtian) low-latitude global (?) glaciation, presumably at ~720 Ma; and 3) correlate Neoproterozoic successions regionally and globally. This new ability to directly date sedimentary rocks will improve dramatically with smaller electron beam diameters and improved count statistics in the next generation of monazite-capable electron microprobes. Continued application of new approaches to geochronology are necessary for understanding Neoproterozoic depositional rates, large-magnitude C-isotope fluctuations and validity of global C-isotope correlations, and quantifying evolutionary rates.