Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 3
Presentation Time: 8:40 AM


SCHMITZ, Mark D., Department of Geosciences, Boise State Univ, 1910 University Drive, Boise, ID 83725,

U-Pb dating of zircons from ash beds interstratified with fossiliferous strata has become an important means of radiometrically calibrating the chronostratigraphic time scales derived by biostratigraphic zonation. The geologically "instantaneous" nature of explosive volcanism makes even distal tuffs excellent event markers in the geological record— markers whose abundance in the stratigraphic record has increasingly come to light. High-precision U-Pb zircon geochronology by the isotope dilution - thermal ionization mass spectrometry (ID-TIMS) method has particularly evolved over the past 15 years into a preeminent tool for calibrating the Mesozoic and older portions of the geological time scale. In complement to the well-known benefits of the dual U-Pb chronometers in zircon, the strength of the ID-TIMS method resides in large part in the isotope dilution method, which can determine 207Pb/235U and 206Pb/238U ratios (and dates) to a precision approaching 0.1%. The ID-TIMS method has also matured through the development of high-efficiency ion emitters and lowering of analytical blanks to the sub-picogram level, allowing even single low-U Phanerozoic zircons (or parts thereof) to be analyzed with high-precision. This ability to measure single grains is important for identifying and mitigating the effects of Pb-loss and inheritance of older zircon cores, which continue to present obstacles to the precise and accurate determination of ash bed ages. This contribution will explore how the ID-TIMS U-Pb zircon method has evolved over the past decade, and what lessons have been learned in its application to time scale calibration problems. The review will highlight the historical analytical trends and ongoing method developments that continue to expand the viability of ID-TIMS U-Pb zircon geochronology for precise and accurate time scale calibration. The ready conclusion of this contribution is that the richness of the stratigraphic record and opportunities for its robust calibration through ID-TIMS U-Pb zircon geochronology challenge both the available analytical infrastructure and human resources dedicated to time scale work, as well as the current mechanisms of cross-disciplinary communication and collaboration between geochronologists, stratigraphers, paleobiologists and paleoecologists.