HR: 16:00h
AN: V22G-01 INVITED     [PDF]
TI: New Advances in Re-Os Geochronology of Organic-rich Sedimentary Rocks.
AU: * Creaser, R A
EM: robert.creaser@ualberta.ca
AF: University of Alberta, Dept Earth and Atmospheric Sciences, 126ESB, Edmonton, AB T6G2E3 Canada
AU: Selby, D
EM: dselby@ualberta.ca
AF: University of Alberta, Dept Earth and Atmospheric Sciences, 126ESB, Edmonton, AB T6G2E3 Canada
AU: Kendall, B S
EM: bkendall@ualberta.ca
AF: University of Alberta, Dept Earth and Atmospheric Sciences, 126ESB, Edmonton, AB T6G2E3 Canada
AB: Geochronology using $^{187}$Re-$^{187}$Os is applicable to limited rock and mineral matrices, but one valuable application is the determination of depositional ages for organic-rich clastic sedimentary rocks like black shales. Clastic sedimentary rocks, in most cases, do not yield depositional ages using other radioactive isotope methods, but host much of Earth's fossil record upon which the relative geological timescale is based. As such, Re-Os dating of black shales has potentially wide application in timescale calibration studies and basin analysis, if sufficiently high precision and accuracy could be achieved. This goal requires detailed, systematic studies and evaluation of factors like standard compound stoichiometry, geologic effects, and the $^{187}$Re decay constant. Ongoing studies have resulted in an improved understanding of the abilities, limitations and systematics of the Re-Os geochronometer in black shales. First-order knowledge of the effects of processes like hydrocarbon maturation and low-grade metamorphism is now established. Hydrocarbon maturation does not impact the ability of the Re-Os geochronometer to determine depositional ages from black shales. The Re-Os age determined for the Exshaw Fm of western Canada is accurate within 2$\sigma$ analytical uncertainty of the known age of the unit (U-Pb monazite from ash, conodont biostratigraphy). This suggests that the large improvement in precision attained for Re-Os dating of black shales by Cohen et al (ESPL 1999) over the pioneering work of Ravizza \& Turekian (GCA 1989), relates to advances in analytical methodologies and sampling strategies, rather than a lack of disturbance by hydrocarbon maturation. We have found that a significant reduction in isochron scatter can be achieved by using an alternate dissolution medium, which preferentially attacks organic matter in which Re and Os are largely concentrated. This likely results from a more limited release of detrital Os and Re held in silicate materials during dissolution, compared with the inverse aqua regia medium used for Carius tube analysis. Using these "organic-selective" dissolution techniques, precise depositional ages have now been obtained from samples with very low TOC contents ($\sim$0.5%), meaning that a greater range of clastic sedimentary rocks is amenable for Re-Os age dating. Well-fitted Re-Os isochrons of plausible geological age have also been determined from low-TOC shales subjected to chlorite-grade regional metamorphism. These results further illustrate the wide, but currently underutilized, potential of the Re-Os geochronometer in shales. The precision of age data attainable by the Re-Os system directly from black shales can be better than $\pm$ 1% uncertainty (2$\sigma$, derived from isochron regression analysis), and the derived ages are demonstrably accurate.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1035 Geochronology
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting