Re-Os Dating of Black Shales: Timing and Duration of Sedimentary Processes

Judith L. Hannah¹, Holly J. Stein¹, Gang Yang¹, Aaron Zimmerman¹, and Bernard Bingen^2
1AIRIE Program, Geosciences, Colorado State University, Fort Collins, CO
²Crustal Processes, Geological Survey of Norway, Trondheim, Norway

Re and Os are redox-sensitive elements fixed in organic material and sulfides in muds deposited in anoxic or euxinic marine environments. Decay of ¹⁸⁷Re to ¹⁸⁷Os provides a radiometric clock that records the time of shale deposition and the ¹⁸⁷Os/¹⁸⁸Os of seawater at that time. Recent studies yield robust ages for shales as old as 2.7 Ga, with resolution comparable to biostratigraphy in Phanerozoic rocks. This new instrument in the isotope geochemist’s toolbox permits us to: (1) place absolute time pins in the sedimentary record, thereby refining the global time scale, constraining the duration of sedimentologic and paleontologic events, and correlating events at different localities in absolute time; (2) establish variations in seawater ¹⁸⁷Os/¹⁸⁸Os through time, thereby recording changes in global patterns of oxidative weathering and Os influx and defining the resolution of the ¹⁸⁷Os/¹⁸⁸Os tracer; and (3) determine Re-Os systematics in migrated hydrocarbons and source rocks, thereby documenting the time of isotopic homogenization of hydrocarbons and establishing ¹⁸⁷Os/¹⁸⁸Os as a tracer for hydrocarbon migration. Case studies demonstrate use of the Re-Os system to (1) date the rise of atmospheric oxygen at >2.32 Ga; (2) record dominance of chondritic, hydrothermal Os in the 2.04 Ga Pechenga rift basin, (3) provide a maximum age of 560 Ma for the Moelv tillite, Norway, and (4) link hydrocarbon maturation and migration to elevated heat flow in the Oslo rift at ~300 Ma. Re-Os dating of black shales complements U-Pb dating of ash layers, providing a new geochronologic method of comparable reliability and precision. Absolute time pins acquired with these two chronometers define rates in sedimentary systems. The Re-Os chronometer offers new potential for addressing fundamental questions in sedimentology, tectonics, paleogeography, paleoecology, and paleoclimatology.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas