Selby, David1, Robert Andrew Creaser1
(1) University of Alberta, Edmonton, AB
ABSTRACT: Application of the Re-Os Chronometer to Hydrocarbon Systems
The rhenium-osmium isotopic (187Re-187Os) chronometer can be currently applied to hydrocarbon systems to obtain absolute depositional ages for organic-bearing sedimentary rocks; a source for natural hydrocarbons. However, we have also found that bitumen formed from migrated oil in both Alberta tar sand deposits and Mississippi Valley type Pb-Zn deposits contains high Re (tens of ppb) and Os (ppt to ppb) abundances inherited from their sources. The 187Re/187Os and 187Os/187Os values in bitumen show significant variation between samples on a deposit and drillhole scale, which make it possible to construct a Re-Os isochron - a requirement for age determination for samples like bitumen that contain common Os. Bitumen Re-Os data from the Polaris MVT Zn-Pb deposit show two isotopic populations, which yield Re-Os dates of 371 ± 26 Ma (Model 3, MSWD=3.9) and 369.5 ± 5.4 Ma (Model 3, MSWD=3.1). These Re-Os age dates are in excellent agreement with the sulphide age constraints at Polaris (Rb-Sr sphalerite age, 366 ± 15 Ma; paleomagnetic age, 367 ± 7 Ma), suggesting that sulphide mineralization and bitumen formation was contemporaneous, being coeval with the Late Devonian Ellesmerian orogeny. The distinct common 187Os/187Os values (~1.4, ~1.6) may suggest different sources for the bitumen. These findings demonstrate, for the first time, the promising utility bitumen has to yield valuable age information on the timing of hydrocarbon migration and further may aid in the identification of hydrocarbon sources for natural hydrocarbons.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.