Hydrocarbons/Rhenium–Osmium (Re–Os): Organic-Rich Sedimentary Rocks

Abstract

The first application of the Re–Os isotope geochronometer to ORR demonstrated that the Re–Os data, although imprecise, could yield the depositional age of sedimentary successions (Ravizza and Turekian 1989), a remarkable outcome given the analytical challenges and data uncertainties at the time. Today, utilizing new analytical and mass spectrometry techniques (see below) and the most accurate Re decay constant (1.666 10 11 year ; Smoliar et al. 1996) has permitted the determination of precise (in some cases 1 % uncertainty) depositional ages for ORR that possess 0.5 wt% total organic carbon (TOC) (e.g., Cohen et al. 1999; Georgiev et al. 2011; Kendall et al. 2009a; Selby and Creaser 2005a; Xu et al. 2009; Cumming et al. 2012). These studies have provided the foundation to apply the Re–Os organic-rich sedimentary rock geochronometer to yield ages throughout geological time for stratigraphic boundary intervals and major Earth events (e.g., rise of oxygen, Proterozoic glaciations etc.; Selby and Creaser 2005a; Selby 2007; Anbar et al. 2007; Xu et al. 2009; Rooney et al. 2011; Yang et al. 2009). Rhenium–osmium geochronology of ORR utilizes the isochron method, which yields the initial Os isotope composition (Os/Os) for the sampled unit, taken to represent the Os/Os composition of the water column at the time of deposition (Ravizza and Turekian 1989; Cohen et al. 1999; Peucker-Ehrenbrink and Ravizza 2000, 2012). In addition to ORR, hydrocarbon products of such matured rocks, for example, crude oil and bitumen, have been shown to also be enriched in both Re and Os at the ppb and ppt level, respectively (Barre et al. 1995; Woodland et al. 2001; Selby and Creaser 2005b; Selby et al. 2007; Finlay et al. 2011). The hydrocarbon Re–Os isotope data from several petroleum systems has shown that Re–Os systematics of hydrocarbons can provide the timing of petroleum generation from an oil source rock (Selby et al. 2005; Selby and Creaser 2005b; Finlay et al. 2011), with the initial Os/Os value derived from Re–Os hydrocarbon data interpreted to reflect the Os/Os inherited from the source rock at the time of petroleum expulsion (Selby and Creaser 2005b; Rooney et al. 2012).