187RE - 187OS NUCLEAR GEOCHRONOMETRY: DATING RADIOACTIVE DEVONIAN BLACK GAS SHALES (USGS SDO-1)

¹⁸⁷Re - ¹⁸⁷Os nuclear geochronometry is a new dating method which combines ideas of nuclear astrophysics with geochronology [1]. Based upon the concept of sudden nucleosynthesis, two-point-isochron (TPI) ages can be calculated using the isotopic signature of an r-process chronometer as one data point in a TPI diagram [2]. Here, a nuclear production ratio ¹⁸⁷Re/¹⁸⁸Os = 5.873 is calculated for an age of 3470 Ma, because an r-process at that time seems to be the most reasonable explanation for subchondritic ¹⁸⁷Os/¹⁸⁸Os ratios [3] and isotopic dichotomy [4]. TPI ages are then calculated for 12 SDO-1 (Devonian Ohio Shale) aliquants, for which repeated Re-Os measurements are reported in the literature [5]. Since SDO-1 is used to establish analytical accuracy of organic- and sulfur-rich sedimentary rocks in trace element analysis, it is a well documented reference sample for the radioactive Upper and Middle Devonian black gas shale. SDO-1 samples the basal 10 ft of a uniform 20.3 ft zone and shows high radioactivity and abundant pyrite and fossils (Tasmanites, conodonts) [6]. It is overlain by an 8.3 ft sequence with remains of Foerstia, an important biostratigraphic marker. TPI ages range from 384.5 ± 2.7 Ma (¹⁸⁷Os/¹⁸⁸Os_i = 0.29413 ± 0.00023) to 387.7 ± 2.1 Ma (¹⁸⁷Os/¹⁸⁸Os_i = 0.29407 ± 0.00019) with a mean of 386.66 ± 0.86 Ma (1SD). This confirms the isochroneous age from the 12 aliquants alone (386 ± 16 Ma, ¹⁸⁷Os/¹⁸⁸Os_i = 0.31), which is bracketed by U-Pb ages for the Belpre Ash (381.1 ± 3.3 Ma) and the Tioga Ash bed (390.0 ± 2.5 Ma) [7]. Hence, SDO-1 can be assigned to the Givetian stage (varcus-zone) of the Middle Devonian, close to the Eifelian/Givetian boundary (388.1 ± 2.6 Ma, using the time-scale of [7]). From these preliminary results it may be concluded that nuclear geochronometry can help to improve the still poorly calibrated Paleozoic biochronometric time-scale and to constrain changes in the ecologic conditions responsible for the rise and fall of Devonian index fossils like Foerstia.

[1] Roller (2014), GSA Abstr. 46, 323. [2] Roller (2015), Geophys. Res. Abstr. 17, EUG2015-35. [3] Morel et al. (2002), Goldschmidt Conference Abstr. A523. [4] Ozdemir et al. (2014), GSA Abstr. 46, 707. [5] Du Vivier et al. (2014), EPSL 389, 23 – 33. [6] Kepferle et al. (1985), USGS Open-File Report 85-145. [7] Kaufmann (2006), Earth-Sci. Revs. 76, 175 – 190.