Recently, 187Re - 187Os nuclear geochronometry, a new dating method combining ideas of
nuclear astrophysics with geochronology, has successfully been used to calculate
two-point-isochron (TPI) ages for Devonian black gas shales using the isotopic signature of
an r-process geochronometer as one data point in a TPI diagram [1]. Based upon a
nuclear production ratio 187Re/188Os = 5.873, TPI ages were calculated for 12 SDO-1
(Devonian Ohio Shale, Appalachian Basin) aliquants, for which repeated Re-Os
measurements are reported in the literature [2]. TPI ages range from 384.5 ± 2.7 Ma
(187Os/188Osi = 0.29413 ± 0.00023) to 387.7 ± 2.1 Ma (187Os/188Osi = 0.29407
± 0.00019) with a mean of 386.67 ± 1.79 Ma). The result is consistent with the
isochronous age from the 12 aliquants alone (386 ± 16 Ma, 187Os/188Osi = 0.31±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) [3] from the Appalachian Basin. Hence, SDO-1
can be assigned to the Givetian stage (varcus-zone) of the Middle Devonian, close
to the Eifelian/Givetian boundary (using the time-scale of [3] or [4]). If an age is
calculated from an isochron diagram for the 12 aliquants including the nuclear
geochronometer, a permil precision can be achieved, an interesting feature with respect
to any effort towards calibrating the Geologic Timescale. Additionally, a Th/U
evolution (or: Th/U-time) diagram can be plotted using U-Pb zircon age data and
Th/U ratios from volcanic rocks and ashes reported in the literature [3] for specific
Devonian samples from the Appalachian Basin. Since the Re-Os age obtained for
SDO-1 can also be connected to its Th/U ratio, it turns out, that Th/U ratios might be
helpful age indicators, as demonstrated for the Devonian using the U-Pb and Re-Os
datasets.
[1] Roller (2015), GSA Abstr. with Programs 47, #248-14. [2] Du Vivier et al.
(2014), Earth Planet. Sci. Lett. 389, 23 – 33. [3] Tucker et al. (1998), Earth Planet.
Sci. Lett. 158, 175 – 186. [4] Kaufmann (2006), Earth-Sci. Revs. 76, 175 – 190. |