RE-OS DEPOSITIONAL AGE OF THE NEOPROTEROZOIC ARALKA FORMATION (AMADEUS BASIN, AUSTRALIA) REVISITED

The Neoproterozoic Aralka Fm (Amadeus Basin, Australia) underlies the glaciogenic Olympic Fm and overlies the glaciogenic Areyonga Fm, which are correlated with the Marinoan and Sturtian glaciations, respectively. However, a recent Re-Os age of 592 ± 14 Ma (n=3, MSWD < 1; Schaefer & Burgess 2003 J. Geol. Soc. Lond.) is not consistent with currently understood age constraints for Marinoan and Sturtian glaciations, suggesting that the proposed correlations may be incorrect, that Marinoan glaciation in different regions is diachronous, or that the Re-Os age is an artifact (Hoffmann et al., in press Geology). We report new Re-Os isotope data on upper Aralka Fm black shales (TOC=0.5-1%) from the same stratigraphic interval as the study of Schaefer & Burgess. However, instead of conventional inverse aqua regia (IAR), our samples were processed with a CrO₃-H₂SO₄ digestion medium, which minimizes release of detrital Os and/or Re by selectively dissolving organic matter that is host to hydrogenous Re and Os (Selby & Creaser 2003 Chem. Geol; Kendall et al. 2004 EPSL). Regression of data for 8 samples from a 2m section (and 2 replicates; n=10) yields a precise Re-Os age of 658.0 ± 5.5 Ma (Model 1, MSWD=1.1). This age is older than the previously reported Re-Os ages of 592 ± 14 Ma or 623 ± 18 Ma (n=7, MSWD=5.2; Schaefer & Burgess 2003). We also performed IAR analyses of Aralka Fm shale; these data fall close to Schaefer & Burgess’s ca. 623 Ma regression. We interpret these results to indicate that conventional IAR analysis releases detrital Os and/or Re, that this Os is less radiogenic than the hydrogenous Os load (Aralka Fm I_Os ~ 0.82; e.g., Ravizza et al. 1991 GCA), and that 658.0 ± 5.5 Ma is the depositional age for the upper Aralka Fm. The 658 Ma age is consistent with the stratigraphic position of the Aralka Fm. between correlated Sturtian and Marinoan glacial deposits, and with global chemostratigraphic correlations. Consequently, the Areyonga Fm is constrained to be older than 658 Ma, and the younger Olympic Fm may be correlative with the Mt Vreeland – Ice Brook glacial deposits of W. Canada (minimum Re-Os age of 607.8 ± 4.7 Ma), the Nantuo diamictite of China (maximum U-Pb and minimum Pb-Pb age constraints of 663 ± 4 Ma and 599.3 ± 4.2 Ma, respectively), and the Ghaub glaciation in Namibia (direct U-Pb age constraint of 635.5 ± 1.2 Ma).