Mathematics – Logic
Scientific paper
Apr 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009e%26psl.280...83y&link_type=abstract
Earth and Planetary Science Letters, Volume 280, Issue 1-4, p. 83-92.
Mathematics
Logic
2
Scientific paper
Rhenium-osmium (Re-Os) dating of carbonaceous rocks using H2SO4-CrO3 dissolution provides a powerful new geochronologic tool for Archean terranes. Here we document the oldest Re-Os depositional ages yet reported for carbonaceous sedimentary rocks. Carbonaceous slates are intercalated with graywackes and mafic tuffs of the Joy Lake sequence in the western Wawa subprovince of the Superior Province. Well-developed D2 cleavage is sub-parallel to bedding. Pyrite occurs in nodules, disseminations, and bedding-parallel stringers. Samples from two drill holes (DH26503 and DH26506) yield isochron ages of 2695 ± 14 Ma and 2684 ± 16 Ma and initial 187Os/188Os ratios (Osi) of 0.15 ± 0.16 and - 0.29 ± 0.20, respectively. The ages, identical within uncertainty, provide the first temporal constraints for the westernmost Wawa subprovince greenstone sequence. The ages are consistent with cessation of mafic/ultramafic volcanism before ca. 2695 Ma in the western Wawa subprovince and D2 deformation before 2685 Ma farther east. The Joy Lake sequence is likely the temporal equivalent of turbiditic graywackes deposited further east, both sourced from arc volcanoes during collision of the Wawa and Wabigoon arcs along the southern margin of the Superior Province. A chondritic Osi (0.109 at 2.69 Ga) is expected for Archean shales, but the negative Osi for DH26506 requires explanation. Re, which is significantly more mobile than Os, may have been released from intercalated high-Re/Os mafic tuffs by local oxidation during recent or Cretaceous exposure to meteoric waters. The adjacent carbonaceous rocks are a ready sink for mobile Re. A small Re addition can shift isochron data points to higher 187Re/188Os ratios with minimal slope change. This increases scatter and uncertainty for the Osi, shifts the isochron to the right, and produces a spuriously low Osi intercept. If Re addition is recent, the age is not significantly affected. Notably, the slates in DH26506 which yield the negative Osi are intercalated with relatively permeable and reactive lithic wackes and mafic tuffs, whereas the slates in DH26503 are in contact with less permeable massive intrusive rocks. This process can be detected only if the true Osi is approximately chondritic, and is more likely where reactive mafic rocks are prevalent—both the case for Archean greenstone belts.
Bekker Alexander
Hannah Judith L.
Stein Holly J.
Yang Guangyu
Zimmerman Aaron
No associations
LandOfFree
Re-Os depositional age for Archean carbonaceous slates from the southwestern Superior Province: Challenges and insights does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Re-Os depositional age for Archean carbonaceous slates from the southwestern Superior Province: Challenges and insights, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Re-Os depositional age for Archean carbonaceous slates from the southwestern Superior Province: Challenges and insights will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1242740