U-Pb DATING OF TRIASSIC PALEOSOL CALCITE IN THE NEW
HAVEN ARKOSE, HARTFORD BASIN, CONNECTICUT
RASBURY, E. T.
HANSON, Gil N..
MEYERS, William J.
All at: Department of Department of Earth and Space
Sciences, State University of New York at Stony
Brook, Stony Brook, NY 11794-2100
We are applying U-Pb dating of paleosol carbonate at paleo-exposure
surfaces to date the time of sedimentation in rapidly deposited sections.
Soil carbonate is a good candidate for U-Pb dating because it (1) can be
rapidly identified geochemically and petrographically; (2) consists of
low-Mg calcite which is stable during diagenesis; and (3) is enriched in
U relative to Pb.
In order to test whether high precision ages would be possible,
multiple aliquots of a ca. 100 gram sample were dated independently by
Mr. Wang and Ms. Rasbury using different chemistries, different size
samples, and different isotopically enriched spikes. This sample is U-
enriched pure pedogenic micritic calcite in a sheet crack about 2000
meters below the Triassic-Jurassic boundary. We have calculated the
ages using two different isochron plots for the two separate aliquots of
the same sample. The uncertainty in the 238U/206Pb - 207Pb/206Pb
isochron (suggested by Rasbury et al., 1996) is about half that of the
238U/204Pb - 206Pb/204Pb isochron (Table 1) because the 207Pb/206Pb
ratio can be measured more precisely than the 206Pb/204Pb ration and
there is less variation in the 207Pb/206Pb initial ratio. The ages are in
good agreement with the published Triassic - Jurassic boundary ages
(Table 2). With uncertainty of less than 3 Ma using the
238U/206Pb - 207Pb/206Pb isochrons for dating the time of sedimentation
this method of dating has high potential for calibrating the geological
Table 1. U-Pb ages of paleosol calcite from
New Haven Arkose
||238U/204Pb - 206Pb/204Pb ages
||238U/206Pb - 207Pb/206Pb ages
||213+/- 5 Ma (2 sigma) (9 points)
||212+/- 3 Ma (2 sigma) (9 points)
||212+/- 13 Ma (2 sigma) (10 points)
||215+/- 10 Ma (2 sigma) (10 points)
||213+/- 5 Ma (2 sigma)
||212+/- 3 Ma (2 sigma)
Table 2. Published Triassic - Jurassic
||Triassic - Jurassic Boundary Ages
|Harland et al. 1990
||208 +/- 16 Ma (2 sigma)
|Odin and Letolle 1982
||204 +/- 4 Ma (2 sigma)
||200 +/- 5 Ma (2 sigma)
||190 +/- 5 Ma (2 sigma)
go back to "MEETING SCHEDULE"