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Old 11-10-2019, 01:58 PM
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AstralTraveller (David)
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Formation of Wolfe Creek Crater dated

Hi people. Here is some news that is so hot it's not even printed, it's still in press. The authors are all well-known in Australian Earth science community.



The age of Wolfe Creek meteorite crater (Kandimalal), Western Australia


Timothy T. BARROWS, John MAGEE, Gifford MILLER, and L. Keith FIFIELD
Meteoritics & Planetary Science1–12 (2019)


Overview:
The Wolfe Creek meteor crater which lies about 160 south of Halls Creek in WA is one of the largest impacts in the past million years. It was formed by a meteor about 15m diameter travelling at about 17km s-1 from the ENE. It was a medium octahedrite of chemical group IIIAB. Previously the age of the impact was poorly constrained. Unpublished ages ranged from 300ka to 30ka (ka = kilo annum = 1,000years). In a paper now in press Barrows et al. publish an age estimate of 120 ± 9ka for the Wolfe Creek impact and revise the age of Meteor Crater (Arizona) to 61.1 ± 4.8ka, which is ~20% older than previous estimates.
The Wolfe Creek age is based on both cosmogenic exposure ages on rock excavated by the meteor and on optically stimulated luminescence (OSL) ages for the burial of sand in the dune which has accumulated on the eastern, upwind side of the crater. Cosmogenic exposure dating measures the accumulation of cosmogenic isotopes (ie isotopes formed by the interaction of high-energy cosmic rays with elements in the rock’s minerals), namely 10Be and 26Al, in previously-buried rock. They produced 6 age estimates on 4 samples and found that they clustered into 2 populations: samples 1&3 have a mean age of 120.0 ± 2.6ka while samples 2&4 have a mean age of 87.7 ± 2.0ka.
OSL dating relies on the existence of crystal defects in naturally-occurring mineral grains (quartz in this case) which produce metastable electron traps. Natural radiation (mostly from K and U) lifts electrons into these traps where they remain until they are exposed to sunlight, which provides enough energy to escape the traps and fall back to the ground state. Therefore samples collected, transported and stored without exposure to light can have their OSL signal measured. The natural radiation is also measured and this yields an age according to:
Age (ka) = Accumulated dose (Gr) ÷ dose rate (Gr ka-1)
It was not possible to date sand from the base of the dune because the presence of ferricrete in the old soil horizon, immediately below the dune, makes determination of the dose rate very difficult. Instead they dated three sample at different depths within the dune and extrapolated those dates to the base. This gives and age estimate of 120ka. After considering all the dates and possible reasons for the discrepancy in exposure ages, which they haven’t resolved, the authors consider that the most likely date for the Wolfe Creek impact is 120 ± 9ka.
They then turn their attention to Meteor Crater, Arizona. This impact has previously been exposure age dated. However this was done when the technique was still very new. Since then there have been improvements in the processing and calibration of the raw data. The author therefore apply modern analysis to the original raw data. This yields an age of 61.1 ± 4.8ka for this event, which is about 20% older than previous estimates.
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