Effects of shock pressure on 40Ar -39Ar radiometric age determinations |
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| Authors: | Paul K Davis |
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| Institution: | Physics Department, University of California, Berkeley, California 94720, U.S.A. |
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| Abstract: | The relation of shock to the drop in the 401Ar/391Ar ratio seen at high release temperatures in some neutron-irradiated lunar samples is investigated through measurements of the 401Ar/391Ar ratio in gas samples released by stepwise heating of rock samples previously subjected to shock, either in the laboratory or in nature.Explosives were used to shock solid pieces and powder of a basalt from a diabase dike in Liberia to calculated pressures of 65, 150 and 270 kbar. These, an unshocked sample of the powder, two naturally shocked samples from the Brent impact crater in Canada, one unshocked sample from near the crater, and appropriate monitors were irradiated. Ar from stepwise heating was analyzed.The unshocked basalt shows a good 401Ar/391Ar plateau at age 198 ± 9 m.y. in agreement with a previous result of 186 ± 2 m.y. The shocked samples contain varying amounts of implanted atmospheric Ar, the isotopes of which have experienced mass fractionation. This effect is small enough in four samples so that the linearity of their graphs of 391Ar/40Ar vs 36Ar/40Ar is evidence of a plateau. The ages of these samples are then 201 ± 10, 205 ± 11, 205 ± 12 and 201 ± 9 m.y. It appears that the shock has had little effect on the 40Ar-39Ar age spectrum, although the release patterns of the 391Ar are shifted downward by the order of 200°C. Shock implantation of Ar was at lower shock pressure, in the presence of less Ar, and into a less porous material than previously demonstrated.The Brent Crater samples do not all show good plateaus, but do indicate an age of ~420 m.y. for the crater event and 795 ± 24 m.y. for the rock formation, in agreement with previous results.None of the 401Ar/391Ar profiles shows a drop at high temperature, but a possible role of shock implantation of Ar is indicated in the production of this effect. Further experiments are suggested. |
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