Mathematics – Logic
Scientific paper
Jun 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981gecoa..45..795b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 45, Issue 6, pp.795-811
Mathematics
Logic
34
Scientific paper
We have selected a variety of minerals from three basic intrusions within the high-grade metamorphic terrane near Haliburton, Ontario, for K-Ar dating. By application of 40 Ar / 39 Ar incremental heating experiments, we conclude that the discordant K-Ar mineral dates can best be explained by a slow-cooling history. From the stepwise heating experiments we calculated isotopic closure temperatures using Dodson's theory for slow-cooling and from these constructed a cooling curve. This curve was then compared to independent constraints to test the overall validity of our procedures and assumptions. The results are geologically reasonable insofar as: 1. (a) the maximum calculated temperatures (from hornblendes) of ~-700°C are consistent with independent, approximate metamorphic grid arguments: 2. (b) the Ar closure temperatures for biotites (350-400°C) are consistent with independent evidence, and 3. (c) minerals with older K-Ar dates have higher calculated closure temperatures. The resultant geological cooling curve implies that a maximum temperature of >700 C was reached before 1000 Ma ago and thereafter the temperature decreased via changes in uplift and cooling rates to below 200°C by 700 Ma ago. We have discovered that plutonic plagioclases are very sensitive indicators (comparable to K-felds-pars) of Ar loss and, therefore, can usefully extend the range of application of the 40 Ar / 39 Ar stepwise heating method. We also describe experiments in which we have tested the effects on the Ar systematics of acid leaching of plagioclases.
Berger Glenn W.
York Derek
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