The Applicability of the 40Ar/39Ar Stepwise Outgassing Method to the Dating of Slates
Savell, Michael J.
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Four samples ranging in composition from a pure slate to a pure metasiltstone, collected from the same location in the Lower Paleozoic Meguma Group, are age dated by the 40Ar/39Ar stepwise outgassing technique. The discordant age spectra record two ages, interpreted as the age of regional metamorphism and a younger thermal metamorphism. The inherited argon recording the regional metamorphic event is released in the high temperature steps. Partial outgassing caused by mild thermal metamorphism, unaccompanied by mineralogical modifications, occurs form the less retentive muscovite flakes. These muscovite flakes may be in phyllosilicate rich laminations; they may have easy access to cleavage fractures or they may be of smaller size. Because the majority of the muscovite in the slate has these characteristics, the larger proportion of the gas released upon heating gives the younger age. This effect of partial outgassing could not have been realized from the conventional K-Ar age determination. It is not known whether the effect is widespread, or occurs infrequently under rare conditions. In order to obtain the apparent age of regional metamorphism, it is desirable to find rocks exhibiting no partial outgassing at all. Since metasiltstones are less easily outgassed, these samples may be better suited for dating regional metamorphism in areas of known intrusive activity of a younger age. However, their small muscovite content may produce less reliable ages. Keywords: Pages: 70 Supervisor: Peter Reynolds / Gunter Muecke