K-Ar-Altersbestimmungen an Ultrabasiten des Baltischen Schildes

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Abstract
The K-Ar-ages of five ultrabasic rocks from the Baltic Shield were determined. K-Ar-ages of 3.7 to 8.8 x 109 y were obtained. Two samples were cut into fractions with different potassium content and a partial correlation of the argon content with the potassium content in the separated fractions was noticed. The question of extraneous argon causing higher age values is discussed. Taking into consideration the previous results obtained by GERLING et al., it may be said, that extremely high Ar40/K-ratios are the rule for ultrabasic rocks from the Baltic Shield. As a consequence of this fact, it is not likely that all these rocks have the age only of the precambrian pluton in which they are incorporated. Assuming that the alpinotype peridotites were transported upwards in the form of solid fractions from the upper mantle, it is possible, that actually radiogenic argon exists more or less in the sample. The highest Ar40/K-ratios (according to ages ranging from 5.9 to 10.9 x 109 y) exhibit the samples with potassium contents below 100 ppm. Samples with potassium contents more than 1‰ have much smaller Ar40/K-ratios (according to ages from 4.2 to 5 x 109 y) . Excess argon probably causes the extremely high “ages” of more than 7 x 109 y. The ages from relative potassium-rich samples will eventuible show the true age. The most likely value for this age is 4.2 x 109 y, measured for a peridotite with 0.49% potassium content.

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