Potassium-argon dating of fine-grained basalts with massive Ar loss: Application of the [formula omitted] technique to plagioclase and glass from the Kirkpatrick Basalt, Antarctica

Abstract

40 Ar 39 Ar incremental-heating measurements are reported for separated plagioclase and matrix fractions and a glass sample of the Kirkpatrick Basalt from Victoria Land, Antarctica. These results are used to address: the limitations of whole-rock analyses of glassy or very fine-grained basalts; the use of plagioclase for determining ages of such rocks; the Ar behavior of matrix, glass and feldspar components; the use of K/Cl ratios in interpretation of step-heating results; and the age of the Kirkpatrick Basalt. The precise age of the Kirkpatrick Basalt has been a long standing problem; the wide range of reported K-Ar dates has been attributed to loss of Ar. Despite massive Ar loss from the matrix, plagioclase is shown to yield good 40 Ar 39 Ar plateaus. Plateaus are observed for increments above 700°C and are defined not only by consistent apparent ages but also appropriately low K/Ca and high K/Cl ratios. Cl relations help identify Ar released from small amounts of matrix which can seriously contaminate a plagioclase separate. An age of 176.8 ± 1.8 Ma defines the time of formation of the youngest lava at Mt. Frustum in north Victoria Land. A glass sample from south Victoria Land gives the same age, indicating that terrestrial basaltic glasses as old as Jurassic do not inherently lose Ar. The step-heating profile of glass with very fine crystals shows discordance which is an artifact of Ar recoil during irradiation.