Laser probe 40Ar- 39Ar studies of the Peace River shocked L6 chondrite

Abstract

Samples of matrix and melt glass from the shocked L6 chondrite Peace River have been subjected to 40 Ar- 39 Ar analysis using both stepped heating and localised outgassing by a laser probe. Stepped heating studies of three distinct samples of matrix gave indistinguishable and unusually well-defined low temperature ( 40Ar/ 39Ar) plateaux, corresponding to an age of 450 ± 30 Ma. In an attempt to investigate whether or not this plateau age is meaningful (i.e. the time of a major collision event involving the parent body) or is an artefact resulting from partial degassing, during the fragmentation of the parent body which commenced the exposure of Peace River to cosmic rays 40 Ma ago, we have used a laser probe to analyse Ar from single 80 μm diameter laser pits which release gas from a few micrograms of sample. On this scale no areas have ages younger than the plateau age, supporting the view that it is not an artefact. Measurements on the melt glass gave variable and meaningless ages, in some cases greater than the age of the solar system, due to retention of 40Ar and loss of K during melting followed by rapid quenching. This observation indicates the danger of attempts to date impact melt glass directly. The observation that argon has been retained by the glass and lost by the matrix can be understood in terms of a range of plausible annealing histories.