Formula omitted] step-heating and laser fusion dating of a quaternary pumice from Neschers, Massif Central, France: The defeat of xenocrystic contamination

Abstract

K-feldspars from a Quaternary pumice flow near Neschers, France, yield 40 Ar 39 Ar age spectra which display a marked “saddle shape” whose minima are well in excess of the age of an underlying basalt unit. The K-feldspars form two populations: (1) a group of euhedral and clear crystals; and (2) a group of rounded cloudy grains. By 40 Ar 39 Ar dating of individual grains by laser fusion it was possible to demonstrate that the clear grains are 0.58 Ma old, while the cloudy grains are contaminating ancient feldspars with ages as old as 330 Ma. Subsequently, hand-picked mineral separates were analysed by step-heating in a radio-frequency fusion system and the results show: (1) the ancient cloudy grains are probably Hercynian-age feldspars which were included in the young pumice during eruption and have only lost modest amounts of 40Ar during the eruptive re-heating; and (2) the saddle shape of the original 40 Ar 39 Ar age spectra was due solely to the different Ar-release patterns of the two populations of feldspars and not due to excess 40Ar incorporated within all of the grains. This study shows that the ability to measure single grains is critical in unravelling the ages of young volcanic materials when country-rock contamination is likely. Conventional KAr ages of bulk samples of hand-picked feldspars may be seriously in error, even when contaminations are at a level of less than one grain in a thousand.