ABSTRACT

In this pilot study an extension of whole-rock K-Ar dating methods of volcanic rocks is established. Whole-rock sieve fractions 80-250 μm of basaltic rocks were etched for durations up to 20 min in 6% hydrofluoric acid at room temperature and cleaned ultrasonically. Aliquots thereof were further separated into magnetic and nonmagnetic subfractions. Conventional K-Ar analyses reveal disturbed K-Ar systems in the rocks and nonmagnetic fractions, as both Ar loss and extraneous Ar are observed. Ar loss occurred in high-potassium matrix phases, while olivine- and clinopyroxene-phenocrysts, which are mostly xenocrysts, are carriers of extraneous Ar. Magnetic fractions etched for 5 min exhibit no significant Ar losses nor extraneous Ar and show internally consistent ages or age concordance with minerals of proven suitable quality for K-Ar dating. The method is suited for unravelling extrusion ages of volcanic rocks in which the whole-rock K-Ar system is disturbed and should be applied whenever adequate minerals cannot be separated for dating purposes. The eight basaltic rocks investigated stem from four localities in the Hocheifel Area, FRG; all of them show significant disturbances in their K-Ar system.