Excess argon and dating of Quaternary Eifel volcanism, IV. Common argon with high and lower-than-atmospheric 40Ar/ 36Ar ratios in phonolitic rocks, East Eifel, F.R.G.

Abstract

Five foid-bearing rocks from the Quaternary East Eifel volcanic field have been investigated using 40Ar/ 39Ar analyses on sanidine, leucite, hornblende and nosean/hauyne for age and Ar isotope systematics. Four of the rocks belong to the 400 ka phase and one to the 10 ka phase of volcanic activity known from this area. Additionally, 40Ar/ 36Ar ratios have been measured on nosean phenocrysts from five supplementary rocks of the 400 ka phase. Nosean and hauyne yielded high amounts of common Ar with an isotopic composition which differs from atmospheric Ar. Two types of common Ar are distinguished: fractionated Ar in the garnet-bearing rocks ( 40Ar/ 36Ar< 290) and Ar enriched in radiogenic 40Ar in the garnet-free rocks ( 40Ar/ 36Ar up to 6500) indicating defferent petrogenetic evolution for these two rock groups. The increased apparent K Ar ages of the noseans from the garnet-free group are attributed to common Ar composed of a mixture of unsupported 40Ar and of Ar atm. The rocks in which nosean/hauyne phenocrysts yield anomalously low 40Ar/ 36Ar ratios are assumed to have interacted with crustal hydrothermal fluids containing large amounts of fractionated Ar with 40Ar/ 36Ar ratios down to 280. Incorporation of such common Ar components results in drastically decreased apparent ages of the noseans/hauynes. Leucite and sanidine phenocrysts may also occasionally be affected, but only to a relatively minor degree. Thus, not only excess 40Ar but also fractionated Ar must be expected to be a severe source of error in K Ar dating of young foid-bearing volcanic rocks and their minerals producing artificially low K Ar dates.