EVIDENCE FOR DISTORTION OF TERTIARY K AR AGES BY EXCESS ARGON - EXAMPLE GIVEN BY 3 ALKALI OLIVINE BASALTS FROM NORTHERN HESSE, GERMANY/

Systematic K/Ar analyses of mineral concentrates from three alkali oli vine basalts from Northern Hesse (Germany) are presented. Two of them (Stellberg, Staufenberg) yielded markedly high whole-rock ages of abou t 22 Ma, suggesting the presence of excess argon, as often observed in Quaternary volcanic rocks. A third rock (Landsburg), with a geologica lly meaningful whole-rock age of 15 Ma, served as a reference sample. Various mineral phases from all three specimens yielded excess ranging from 0.1 to 1 nl/g. Linear arrays were obtained on K-Ar isotope corre lation diagrams, indicating ages of 20, 10 and 13 Ma and Ar-40/Ar-36 i nitial ratios of 311, 500 and 298, respectively. Compared to these age s, the whole-rock results are too high by 35, 58 and 8 %. In the refer ence sample (Landsburg) the excess argon is observed only in the conce ntrates of mafic phenocrysts, whereas in the rest of the analysed phas es, it is below the limits of detection. In the Stellberg sample, the excess argon is distributed over all the analysed phases. In the Stauf enberg sample, two excess argon components are observed, one in the ma fic phases (as in the other two samples, probably derived from the mag ma) and another component in the least dense minerals (plagioclase, al teration products, resorbed quartz), which is thought to result from c rustal contamination or a hydrothermal overprint. Excess argon often m akes K/Ar work in Quaternary rocks problematic, but it may also lead t o dating problems in Upper Tertiary volcanic rocks. The analysis of di fferent mineral phases leads to a clearer view of the K-Ar systematics , enabling the discrimination of the excess argon carriers. This helps to establish eruption ages even for volcanic rocks that are candidate s for containing excess argon.