Evidence for distortion of Tertiary K/Ar ages by excess argon - example given by three alkali olivine basalts from Northern Hesse, Germany
Rittmann, Ursula; Lippolt, Hans Joachim
published: Jan 26, 1998
manuscript accepted: Sep 19, 1997
manuscript received: May 10, 1996
ArtNo. ESP147051001010, Price: 29.00 €
Abstract Systematic K/Ar analyses of mineral concentrates from three alkali olivine basalts from Northern Hesse (Germany) are presented. Two of them (Stellberg, Staufenberg) yielded markedly high whole-rock ages of about 22 Ma, suggesting the presence of excess argon, as often observed in Quaternary volcanic rocks. A third rock (Landsburg), with a geologically meaningful whole-rock age of 15 Ma, served as a reference sample. Various mineral phases from all three specimens yielded excess argon ranging from 0.1 to 1 nl/g. Linear arrays were obtained on K-Ar isotope correlation diagrams, indicating ages of 20, 10 and 13 Ma and 40Ar/ 36Ar initial ratios of 311, 500 and 298, respectively. Compared to these ages, the whole-rock results are too high by 35, 58 and 8 %. In the reference sample (Landsburg) the excess argon is observed only in the concentrates of mafic phenocrysts, whereas in the rest of the analysed phases, it is below the limits of detection. In the Stellberg sample, the excess argon is distributed over all the analysed phases. In the Staufenberg sample, two excess argon components are observed, one in the mafic phases (as in the other two samples, probably derived from the magma) and another component in the least dense minerals (plagioclase, alteration products, resorbed quartz), which is thought to result from crustal contamination or a hydrothermal overprint. Excess argon often makes K/Ar work in Quaternary rocks problematic, but it may also lead to dating problems in Upper Tertiary volcanic rocks. The analysis of different 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 candidates for containing excess argon.