Pitfalls in 230Th– 238U dating of young Quaternary volcanic rocks: : a case study from Fornicher Kopf (East Eifel Volcanic Field, Germany)

Abstract

The eruption age of the Fornicher Kopf basanite lava flow (East Eifel Volcanic Field, Germany) is of interest because it can provide an age constraint for part of the Quaternary Rhine river terrace chronology. Previous dating efforts by K–Ar and α-counting 230Th– 238U methods produced a wide dispersion of ages. This paper presents a fresh attempt to date this lava flow by the potentially more precise mass spectrometric 230Th– 238U mineral isochron technique. Unfortunately, this study also proved unsuccessful because it did not produce a meaningful mineral isochron: the only age information is provided by the fact that the groundmass shows 230Th– 238U disequilibrium, which implies that the eruption age was <350 ka. However, the mineral data clearly highlight some of the problems that can plague efforts to date young lavas by the 230Th– 238U method: (i) inherited older crystals; magnetite and pyroxene plot on the equiline (i.e. 230Th and 238U in secular equilibrium) and tielines to the groundmass data produce impossible ages, indicating that these crystals are older than the host magma, and (ii) open system behaviour; phlogopite has experienced variable, recent U exchange, which has implications for the reliability of this phase for alpha-recoil dating.