Beryllium-10 concentrations of tektites from the Ivory Coast and from Central Europe: Evidence for near-surface residence of precursor materials

By using accelerator mass spectrometry, we measured ¹⁰Be ( T_1/2 = 1.5 Ma) concentrations in nine Ivory Coast (IVC) tektites, in six soil samples collected near the Bosumtwi impact crater, the likely source region, and in a depth profile taken through a 23 g moldavite. In the core of the moldavite sample we also measured an upper limit on the ³⁶Cl ( T_1/2 = 0.3 My) concentration. The average ¹⁰Be concentration in IVC tektites of (22 ± 11) × 10 ⁶ atom/g exceeds reasonable limits for a meteoritic component or cosmic-ray production in situ after tektite formation. The ¹⁰Be must be meteoric, which implies that IVC tektites formed from soils or sediments. Corrected to the time of formation (ToF) 1.07 Ma ago and for a small in situ component, the average ¹⁰Be concentration of (35 ± 7) × 10 ⁶ atom/g (1 - σ mean) is considerably lower than those of contemporary Bosumtwi soils, ∼250 × 10 ⁶ atom/g, or of Australasian tektites at their ToF, 0.8 Ma B.P. near Lake Bosumtwi today the soil column is only ∼1 m thick. If the landscape was similar 1.07 Ma ago, then the total thickness of the tektite formation zone probably did not exceed 10 m. With increasing depth below the surface of the moldavite, the ¹⁰Be concentrations decrease rapidly owing to the presence of a surface component, probably of recent origin. The main interior mass of the sample contains ∼0.8 × 10 ⁶ atom ¹⁰Be/g and fewer than 0.1 × 10 ⁶ atom ³⁶Cl/g, little of which can be meteoritic. Although not definitive, consideration of several possible cosmic-ray exposure histories suggests that about half the interior ¹⁰Be has a meteoric origin, which if corrected to the time of formation yields a concentration compatible with those measured in typical contemporary soils. The observations are consistent with the formation of three of the four main tektite groups from surface soils or sediments.