Determination of cosmogenic ⁴¹Ca in a meteorite with tandem accelerator mass spectrometry

Abstract

Several long-lived cosmogenic radioisotopes- ¹⁴C, ³⁶C1, ²⁶Al, ¹⁰Be, ⁵³Mn and ¹²⁹I-have been used to investigate the history of meteoric material and lunar samples. One of the most interesting results has been the determination of the residence time or ‘terrestrial age' of meteorites found on ablation areas of the Antarctic ice sheets. As these ages range up to 700,000 years¹, ³⁶CI, with a half-life of 3.01 × 10⁵ yr has been the most useful isotope, but for reliable measurements of terrestrial ages below 300,000 yr, a radio-isotope with a half-life between those of ³⁶C1 and ¹⁴C (T_½ = 5730 yr) is needed. Calcium-41 is a suitable candidate, with T_½ = 1.03 × 105 yr, but it occurs in concentrations below the detection limit of conventional techniques. Here we report the first use of tandem accelerator mass spectrometry (JAMS) to measure 41Ca in a natural sample—the Bogou iron meteorite. We find a ⁴¹Ca/Ca ratio of (3.8 ± 0.6) × 10⁻¹² which corresponds to a ⁴¹Ca activity of 6.9 ±1.1 d.p.m. per kg meterorite. This implies that ⁴¹Ca can be successfully used to determine the irradiation history of meteorites.