Isotopic analysis of early solar system objects by an ion microprobe: parametric studies and initial results

Abstract

Magnesium, potassium and calcium isotope compositions in terrestrial samples and refractory phases from primitive meteorites are determined using an ion microprobe. A thorough investigation of the different instrument parameters is carried out to ensure that conditions necessary for high mass resolution and high precision isotopic studies are adequately satisfied. The instrument can be tuned to achieve mass resolution (M/ΔM) of up to 10,000 (M≤60); it has a very good dynamic stability (ΔB/B#8804;10 ppm over durations of #8804;40 minutes) and the counting system has an effective dead-time of #8804;25 nsec and a dynamic background of #8804;0.01 c/s. Reproducibility and precision of isotopic measurements are checked by analyzing magnesium and titanium isotopic compositions in terrestrial standards and isotopically doped silicate glasses. A precision of 2% (2σ m ) was achieved during magnesium isotopic analysis in samples with low Mg content (200 ppm). Results from studies of magnesium and potassium isotopic compositions in several Ca-Al-rich refractory inclusions (CAIs) from the primitive meteorites Efremovka and Grosnaja, representing some of the early solar system objects, are presented. The well-behaved Mg-Al isotopic systematics confirm the pristine nature of the Efremovka CAIs inferred earlier from petrographic and trace element studies. The Grosnaja CAIs that have experienced secondary alterations show disturbed magnesium isotopic systematics. Observation of excess ²⁶Mg in several of the analyzed CAIs confirms the presence of the now extinct ²⁶Al (t ½ =7×10⁵ years) in the solar nebula at the time of CAI formation. Our data also suggest a relatively uniform distribution of ²⁶Al in the solar nebula. Several Efremovka CAIs with excess ²⁶Mg also have excess ⁴¹K resulting from the decay of ⁴¹Ca (t ½10⁵ years). This observation constrains the time interval between cessation of nucleosynthetic input to the solar nebula and the formation of some of the first solar system solids (CAIs) to less than a million years.