Combined Separation Of Iron, Calcium, And Magnesium From Composite Lunar Samples For High-Precision Isotope Analyses

Abstract

: Fe, Ca, and Mg isotopes have been applied to Apollo and Luna samples in order to explore the origin and evolution of the Moon. In most previous studies, only one specific element was separated and measured for its isotope compositions, thereby consuming different splits of samples. Considering the rarity and preciousness of lunar samples, herein we present a three-step purification scheme to separate Fe, Ca, and Mg from a single aliquot of sample solutions with the amounts of these elements down to 20, 15, and 10 μg, respectively. Fe was first collected using 0.4 mL of AG1-X8 resin in a HCl medium. Matrices from the first column containing Ca and Mg were then passed through columns filled with 0.4 mL of DGA resin, with Mg being eluted by 11.5 mL of 4N HNO 3 , and Ca being quantitatively recovered by 10 mL of 8N HNO 3 + 0.2N HF. Mg was then purified using AG50W-X8 in a 1N HNO 3 medium. Fe and Mg isotope analyses were conducted by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), and Ca isotope data were obtained by Triton Plus thermal ionization mass spectrometry (TIMS) following procedures previously established in the same laboratory. Quantitative recovery of Fe, Ca, and Mg for the entire procedure was demonstrated by ten composite samples analogous to the representative lunar samples from the Apollo missions. The shifts in δ 56 Fe, δ 44/42 Ca, and δ 26 Mg of the composite samples were -0.003 ± 0.013‰ (2SD, N = 30), -0.007 ± 0.028‰ (2SD, N = 7), and 0.006 ± 0.015‰ (2SD, N = 29), respectively, ensuring high precision and accuracy of the procedure for combined Fe, Ca, and Mg isotope analyses. The robustness of the procedure was further assessed by replicate analyses of three well-characterized international reference materials (BCR-2, BHVO-2, and W-2a), and the results were consistent with previously published values. The procedure established herein allowed us to obtain combined Fe, Ca, and Mg isotopic datasets from single splits of precious extra-terrestrial samples, thereby providing new insights into the origin and evolution of their parent bodies ( e.g. , the Moon).