Low-level Magnesium Isotopic Analysis for the Genesis Mission

Introduction: The Sun includes 99.9% of the mass of the solar system, so analysis of the isotopic compositions of elements from the Sun should be representative of the initial and/or bulk isotopic composition of planetary materials. Gravitational settling from the Outer Convection Zone (OCZ) of the Sun may contribute towards systematic light isotope enrichment of a few per mil detectable in the solar wind [1]. During the generation of the solar wind, isotopic fractionation of accelerated ions is expected due to inefficient coulomb drag [1, 2]. Enrichment of the lighter isotopes of noble gases has been reported in GENESIS DoS wafers [3], but not confirmed in AloS [4]. Magnesium isotopic composition is a major target for solar astrophysics because it is a low FIP (7.6 eV) element with three isotopes that have favorable abundances for precise determination (Mg: 78.99 %, Mg: 10.00 %, Mg: 11.01 %) and no atomic isobaric interferences [5, 6, 7]. The predicted mass fractionation in Mg is on the order of tens of per mil (dominantly from inefficient coulomb drag) [1], but spacecraft-borne measurements of Mg isotopic composition do not have sufficient precision to confirm a solar wind-terrestrial difference in isotopic composition of Mg [5, 6, 7]. The Mg isotopic composition is likely to be uniform in bulk chondrites and planets [8, 9], and by inference in the bulk Sun [1]. The GENESIS mission has directly sampled implanted solar wind in high-purity wafers [10]. The isotopic measurements of Mg in the collected solar wind is challenging for two reasons. The implant solar wind levels are very low for Mg (2x10 atoms/cm) [10], and the removal of post-crash UTTR contaminants, established to be about a hundred times higher than the solar wind implant [11, 12], needs to be >99.9 %. Determination of the solar wind Mg isotopic composition requires the ability to precisely measure the Mg isotopic composition of picogram quantities of Mg. Here, we report a method developed for low-level Mg isotopic analysis, and some initial results. Analytical Methods: Measurements of Mg isotopic composition were performed at the Plasma Analytical Facility, Florida State University, on a ThermoFinnigan NeptuneTM multicollector ICP-MS equipped with 8 movable and one fixed (axial) faraday cup detectors. An ESITM Apex nebulizer (80 μl/min) was used for sample introduction. The masses 24, 25, 26 and 27 were simultaneously collected on the L4, L2, Axial, and H2 faraday cups, respectively. There are no atomic isobaric interferences on Mg or Al. Because of a molecular isobaric interference from CN on Mg, that is significant at these low intensities, we measured all peaks using the high-resolution offset technique [13]. At high-resolution on the NeptuneTM, the molecular isobar is offset from the atomic peak to higher mass, such that a flat shoulder of the atomic peak is available, and the atomic peak is then measured on the flat portion of the shoulder instead of on the peak center. We have determined the precision with which Mg isotopic composition could be determined for small quantities of Mg (~10 atoms) using 2 ppb (5E13 atoms/ml) and 10 ppb (2.5E14 atoms/ml) reagent Mg standard solutions. These solutions were prepared by dilution with 2% HNO3 of an in-house 10 ppm Mg solution prepared from high-purity Mg metal for calibration of our Mg spike [12]. Figure 1 shows the 2σ relative standard error of the mean (‰) as a function of the number of atoms consumed by the measurement using2 ppb (black) and 10 ppb (red) Mg solutions. Each data point represents a 4-second integration of the beam, and a total of 300 ratios were obtained. The theoretical performance curve is determined by calculating the standard error of the mean of N ratios using the standard deviation of the 300-ratio dataset. The actual performance (crosses) for the test performed here is shown as the cumulative standard deviation divided by the square root of N=2, 3, . . . . 300 ratios.