5:15 PM - 7:15 PM
[PPS07-P13] Quantitative investigation of useful yields for Mg-isotope measurements with SIMS
Keywords:SIMS, Al–Mg system, dating, CAI
The early solar system materials such as CAIs and chondrules contained 26Al at the time of their formation. 26Al is a short-lived radionuclide that decays to 26Mg with a half-life of 0.705 Myr and the 26Al–26Mg systematics has been applied to determine their relative formation ages (e.g., Kita et al., 2013). Secondary Ion Mass Spectrometry (SIMS), which is capable of highly sensitive in situ analysis, has been used to perform high time resolution dating with the 26Al–26Mg systematics for the small rocks and minerals from the early solar system. The 16O– primary beam has been used for the 26Al–26Mg systematics with SIMS because of its high ionization efficiency (e.g., Hutcheon et al., 1989). Recently, it has been reported that higher secondary ion intensities can be obtained when 16O2– is used rather than 16O–, and the use of 16O2– is being used for the 26Al–26Mg systematics (e.g., Siron et al., 2021; Kawasaki et al., 2024). However, it is not clear whether the increased secondary ion intensity when using 16O2– is due to an increase in the sputtering rate or the ionization rate. In this study, we investigate the secondary ion yields (the number of detected secondary ions / the number of sputtered atoms) of Mg isotopes in order to optimize the conditions for the 26Al–26Mg systematics with SIMS (Cameca ims-1280HR).
We used the following standard materials for the 26Al–26Mg systematics of the early solar system materials: San Carlos olivine, Russian spinel, two synthetic anorthite glasses and Miyakejima anorthite, three synthetic polycrystalline hibonite, six synthetic melilite glasses, and nine synthetic fassaite glasses (Kawasaki et al., 2019, 2021, 2024). First, we measured the sputtering rates of each mineral using both 16O– and 16O2– primary ion beams. The results show that, for all minerals, the sputtering rate was approximately three times higher when using 16O2– than when using 16O–.
Next, secondary ion intensities under actual isotopic analysis conditions (e.g., Kawasaki et al., 2020, 2021, 2024) were measured, and the secondary ion yields were calculated using the secondary ion intensities. Based on the secondary ion yields of each mineral, the minerals were classified into three groups, (a), (b), and (c). In group (a), the secondary ion yield is higher when using 16O2– as the primary ion beam than when using 16O–. This group includes olivine and spinel. It is ~20% higher for olivine and ~10% higher for spinel. In group (b), the secondary ion yield is higher when using 16O– compared to 16O2–. This group includes anorthite, hibonite and some melilite. It is ~50% higher for anorthite, ~5-20% for hibonite and ~20-35% for melilite. Group (c) has similar secondary ion yields between 16O2– and 16O–. This group includes some melilite and fassaite. In each group, there are significant differences in analytical conditions due to the use of electron multipliers and Faraday cups depending on the secondary ion intensity. The primary ion beam current and analysis time for 16O2– are as follows: (a) 1 nA, 8 minutes, (b) 20–75 pA, 40 minutes, (c) 15 nA, 6 minutes. Group (b) has the smallest primary ion beam current and the longest analysis time, while group (c) has the largest primary ion beam current and the shortest analysis time. When the same mineral is compared under the conditions of (b) and (c), the secondary ion yield under (c) is ~60% higher than under (b) when a 16O2– beam is used, and ~40% higher when a 16O– beam is used. Our results show that it is important to select the appropriate primary ion beam type according to the mineral to be analyzed and the analysis conditions.
We used the following standard materials for the 26Al–26Mg systematics of the early solar system materials: San Carlos olivine, Russian spinel, two synthetic anorthite glasses and Miyakejima anorthite, three synthetic polycrystalline hibonite, six synthetic melilite glasses, and nine synthetic fassaite glasses (Kawasaki et al., 2019, 2021, 2024). First, we measured the sputtering rates of each mineral using both 16O– and 16O2– primary ion beams. The results show that, for all minerals, the sputtering rate was approximately three times higher when using 16O2– than when using 16O–.
Next, secondary ion intensities under actual isotopic analysis conditions (e.g., Kawasaki et al., 2020, 2021, 2024) were measured, and the secondary ion yields were calculated using the secondary ion intensities. Based on the secondary ion yields of each mineral, the minerals were classified into three groups, (a), (b), and (c). In group (a), the secondary ion yield is higher when using 16O2– as the primary ion beam than when using 16O–. This group includes olivine and spinel. It is ~20% higher for olivine and ~10% higher for spinel. In group (b), the secondary ion yield is higher when using 16O– compared to 16O2–. This group includes anorthite, hibonite and some melilite. It is ~50% higher for anorthite, ~5-20% for hibonite and ~20-35% for melilite. Group (c) has similar secondary ion yields between 16O2– and 16O–. This group includes some melilite and fassaite. In each group, there are significant differences in analytical conditions due to the use of electron multipliers and Faraday cups depending on the secondary ion intensity. The primary ion beam current and analysis time for 16O2– are as follows: (a) 1 nA, 8 minutes, (b) 20–75 pA, 40 minutes, (c) 15 nA, 6 minutes. Group (b) has the smallest primary ion beam current and the longest analysis time, while group (c) has the largest primary ion beam current and the shortest analysis time. When the same mineral is compared under the conditions of (b) and (c), the secondary ion yield under (c) is ~60% higher than under (b) when a 16O2– beam is used, and ~40% higher when a 16O– beam is used. Our results show that it is important to select the appropriate primary ion beam type according to the mineral to be analyzed and the analysis conditions.