10:45 AM - 11:00 AM
[MIS23-07] The latest stage of zircon trace element analysis by SHRIMP
★Invited Papers
Zircon (ZrSiO4) is the most widely used mineral for geochronological and geochemical investigations owing to its favorable properties, such as high physicochemical stability, adequate content of trace elements (TE) such as U, rare-earth elements (REE), and Ti (e.g., Armstrong–Altrin et al., 2018 and references therein), and exhibits better retention of radiogenic Pb (>900 ℃; e.g., Cherniak, 2010 and references therein). TE in zircon, especially Nb and Sc, serve as indicators for estimating the source melt and the environment where it crystallized (Grimes et al., 2015). U–Pb zircon geochronology combined with TE geochemistry is a powerful tool for investigating the geochemical evolution of the source magma. Secondary ionization mass spectrometry (SIMS) is one of the suitable methods for analyzing multiple element contents and isotopic ratios with high spatial resolution and minimal sample loss. A sensitive high-resolution ion microprobe (SHRIMP) is one of the large-geometry SIMS with a cylindrical electrostatic field-quadratic lens-homogeneous magnetic sector field (CQH), which was developed by the team led by the Australian National University (ANU) and the Australian Scientific Instruments (ASI). We applied CQH SHRIMP (SHRIMP-IIe) analysis to the TE, including Nb and Sc, of zircons from a hornblende peridotite collected from the Hida Belt, Japan (Itano et al., 2023). This presentation will focus specifically on the methodology of SHRIMP-IIe for the TE analysis of zircon in this application.
The zircon TE analysis, such as Nb and Sc, often has difficulty in accurate measurement due to isobaric interference derived from the major elements. For example, the mass spectrum of the Sc cation (45Sc+) is very close to that of the Zr doubly charged cation (90Zr2+), and the mass spectrum of the Nb cation (93Nb+) is also close to that of the Zr hydride cation (92ZrH+). Therefore, the analysis of these targeting cations cannot avoid these isobaric interferences, namely overcounting, with the mass resolution sufficient for U-Th-Pb dating (M/DM ~5500 at 1% height). The following two methods are generally implemented to avoid isobaric interference in SHRIMP analysis: energy-filtering and high-mass-resolution modes. The secondary ion energy distribution of monoatomic ions spreads to the high energy side rather than that of molecular and doubly charged ions. Therefore, an energy offset effectively decreases the yields of molecular and doubly charged ions over atomic ions. However, the energy-filtering method is not suitable for these elements because molecular and doubly charged ions of Zr are much more than the monoatomic ions of Sc and Nb. The stronger energy offset to avoid the larger interference leads to a reduction in the secondary ions, including the monoatomic ions. On the other hand, in the high-mass-resolution method, which is to narrow the mass spectra with flat tops by mainly adjusting the two slit widths in the secondary ion beam path, the SHRIMP’s high transparency minimizes attrition of the secondary ions. Therefore, the high-mass-resolution method should be applied to the zircon TE analysis, such as Nb and Sc, which achieves high mass resolution enough to analyze them (M/DM ~14000 at 1% height).
The zircon TE analysis, such as Nb and Sc, often has difficulty in accurate measurement due to isobaric interference derived from the major elements. For example, the mass spectrum of the Sc cation (45Sc+) is very close to that of the Zr doubly charged cation (90Zr2+), and the mass spectrum of the Nb cation (93Nb+) is also close to that of the Zr hydride cation (92ZrH+). Therefore, the analysis of these targeting cations cannot avoid these isobaric interferences, namely overcounting, with the mass resolution sufficient for U-Th-Pb dating (M/DM ~5500 at 1% height). The following two methods are generally implemented to avoid isobaric interference in SHRIMP analysis: energy-filtering and high-mass-resolution modes. The secondary ion energy distribution of monoatomic ions spreads to the high energy side rather than that of molecular and doubly charged ions. Therefore, an energy offset effectively decreases the yields of molecular and doubly charged ions over atomic ions. However, the energy-filtering method is not suitable for these elements because molecular and doubly charged ions of Zr are much more than the monoatomic ions of Sc and Nb. The stronger energy offset to avoid the larger interference leads to a reduction in the secondary ions, including the monoatomic ions. On the other hand, in the high-mass-resolution method, which is to narrow the mass spectra with flat tops by mainly adjusting the two slit widths in the secondary ion beam path, the SHRIMP’s high transparency minimizes attrition of the secondary ions. Therefore, the high-mass-resolution method should be applied to the zircon TE analysis, such as Nb and Sc, which achieves high mass resolution enough to analyze them (M/DM ~14000 at 1% height).