High Resolution Cl and S Isotope Analyses in Rocks and Minerals Using NanoSIMS

Abstract:

We have developed highly spatial resolution isotope analysis with cycling magnetic fields on NanoSIMS 50 (Cameca, Gennevilliers, France) housed at Atmosphere and Ocean Research Institute, Japan. This technique requires two unmovable EM detectors (EM1 and EM2) and three magnetic fields (B1, B2, and B3). For Cl measurements, the following configuration was applied: (B1-EM1) ³⁵Cl^-; (B2-EM1) ³⁷Cl^-; (B2-EM2) ³⁵Cl^-; (B3-EM2) ³⁷Cl^-, in the form of “(B and EM configurations) target ions” (Table 1). When magnetic fields are in a cycling mode and moving next to next, we can obtain three isotopic ratios from the same spot: ³⁷Cl_EM1/³⁵Cl_EM1, ³⁷Cl_EM2/³⁵Cl_EM2, and ³⁷Cl_EM1/³⁵Cl_EM2 where subscripts show corresponding detectors. Considering the average of ³⁷Cl_EM1/³⁵Cl_EM1 and ³⁷Cl_EM2/³⁵Cl_EM2, reproducibility gets higher because total counts increase and analytical artifacts due to difference between detectors’ sensitivity are offset. 10-μm spots were used for analyses. Reproducibility of ³⁷Cl/³⁵Cl in an apatite crystal from Imilchil/Errachidia region in Morroco was 1.1‰ (1σ; N=10), and that in RY380-R03b (EPR basalt: 17.4S, 113.2W) was 1.3‰ (1σ; N=8). These values may be small enough to evaluate aqueous alteration on mantle-derived materials because it causes 6‰ variation in reaction related reservoirs such as sediments and pore fluids^[1]. There were no apparent correlations between Cl contents and ³⁷Cl/³⁵Cl in RY380-R03b, supporting that strong correlations between them found in MORB by a pyrohydrolysis method^[2] may be analytical artifacts^[1]. We also measured S isotope ratios based on the same method as Cl. 5-μm spots were used for analyses. Reproducibility of ³⁴S/³²S in a pyrite crystal from Udo mine in Japan^[3] was 1.8‰ (1σ; N=6), and that in CH98 DR12 (MAR basalt: 30.1N, 41.9W) was 1.7‰ (1σ; N=5). These values are small enough to evaluate S fractionation or mixing in the basalt-seawater system which have 20‰ difference in δ³⁴S. This technique will enable us to study magma compositions through the geological time by inclusion analyses which cannot be achieved by bulk measurements such as combustion and pyrolysis methods.

[1] Sharp et al. (2013) GCA 107, 189-204. [2] Bonifacie et al. (2008) Science 319, 1518-1520. [3] Nishizawa et al. (2010) Rapid Commun. Mass Spectrom. 24, 1397-1404.