2.2 Secondary Ion Mass Spectrometry (SIMS)

The O isotopic compositions of carbonate and magnetite grains were measured in situ using the University of Hawai’i Cameca ims-1280 ion microprobe. Two different analytical setups based upon the methods of Makide et al. (2009) and Nagashima et al. (2015) were used—one for coarse grains (>15 μm) in Renazzo N1127 and GRO 95577, and one for grains smaller than 15 μm, used for the remaining thin sections. For both procedures, a Cs⁺ primary beam with total impact energy of 23 keV was used to sputter O isotopes from the sample.

Large grains were presputtered for 90 seconds to remove the carbon coating and surface contamination using a ~600 pA primary beam focused to ~10 μm and a 10 × 10 μm² raster. The raster size was then reduced to 7 × 7 μm² for data collection. Masses ¹⁶O⁻, ¹⁷O⁻ and ¹⁸O⁻ were measured in multicollection mode using a Faraday cup (FC), the axial electron multiplier (EM) and a FC, respectively. The mass resolving power was set to ~2000 for ¹⁶O⁻ and ¹⁸O⁻_, and to ~5000 for ¹⁷O⁻, sufficient to resolve the interference from ¹⁶OH⁻. Data were collected over 30 cycles to permit monitoring of the ion signals with time and to evaluate their stability. Each cycle ran for 16 s, for a total run time of ~10 min. For grains smaller than ~15 μm, a ~25 pA primary beam was used to obtain a spot size of ~3 μm. All grains were pre-sputtered for 180 s. Because of the low primary beam current, ¹⁸O⁻ was measured with an EM, instead of the FC. Each run consisted of 30 cycles of 46 s each, with a total run time of ~25 min. No raster was used for the reduced-beam-size procedure. In both procedures, the ¹⁶OH⁻ peak was measured at the end of each measurement. Values of ¹⁷O⁻ were corrected for the tail of ¹⁶OH⁻ using a tail/peak ratio of 20 ppm, with a typical contribution of ~0.6‰. To assure accuracy of the final results, data gathered by each method was reduced against standard data gathered in the same way.

All ion-probe pits were imaged using the SEM after measurement to ensure that no cracks or different phases were included in the analysis. Any analyses that were clearly mixed or missed the target mineral were discarded. The data were reduced using the method of total counts to minimize statistical bias (Ogliore et al., 2011). All measurement data were converted into delta notation with units of parts per mil (‰), relative to the composition of Vienna standard mean ocean water (VSMOW; Baertschi, 1976; Fahey et al., 1987):

The data were also expressed as Δ¹⁷O, representing the vertical displacement from the mass-dependent TFL, given in ‰:

Instrumental mass fractionation (IMF) was corrected for via sample-standard bracketing using terrestrial calcite (UWC-1, δ¹⁸O = 23.36‰ and UWC-3, δ¹⁸O = 12.49‰), terrestrial dolomite (UW6250, δ¹⁸O = −21.61‰), and terrestrial magnetite (δ¹⁸O = −6.05‰) standards. Standard reproducibility for δ¹⁸O was 0.5‰, 0.4‰, 0.7‰, and 1.1 to 2.0‰ for UWC-1, UWC-3, UW6250, and magnetite standards, respectively; reproducibility for δ¹⁷O was 0.6 to 0.9‰, 1.2‰, 0.8‰, and 1.2 to 1.8‰ (2σ standard deviation). Reported uncertainties on the meteoritic mineral data reflect the propagation of both the internal analytical precision and the external reproducibility of the standards.