# Oxygen Isotopic Compositions of San Carlos Olivine Standard NMNH 111312-42
## 2 Materials and Methods  
Secondary ion mass spectrometer (SIMS) analysis is an in situ analytical technique for measuring trace element abundance and isotopic ratios of geological samples at \(\upmu\)m size. Along with the development of this analytical method, in situ \(\delta^{18}\)O measurements have been extensively applied on silicate minerals and glasses (e.g., zircon, olivine, and quartz) [8]. During in situ analysis, standard samples were used to calibrate \(\delta^{18}\)O results of unknown samples. Therefore, standard samples are crucial in the application of in situ oxygen isotopic analysis.  
This study analyzed the oxygen isotopic compositions of -42 and -44, using a CAMECA IMS-1280 secondary ion mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences, China. The CAMECA IMS-1280 SIMS is a large-geometry, double-focusing instrument, which has the same radius (585 mm) for both the electrostatic sector analyzer (ESA) and the sector magnet. It is equipped with two primary ion sources, the Duoplasmatron source, which uses oxygen gas, and the cesium ion source [9]. The Gaussian primary beam size could be adjusted to determine the \(\delta^{18}\)O value. Randomly selected -42 and -44 olivine grains, which were both provided by the Smithsonian Institution, were mounted in epoxy resin and polished. The polished mount is cleaned in high-purity ethanol using an ultrasonic bath and then vacuum-coated with high-purity gold prior to the SIMS analysis [10]. The oxygen isotopes of both -42 and -44 olivine grains were measured in two sessions (July and October 2020) to check their homogeneity and consistency. The instrument configuration and analytical protocol are similar to that of [PERSON] et al. [11]. The Cs\({}^{+}\) ions were used as the primary beam and worked in Gaussian mode to sputter olivine for oxygen isotope analysis. The primary beam size was -15 \(\upmu\)m in diameter, and the intensity was 3.8-6.5 nA. The primary beam worked in raster mode with an area of 10 \(\times\) 10 \(\upmu\)m\({}^{2}\) during analysis. The \({}^{16}\)O and \({}^{18}\)O ions were detected simultaneously by two Faraday cups, and the signals were amplified  
Figure 1: Primitive mantle-normalized average concentrations of the NMNH 111312–42 reference material (orange) [5] compared to the concentrations of the NMNH 111312–44 reference material (light blue) [6,7].  
by 109 ohms and 1012 ohms resistors, respectively. A normal electron gun was used to compensate for the charging effect in the bombarding area. The width of the entrance slit was -200 um; the size of the field aperture was 5000 \(\times\) 5000 um\({}^{2}\); the width of the energy slit was 40 eV. The magnification of the transfer system was configured as -133. Each analysis consists of pre-sputtering, beam centering, and signal collecting processes. The collecting process consists of 12 cycles, and each cycle lasts 4 s. Typical \({}^{18}\)O peak ion intensities of 1.1-1.8 \(\times\) 10\({}^{7}\) counts per second yield an internal 1\(\sigma\)-uncertainty of less than 0.1%.  
The measured \({}^{18}\)O/\({}^{16}\)O ratios were normalized using Vienna Standard Mean Ocean Water compositions (VSMOW, \({}^{18}\)O/\({}^{16}\)O = 0.0020520) [6]:  
\[\left(\delta^{18}\mathrm{O}\right)_{\mathrm{M}}=\left(\frac{\left({}^{18} \mathrm{O}/^{16}\mathrm{O}\right)_{\mathrm{M}}}{0.0020052}-1\right)\times 1000 \% \tag{1}\]