Microscale Simultaneous Measurement of Carbon and Nitrogen Isotopes on Natural Diamond

Abstract

Simultaneous analysis of carbon and nitrogen isotope ratios by SIMS was applied for the first‐time to a natural diamond from the Kelsey Lake kimberlite, State Line Distinct, Colorado (UWD‐1). This in situ procedure is faster, reduces sample size for analysis, and measures both isotope ratios from a single ~ 10 μm diameter pit, a critical advantage for zoned diamonds. The carbon isotope ratio (expressed as δ13CVPDB) of the bulk UWD‐1 crystal, determined by the conventional combustion method in the present study, is ‐5.9‰ ± 0.2‰ (VPDB, 2s). Nitrogen mass fraction ([N]) and isotope ratio (expressed as δ15NAir) were determined by stepwise combustion and gas‐source mass‐spectrometry, resulting in 553 ± 64 μg g‐1 and ‐6.7‰ ± 1.1‰ (Air, 2s), respectively. Secondary ions of 12C2‐, 12C13C‐, 12C14N‐, and 12C15N‐ were simultaneously measured by SIMS using three Faraday cups and one electron multiplier. The spot‐to‐spot reproducibility of δ13C and δ15N values for the UWD‐1 (178 spots on sixteen chips, 10 μm spots), were 0.3‰ and 1.6‰, respectively (2s). While 12C14N‐/12C2‐ ratios, which are an indicator for [N], varied up to 12% among these sixteen chips, such variation did not correlate with either δ13C or δ15N values. We propose that UWD‐1 is a suitable reference sample for microscale in situ analysis of δ13C and δ15N values in diamond samples.