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 δ¹³C_VPDB) 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 δ¹⁵N_Air) 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.