Nitrogen and Carbon Isotopic Composition of the Sun Inferred from a High-Temperature Solar Nebular Condensate

Abstract

We report high-precision measurements of nitrogen and carbon isotopic compositions of a carbon-bearing titanium-nitride (osbornite) in a calcium-aluminum-rich inclusion (CAI) from the CH/CB-like carbonaceous chondrite Isheyevo. The mineralogy and petrography of the CAI and thermodynamic calculations indicate that the osbornite formed by gas-solid condensation in a high-temperature (~2000 K) region of the solar nebula. Because isotopic fractionation at high temperature is small, the measured nitrogen [15N/14N = (2.356 ± 0.018) × 10 -3] and carbon [13C/12C = 0.01125 ± 0.00008; 1 σ] isotopic compositions of the Isheyevo osbornite are representative of the solar nebula and, hence, of the Sun. This conclusion is supported by the observations that (1) the measured 13C/ 12C ratio is indistinguishable from the spectroscopic determination of the 13C/12C ratio of the solar photosphere and (2) the measured 15N/14N ratio of osbornite is in excellent agreement with the Galileo spacecraft measurement of the nitrogen isotopic composition of the Jovian atmosphere, the second largest reservoir of nitrogen in the solar system. The inferred 15N/ 14N ratio of the solar nebula is also similar to the nitrogen isotopic composition of the vast majority of chondritic nanodiamonds, suggesting their solar nebula origin.