Common metamorphic quartz samples have abundant fluid inclusions that contain C, N, H2O etc. Understanding the nitrogen content in quartz is especially important since neutron capture by nitrogen would contribute a significant amount of the total in situ 14C production in cosmic ray exposed quartz [1]. To determine carbon and nitrogen content experimentally, we applied nuclear reaction analysis (NRA) using a 1.4 MeV deuteron beam [2]. Sample pellets were made with about ~50 mg of each sample. The nuclear reactions 14N(d,α0) and 12C(d,p0) were used to estimate atomic concentrations of N and C, respectively. In our six samples, the range of nitrogen concentration was 0.008 ± 0.001 to 0.050 ± 0.002 at%, and the carbon content ranged from 0.0005 ± 0.0003 to 0.0017 ± 0.0003 at%. The large errors associated with these estimates are due to low counting statistics from measurements of trace concentrations near the limits of detection.
We also used a Costech Elemental Analyzer to determine nitrogen and carbon content. About 100 mg of each sample was heated rapidly to 1030°C, resulting in the decrepitation of fluid inclusions and the release of C and N gaseous species into the He carrier gas. The nitrogen content of the quartz varies from 0.001 to 0.002 wt%, consistent with that of NRA, but with poor precision because measurements are at the limit of detection. The carbon concentrations of our samples ranges from 0.0086 to 0.0242 wt% with better-defined chromatogram peaks than those of nitrogen. The ratios of C/N concentration in the fluid inclusion of quartz from this method are consistent with published results [3]. Further investigation is needed for the development of a reliable method of determination of C and N in quartz samples that could be used for in situ 14C analysis for exposure dating applications.
[1] Lal and Jull, NIM B92 (1994) 291-296; [2] Vickridge et al., NIM B118 (1996) 608-612; [3] de Ronde et al., Econ. Geol. (2000) 1025-1045.
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See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)