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Geochronology Advances in geochronological science
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Discussion papers
https://doi.org/10.5194/gchron-2019-7
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gchron-2019-7
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 27 Jun 2019

Submitted as: research article | 27 Jun 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Geochronology (GChron).

Isolation of quartz for cosmogenic in situ 14C analysis

Keir A. Nichols and Brent M. Goehring Keir A. Nichols and Brent M. Goehring
  • Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA, 70118, USA

Abstract. Froth flotation is a commonly used procedure for separating feldspars and micas from quartz for the preparation of quartz mineral separates to carry out cosmogenic nuclide analysis. Whilst extracting carbon from quartz we observed in situ carbon-14 (14C) concentrations which were anomalously high and in excess of theoretical geological maximum concentrations. Further etching of sample material reduced carbon yields and 14C concentrations, yet the latter remained unrealistically high. When quartz from the original whole rock sample was isolated in our laboratory, we observed even lower carbon yields and geologically plausible in situ 14C concentrations. After ruling out unlikely geological scenarios and systematic measurement issues, we decided to investigate the quartz isolation procedure as a potential source of 14C contamination. We hypothesised that laurylamine (dodecylamine), an organic compound used as part of the froth flotation procedure, elevates 14C concentrations if residual laurylamine is present. We demonstrate that laurylamine has a 14C modern carbon source and thus has the potential to influence in situ 14C measurements if present in minute but measurable quantities. Furthermore, we show that insufficient sample etching results in laurylamine-derived carbon persisting through step heating of quartz and is subsequently collected with the in situ component released at 1100 °C. We therefore demonstrate that laurylamine contaminates in situ 14C measurements. We provide guidelines for the preparation of quartz based on methods developed in our laboratory and demonstrate that all laurylamine derived carbon and 14C is removed when applied. We recommend that the procedures presented be used at a minimum when using froth flotation to isolate quartz for in situ 14C measurements.

Keir A. Nichols and Brent M. Goehring
Interactive discussion
Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Keir A. Nichols and Brent M. Goehring
Keir A. Nichols and Brent M. Goehring
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Latest update: 19 Aug 2019
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Short summary
We describe observations of anomalously high measurements of C-14 made from geologic material. We undertake a systematic investigation to identify the source of contamination, which we hypothesise is sourced from a commonly used method that is used prior to sample analysis. We find that the method does introduce modern carbon to samples and elevates C-14 measurements. We describe a standard procedure that effectively removes contamination from the aforementioned method.
We describe observations of anomalously high measurements of C-14 made from geologic material....
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