Isolation of quartz for cosmogenic in situ <sup>14</sup>C analysis

Abstract

Froth flotation is a commonly used procedure for separating feldspars and micas from quartz for the preparation of 5 quartz mineral separates to carry out cosmogenic nuclide analysis. Whilst extracting carbon from quartz we observed in situ carbon-14 ( 14 C) concentrations which were anomalously high and in excess of theoretical geological maximum concentrations.Further etching of sample material reduced carbon yields and 14 C 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 14 C concentrations. After ruling out unlikely geological scenarios and systematic measurement 10 issues, we decided to investigate the quartz isolation procedure as a potential source of 14 C contamination. We hypothesised that laurylamine (dodecylamine), an organic compound used as part of the froth flotation procedure, elevates 14 C concentrations if residual laurylamine is present. We demonstrate that laurylamine has a 14 C modern carbon source and thus has the potential to influence in situ 14 C 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 15 subsequently collected with the in situ component released at 1100 °C. We therefore demonstrate that laurylamine contaminates in situ 14 C measurements. We provide guidelines for the preparation of quartz based on methods developed in our laboratory and demonstrate that all laurylamine derived carbon and 14 C 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 14 C measurements. Introduction 20In the course of extracting carbon from quartz we have, on multiple occasions, observed concentrations of in situ 14 C that were anomalously high and in excess of geologically plausible maximum concentrations. We hypothesise that the elevated in situ 14 C concentrations are sourced from part of the widely used mineral separation procedure known as froth flotation, a process that relies on the long-chain organic compound laurylamine. Our observations, combined with a desire to continue use of froth flotation for the benefits it provides during quartz separation, form the motivation for this paper. In this study we 25 explore both the potential influence that quartz isolation procedures have on resulting measured 14 C concentrations as well as procedures to minimise potential contamination during use of froth flotation.Froth flotation is a method by which feldspars and, to a lesser extent, micas are separated from quartz (Herber, 1969).The method precedes acid etching as part of the quartz isolation process for cosmogenic nuclide analysis and is used by