Preliminary Results for the Extraction and Measurement of Cosmogenic in Situ 14C from Quartz

Abstract

Radiocarbon is produced within minerals at the earth's surface (in situ production) by a number of spallation reactions. Its relatively short half-life of 5730 yr provides us with a unique cosmogenic nuclide tool for the measurement of rapid erosion rates (>10^-3 cm yr-1) and events occurring over the past 25 kyr. At SUERC, we have designed and built a vacuum system to extract 14C from quartz which is based on a system developed at the University of Arizona. This system uses resistance heating of samples to a temperature of approximately 1100 degrees C in the presence of lithium metaborate (LiBO2) to dissolve the quartz and liberate any carbon present. During extraction, the carbon is oxidized to CO2 in an O2atmosphere so that it may be collected cryogenically. The CO2 is subsequently purified and converted to graphite for accelerator mass spectrometry (AMS) measurement. One of the biggest problems in measuring in situ 14C is establishing a low and reproducible system blank and efficient extraction of the in situ 14C component. Here, we present initial data for 14C-free CO2, derived from geological carbonate and added to the vacuum system to determine the system blank. Shielded quartz samples (which should be 14C free) and a surface quartz sample routinely analyzed at the University of Arizona were also analyzed at SUERC, and the data compared with values derived from the University of Arizona system.