Extraction and measurement of cosmogenic in situ 14C from quartz

QC Physics;T Technology (General)

Naysmith, Philip ; Gordon, Cook

University:University of Glasgow

Department:School of Geographical and Earth Sciences

关键词: QC Physics;    T Technology (General);   

Others  :  http://theses.gla.ac.uk/548/1/2007naysmithmsc.pdf

来源: University of Glasgow

【 摘 要 】

Unlike 14C that is produced in the upper atmosphere by the 14N(n,p)14C reaction, in situ 14Cis produced within minerals at the earth’s surface by a number of spallation reactions including 17O(n,α)14C,16O(n,2pn)14C and 14N(n,p)14C (Gosse & Phillips, 2001).A range of cosmic-ray produced radionuclides including 10Be, 26Al and 36Cl, which are formed in surface minerals, are now used to establish ages for formerly un-dateable deposits, however, their long half-lives render them insensitive to recent events and rapidly eroding deposits.Pure quartz (SiO2) is an ideal mineral for in situ 14C dating due to its lack of cleavage in the mineral grains, ensuring resistance to contamination by atmospheric 14C.This resistance to weathering under surface conditions, coupled with the relatively short half-life of 5730 years, provides a unique cosmogenic nuclide tool for the measurement of rapid erosion rates (>10-3 cm yr-1) and events occurring over the past 25,000 yr (Lal, 1991).Furthermore, recent advances in 14C dating by AMS have provided the opportunity to measure the very small quantities of carbon that can be extracted from quartz. The vacuum system that I have designed and built to extract carbon from quartz is based on that used at the University of Arizona (Lifton 1997), which uses resistance heating of samples to a temperature of approximately 1100ºC in the presence of lithium metaborate (LiBO2) to fuse the quartz. In the presence of O2, any carbon present is released and oxidised to CO2, which is subsequently cryogenically trapped and graphitised for AMS measurement. In previous work (Naysmith et al., 2004) it has been shown that the extraction system produced a stable blank value but when running Lifton’s PP-4 standard sample, the system generated larger volumes of CO2 but only half the number of carbon atoms compared to Lifton.In this study, new data for CO2 blank values, system blank values and new PP-4 data will be presented.The original vacuum system has been modified to try and reduce the volume of CO2 produced from each combustion.Further improvements in the cleaning and handling of the quartz sleeves before they were used in the extraction process were implemented in an attempt to reduce the contamination associated with the combustion stage of the process.The CO2 purification has been improved and results show that realistic volumes of CO2 are being generated from quartz samples.A new-shielded quartz sample has been obtained from a depth of greater than 250 m. The results from this show it to have a very low 14C atom content.A new sample of PP-4 quartz was obtained from the University of Arizona and the results (in 14C atoms g-1 SiO2) agree with values published by Lifton (Miller et al., 2006) for this sample.The data from both these samples are included in this study.