Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v13a2088a&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V13A-2088
Mathematics
Logic
1000 Geochemistry, 1094 Instruments And Techniques, 1100 Geochronology, 1115 Radioisotope Geochronology, 9805 Instruments Useful In Three Or More Fields
Scientific paper
Multicollector ICP-MS is a mainstream method for precise isotopic analyses of large (over 10-8 g) quantities of Pb, and is becoming increasingly popular for very rapid, even if relatively imprecise, U-Pb dating of U-bearing minerals using laser ablation. At the same time, high precision U-Pb geo- and cosmo- chronology mainly utilizes isotope dilution thermal ionization mass spectrometry, recently enhanced by application of double spikes for both Pb and U. Here we explore the suitability of MC-ICPMS for analysis of 10-11-10-9 g quantities of radiogenic Pb, contained in small single grains of zircon and other U- bearing minerals, and in chondrules, refractory inclusions and mineral fractions from meteorites. Analyses were performed at the Geological Survey of Canada using a Nu Plasma with DSN-100 desolvating nebulizer, at Arizona State University using a Neptune with Apex nebulizer, and at Harvard University using an Isoprobe P with Apex nebulizer. A total ion yield of 0.4-0.5% was achieved in all three instruments in 2.5-4 minute analyses. The fractions of SRM-981 and SRM-983 standards, spiked with 202Pb-205Pb- 233U-235U [1], containing between 3*10-11 and 10-9 Pb, were analyzed in all three labs. Precision of 207Pb/206Pb ratios in SRM-981 was 0.1-0.3% for 3*10-11 g fractions, 0.03-0.1% for 10-10 g fractions, and 0.006-0.013% for 10-9 g fractions. Precision of the best MC-ICPMS analyses was similar to precision of average TIMS analyses from the same quantities of Pb. Reproducibility of analyses depends on accurate blank and background subtraction as much as on the counting statistics. A series of analyses of the same solution run within a short period of time (i.e. with constant background) yielded a reproducibility similar to that of TIMS, whereas the analyses of a series of separately prepared aliquots were less reproducible. Our data demonstrate that the quality of analyses of 10^11 - 10^9 g Pb fractions by modern MC-ICPMS approaches the quality of TIMS analyses. Further enhancement in precision can be expected when the total ion yield is increased to 1.5-1.7%, the highest values reported for MC-ICPMS [2, 3], and with using ion beam detection systems optimized for measuring medium to small signals. [1] Amelin Y. and Davis W.J. (2006) JAAS 21, 1053-1061. [2] Thirlwall M.F. (2002) Chem. Geol. 184, 255-279. [3] Makishima A. et al. (2008) JAAS 23, 1102-1107
Amelin Yu.
Chakrabarti Raj
Jacobsen Stein B.
Janney Philip
Wadhwa Meenakshi
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