Multi-Daly Isotope Measurements by MC-ICP-MS
Abstract
One of the latest developments in multi-collector mass spectrometry is the incorporation of multiple high-sensitivity Daly detectors among a suite of Faraday detectors. The dynamic range of a Daly of 100-106 counts per second (cps) overlaps the 104-1010 cps range of a Faraday, but extends to much lower signal intensities. Multiple Dalys enable the simultaneous measurement of multiple low-abundance isotopes in addition to higher abundance isotopes in Faradays. The Jackson School of Geosciences at The University of Texas at Austin has one of the first Nu Plasma 3D MC-ICP-MS with 6 Dalys and 16 Faradays. The collector housing is designed to facilitate multi-Daly analysis of Pb, Li, U and Th isotopes.
Method development began with Pb and Li isotopes using standard-sample bracketing to correct for mass bias and detector gain. Analyses (n=21) of Pb standard NBS981 at 0.5 ppb and 0.125 ppb, bracketed by 0.25 ppb solutions, yield average values of 206Pb/204Pb = 16.95 ± 0.05, 207Pb/204Pb = 15.49 ± 0.02, and 208Pb/204Pb = 36.72 ± 0.06. This reproducibility is better than 0.3% (2SD) and within 0.1% of the accepted values. In-run precision on individual measurements exceeds 0.05% (2SE). For mineral separates, ratios generated by multi-Daly analysis are indistinguishable, within the 0.3% reproducibility of the multi-Daly measurements, from ratios generated by Faraday analysis with Tl normalization of mass bias. In preliminary work on multi-Daly Li analyses, six measurements of 0.8 ppb solutions of L-SVEC, bracketed by 0.8 ppb L-SVEC, yields average δ 7 LiL-SVEC = 0.07 ± 2‰. During the same analytical session two aliquots of in-house seawater standard yielded δ 7 LiL-SVEC of 29.02‰ and 31.67‰ and one aliquot of JB-2 yielded δ 7 LiL-SVEC = 4.61‰. In-run precision (2SE) is better than 0.6‰ and values for all measurements of the three standards are within 2‰ of accepted values. Our multi-Daly measurements, thus far, yield order-of-magnitude worse in-run precision and reproducibility than typical Faraday measurements, but multi-Daly analyses provide data for samples too small for Faraday analysis. Pb and Li solution multi-Daly work has characterized multi-Daly behavior in preparation for U and Th isotope analyses for U-Th geochronology and ocean anoxia studies and is the first step in developing multi-Daly laser ablation techniques.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V14B..02L
- Keywords:
-
- 1040 Radiogenic isotope geochemistry;
- GEOCHEMISTRY;
- 1041 Stable isotope geochemistry;
- GEOCHEMISTRY;
- 1065 Major and trace element geochemistry;
- GEOCHEMISTRY;
- 1094 Instruments and techniques;
- GEOCHEMISTRY