Effects of simple acid leaching of crushed and powdered geological materials on high-precision Pb isotope analyses

We present new results of simple acid leaching experiments on the Pb isotope composition of USGS standard reference material powders and on ocean island basalt whole rock splits and powders. Rock samples were leached with cold 6 N HCl in an ultrasonic bath, then on a hot plate, and washed with ultra...

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Bibliographic Details
Published in:Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2015-07, Vol.16 (7), p.2276-2302
Main Authors: Todd, Erin, Stracke, Andreas, Scherer, Erik E.
Format: Article
Language:English
Subjects:
Acid leaching
Basalt
Chemical composition
Contaminants
Digestion
Heterogeneity
Isotope composition
Isotopes
Leaching
Lead (metal)
Lead isotopes
MC-ICPMS
Pb isotopes
Ratios
Reproducibility
Rock
Rocks
Sediment samples
Solutions
Uncertainty
USGS reference material
Water purification
Yields
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Summary:We present new results of simple acid leaching experiments on the Pb isotope composition of USGS standard reference material powders and on ocean island basalt whole rock splits and powders. Rock samples were leached with cold 6 N HCl in an ultrasonic bath, then on a hot plate, and washed with ultrapure H2O before sample digestion in HF‐HNO3 and chromatographic purification of Pb. Lead isotope analyses were measured by Tl‐doped MC‐ICPMS. Intrasession and intersession analytical reproducibilities of repeated analyses of both synthetic Pb solutions and Pb from single digests of chemically processed natural samples were generally better than 100 ppm (2 SD). The comparison of leached and unleached samples shows that leaching consistently removes variable amounts of contaminants that differ in Pb isotopic composition for different starting materials. For repeated digests of a single sample, analyses of leached samples reproduce better than those of unleached ones, confirming that leaching effectively removes most of the heterogeneously distributed extraneous Pb. Nevertheless, the external reproducibility of leached samples is still up to an order of magnitude worse than that of Pb solution standards (∼100 ppm). More complex leaching methods employed by earlier studies yield Pb isotope ratios within error of those produced by our method and at similar levels of reproducibility, demonstrating that our simple leaching method is as effective as more complex leaching techniques. Therefore, any Pb isotope heterogeneity among multiple leached digests of samples in excess of the external reproducibility is attributed to inherent isotopic heterogeneity of the sample. The external precision of ∼100 ppm (2 SD) achieved for Pb isotope ratio determination by Tl‐doped MC‐ICPMS is thus sufficient for most rocks. The full advantage of the most precise Pb isotope analytical methods is only realized in cases where the natural isotopic heterogeneity among samples in a studied suite is substantially below 100 ppm. Key Points: Ideal external precision by Tl‐doped MC‐ICPMS is ∼100 ppm Multiple digests reproducibility exceeds ideal precision by order of magnitude Leaching improves precision; sample heterogeneity adds uncertainty above ideal
ISSN:1525-2027
1525-2027
DOI:10.1002/2015GC005804