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TitleAlpha recoil loss of Pb from baddeleyite evaluated by High Resolution Ion Microprobe (SHRIMP II) depth profiling and numerical modelling: implications for the interpretation of U-Pb ages in small baddeleyite crystals
 
AuthorDavis, W JORCID logo; Davis, D W
SourceMicrostructural geochronology: planetary records down to atom scale; by Moser, D E (ed.); Corfu, F (ed.); Darling, J R (ed.); Reddy, S M (ed.); Tait, K (ed.); American Geophysical Union Geophysical Monograph 232, 2017 p. 247-259, https://doi.org/10.1002/9781119227250.ch11
Year2017
Alt SeriesEarth Sciences Sector, Contribution Series 20160336
PublisherJohn Wiley & Sons, Inc.
Documentbook
Lang.English
Mediapaper; on-line; digital
RelatedThis publication is accompanied by Alpha recoil loss of Pb from baddeleyite evaluated by High Resolution Ion Microprobe (SHRIMP II) depth profiling and numerical modelling: implications for the interpretation of U-Pb ages in small baddeleyite crystals - supplemental data files
File formatpdf
Subjectsgeochronology; radiometric dating; uranium lead dating; lead lead dates; baddeleyite; modelling; Methodology
Illustrationsphotomicrographs; tables; profiles; plots; graphs
ProgramScience laboratory network
Released2017 12 01
Abstract206Pb/238U ratios have been measured as a function of distance beneath natural and polished surfaces of baddeleyite crystals using a high resolution ion microprobe (SHRIMP II) for depth profiling. The experimental data are compared to model profiles calculated using a stochastic averaging algorithm to constrain the average alpha recoil distance in baddeleyite and the degree of alpha recoil-induced Pb loss. The precision and accuracy of measured profiles are hampered by relatively high common Pb corrections near the surface and the difficulty in estimating penetration depth at the start of measurements. Uncertainties in uranium concentration gradients within the first 200 nanometers of the natural crystal surfaces may also influence the model profiles. Despite these difficulties, results indicate that the measured U-Pb depth profiles are consistent with an alpha recoil distance in baddeleyite of about 20 ± 5 nanometers, consistent with a linear relationship between recoil distances and specific volume calculated for zircon and phlogopite. Assuming this recoil distance, Pb-loss from near surface alpha recoil becomes analytically resolvable for tabular crystals with thickness of less than 15-20 microns. This will not significantly affect ages based on 207Pb/206Pb ratios but 206Pb/238U ages may require corrections of 0.1 to 0.5% depending of grain size and shape. For typical tabular crystals, alpha recoil loss is linearly correlated with surface area to volume ratios allowing simple corrections to be applied if the dimensions of the crystal are accurately determined prior to analyses.
Summary(Plain Language Summary, not published)
Measuring the age of rocks is key to understanding a range of geological processes. The process of radioactive decay is fundamental to determining accurate ages. Improved analytical techniques now allow the analysis of very small samples for which assumptions about retention of radiogenic daughter decay products may not be valid. This paper uses the unique capabilities of the SHRIMP ion microprobe to assess loss of radiogenic daughter products from the surface of crystals. The results indicate that daughter product loss may be significant and should be corrected for in order to determine accurate ages.
GEOSCAN ID299682

 
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