Evaluating the impact of Pb volatilization during 40Ar/39Ar CO2 laser fusion upon LA-ICP-MS measurement of Pb isotopic composition of detrital K-feldspar

• Published in: Chemical geology 2018-04, Vol.483, p.78-87
• Main Authors: Shulaker, Danielle Ziva, Hourigan, Jeremy K., Grove, Marty
• Format: Article
• Language: English
• Subjects: 40Ar/39Ar; Detrital; K-feldspar; LA-ICP-MS; Pb isotopes; Provenance
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2018.02.021

The information provided by detrital studies can be greatly enhanced by coupling geochronology and isotopic measurements from the same grains. While accessory minerals like zircon and apatite have been widely exploited in this regard, modal phases have been under-utilized. The benefits of using K-feldspar in detrital studies are twofold: K-feldspar 40Ar/39Ar has shown promise as a detrital thermochronometer and incorporation of common Pb into K-feldspar allows its Pb isotope composition to be used as a provenance tool. In this paper, we quantitatively investigate the effects of laser fusion during 40Ar/39Ar analyses of K-feldspar upon subsequent LA-ICP-MS measurement of Pb isotopic ratios. The results reveal that significant Pb volatilization occurs during laser fusion of K-feldspar. While Pb isotopes do not appear to be significantly fractionated, counting statistics suffer as Pb concentrations are lowered. Conversely, neutron irradiation appears to have a small though inconsequential impact upon Pb isotopic measurement of K-feldspar. Consequently, K-feldspar can be analyzed for Pb isotopes prior to total fusion 40Ar/39Ar analysis. Despite this artifact, coupled 40Ar/39Ar and Pb isotopic measurements constitute a promising approach for provenance studies that seek to constrain the geochemical nature and thermal evolution of basement source regions.