In situ beta decay dating by LA-ICP-MS/MS: applications
Date
2024
Authors
Gilbert, S.
Glorie, S.
Zack, T.
Editors
Shellnutt, J.G.
Denyszyn, S.W.
Suga, K.
Denyszyn, S.W.
Suga, K.
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Book chapter
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Methods and Applications of Geochronology, 2024 / Shellnutt, J.G., Denyszyn, S.W., Suga, K. (ed./s), Ch.8, pp.243-295
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Sarah E. Gilbert, Stijn Glorie and Thomas Zack
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Abstract
The development of in situ laser ablation tandem mass spectrometry (LA-ICP-MS/MS) has made a new range of isotopic systems readily accessible to geologists for dating minerals: Rb–Sr, Lu–Hf, K–Ca, and Re–Os. In these β-decay systems, the parent–daughter isotopes have the same atomic mass (e.g., 87Rb and 87Sr) which are not resolvable with single-quadrupole ICP-MS, and some are difficult or impossible to resolve with high resolution MC-ICP-MS/MS. Analysis of these geochronometers now enables direct dating of a wide range of geological processes in sedimentary and igneous settings, metamorphic terrains, and ore deposit systems. These isotopic systems also highlight the versatility of LA-ICP-MS/MS which utilizes a range of reaction gases to effectively resolve the isobaric interferences. Lu–Hf dating of garnet, apatite, and calcite uses NH3 to efficiently separate Lu from Hf as they form large cluster ions with different isotopic masses. Combining U–Pb and Lu–Hf isotopic systems for double-dating of apatite can be used to differentiate the high-temperature crystallization age by Lu–Hf from subsequent lower temperature overprinting by U–Pb. Rb–Sr dating of micas and feldspars has gained recent popularity, where Sr readily reacts with N2O or SF6 while Rb is unreactive. K–Ca isotopes in feldspars and K-bearing micas can be separated from the additional 40Ar isobaric interference using a combination of SF6 and H2 gases. Finally, Re–Os isotopes can be measured in molybdenite in ore deposits using CH4 as the reaction gas with the addition of He to suppress the Re reaction.
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