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Assessment of Five Monazite Reference Materials for U-Th/Pb Dating Using Laser-Ablation ICP-MS

Artikel i vetenskaplig tidskrift
Författare M. Richter
Y. Nebel-Jacobsen
O. Nebel
Thomas Zack
R. Mertz-Kraus
M. Raveggi
Delia Rösel
Publicerad i Geosciences
Volym 9
Nummer/häfte 9
Publiceringsår 2019
Publicerad vid Institutionen för geovetenskaper
Språk en
Länkar dx.doi.org/10.3390/geosciences90903...
Ämnesord monazite, reference material, Moacir, USGS 44069, TMM, MAdel, 94-222, U-Th/Pb dating, LA-ICP-MS, northern gawler craton, th-pb, central australia, blue-ridge, geochronology, constraints, zircon, complex, rocks, metamorphism, Geology, eiger rh, 1977, earth and planetary science letters, v36, p359
Ämneskategorier Geovetenskap och miljövetenskap

Sammanfattning

Monazite is a common accessory phosphate mineral that occurs under a wide range of pressure and temperature conditions in sedimentary, metamorphic and igneous rocks. Monazite contains high amounts of Th and U, rendering single monazite grains suitable for in-situ U-Th/Pb dating using laser ablation inductively-coupled mass spectrometry (LA-ICP-MS). Two key aspects of monazite dating that are critical for accurate age data with maximum precision are (i) optimized instrumental conditions to minimize analytical scatter and (ii) a well characterized reference material to ensure the accuracy of the obtained aged. Here, we analyzed five monazite reference materials (USGS 44069, 94-222, MAdel, Moacir and Thompson Mine Monazite) for their U-Th/Pb ages using LA-ICP-MS technique and applied a variety of laser spot diameters and repetition rates to find the best operational conditions to achieve accurate age data while maintaining maximum precision. We find that a spot diameter of 10 mu m and a repetition rate of 10 Hz yield the most precise ages with a deviation of +/- 2.0% from their respective high-precision U/Pb literature age data. Ages were reproduced in three different LA-ICP-MS laboratories using these parameters. Each reference material was tested for its suitability as a matrix-matched age reference material. For this, a rotating, iterative approach was adopted in which one reference monazite was used as calibration reference material against all others, which were treated as unknowns. The results reveal that USGS 44069, 94-222, Thompson Mine Monazite and MAdel all agree with their respective calculated ages and ID-TIMS reference ages and thus are suggested as suitable calibration reference materials. Moacir, however, appears slightly older than previously suggested (up to 4%), thus, caution is advised here when using Moacir as reference material for U-Th/Pb LA-ICP-MS dating in the absence of further absolute age calibration.

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