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

Marianne Richter; Yona Nebel-Jacobsen; Oliver Nebel; Thomas Zack; Regina Mertz-Kraus; Massimo Raveggi; Delia Rösel

doi:10.3390/geosciences9090391

Geosciences (Sep 2019)

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

Marianne Richter,
Yona Nebel-Jacobsen,
Oliver Nebel,
Thomas Zack,
Regina Mertz-Kraus,
Massimo Raveggi,
Delia Rösel

Affiliations

Marianne Richter: Isotopia Laboratory, School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, 3800 Clayton, VIC, Australia
Yona Nebel-Jacobsen: Isotopia Laboratory, School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, 3800 Clayton, VIC, Australia
Oliver Nebel: Isotopia Laboratory, School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, 3800 Clayton, VIC, Australia
Thomas Zack: Department of Earth Sciences, University of Gothenburg, Box 460, SE-405 30 Göteborg, Sweden
Regina Mertz-Kraus: Institut für Geowissenschaften, Johannes Gutenberg-Universität Mainz, J.-J.-Becher-Weg 21, D-55128 Mainz, Germany
Massimo Raveggi: Isotopia Laboratory, School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, 3800 Clayton, VIC, Australia
Delia Rösel: Department of Earth Sciences, University of Gothenburg, Box 460, SE-405 30 Göteborg, Sweden

DOI: https://doi.org/10.3390/geosciences9090391
Journal volume & issue: Vol. 9, no. 9
p. 391

Abstract

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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 µm 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.

Published in Geosciences

ISSN: 2076-3263 (Online)
Publisher: MDPI AG
Country of publisher: Switzerland
LCC subjects: Science: Geology
Website: https://www.mdpi.com/journal/geosciences

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