Abstract
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.
Highlights
Three commonly used and two in-house monazite reference material (RM) were chosen to assess their suitability as RM for U-Th/Pb age dating for LA-ICP-MS analyses
After determining suitable operational settings during the first two analytical sessions, the refinement of the ablation time to 15 s and the suitability of each individual RM was assessed in the third analytical session 130115, in which each RM was used as calibration RM by treating the others as unknown
To calculate the U-Th/Pb monazite ages, the mean measured ratio of each spot is normalized to a correction factor that is based on the measured ratio of all the analyzed USGS 44069 relative to the preferred ratio calculated for the given TIMS
Summary
Monazite [(Ce, La, Nd, Th)PO4 ] is an accessory mineral that can contain large amounts of U (up to wt.% UO2 ), Th (< 1 to 30 wt.% ThO2 , common 4–12 wt.%) and light-rare earth elements (LREE) [1,2,3]. Monazite U-Th/Pb ages can be determined using isotope dilution thermal ionization mass spectrometry (ID-TIMS), secondary ion mass spectrometry (SIMS), electron microprobe analysis (EMPA) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) technique, with each method having their own advantages and disadvantages [10,11]. ID-TIMS analyses achieve an age resolution of < ±0.1% [12], but require the dissolution of whole grains, resulting in the loss of spatial resolution for a mineral that commonly exhibits strong chemical zonation. The age resolution of all three analytical techniques is lower than for ID-TIMS and ranges between ±0.4% for SIMS, to circa. Three commonly used and two in-house monazite RMs were chosen to assess their suitability as RM for U-Th/Pb age dating for LA-ICP-MS analyses. The assessment was performed by determining an analytical protocol, using a variety of operational settings, which was applied to three different LA-ICP-MS laboratories
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