Enhanced Chlorine Content Calibration Method for Simultaneous Determination of Apatite U-Pb Age and Trace Elements by LA-ICP-MS

The high background of Chlorine (Cl) signals in the carrier gas and the influence of apparent halogen signals have posed challenges for determining Cl content in apatite using Laser Ablation Inductively Coupled Plasma (quadrupole) Mass Spectrometry (LA-(Q)ICP-MS), which hinder simultaneous determination of U-Pb ages, trace element in apatite.In this contribution, we present an enhanced Cl content calibration method using Iolite software 3D Trace Elements DRS.We analyzed six reference materials (RMs) at spot sizes of 44 m, 32 m, and 24 m.The U-Pb ages (corrected against Madagascar apatite), and the trace element concentrations (calibrated using NIST SRM 610), respectively, were consistent with the recommended values.We thus compared two methods for calibrating Cl content: (1) using a single RM (Durango with analyzed Cl content) and (2) employing multiple RMs (Madagascar, Mud Tank, Durango, Otter Lake with Slyudyanka as monitor).The former method yielded Cl contents that deviated from Electron probe microanalyzer analysis (EPMA) results due to the "apparent halogen signal," while the latter provided consistent results across all spot sizes.As analytical uncertainties increase with decreased spot sizes, we recommend 44 m spot size for routine analysis.Our findings suggest that combinations of Madagascar, Mud Tank, Durango, Otter Lake and NIST SRM 610 can be used to determine U-Pb ages, trace element concentrations, and Cl contents simultaneously using LA-(Q)ICP-MS, where Madagascar and NIST SRM 610 are recommend to corrected U-Pb age and calibrate trace element contents, respectively. EXPERIMENTALSamples.The summarized U-Pb ages, trace element concentrations, and Cl contents for Mud Tank, Otter Lake, Slyudyanka, Durango, Madagascar, and McClure Mountain apatite, based on previous studies, are presented in Tables S1 to S3. Mud Tank apatite megacrysts show homogeneous compositions in backscatter electron (BSE) images (Fig. 1a), with U concentrations ranging from 3 to 9 g/g, U-Pb ages between 355 30 Ma 30 and 464 10 Ma 28 , and Cl content of approximately 400 g/g 13 .Otter Lake (Fig. 1b) and Slyudyanka (Fig. 1c) apatites exhibit pronounced impurity components, characterized by distinct bright domains and intricate dark bands in BSE images.These features, previously observed by Xiang et al. 31 , are likely the result of complex thermal histories 25,30,31 .Otter Lake grains 1 and 2 have U-Pb ages of 865 10 Ma 30 and 920 4 Ma 31 , respectively, with variable Cl contents ranging from 90 40 g/g to 500 g/g 13,26,33,34 .In contrast, Slyudyanka apatite shows variable U-Pb ages between 386 11 Ma 31 and 448 7 Ma 30,32 , but with consistent Cl contents of 1300 g/g 13 .Durango apatite displays a few dots in the BSE image (Fig. 1d) and relatively homogeneous trace element compositions.Its U-Pb age is 32.0 0.6 Ma 17 , with Cl contents ranging from 3760 600 g/g to 4600 736 g/g 17,35 .Madagascar apatite (Fig. 1e) exhibits large grain sizes and shows excellent internal consistency in terms of U content (15-23 g/g) and Th/U ratio (6.7-33.3) 26.Its variable ID-TIMS U-Pb ages range from 473.5 0.7 Ma to 485.9 0.9 Ma 28 , with Cl content ranging from 1920 160 g/g to 2200 100 g/g 17,26 The Madagascar sample used in this study is MAD2, with a U-Pb age of 473.5 0.7 Ma 28 .McClure Mountain apatite (Fig. 1f) consists of small euhedral grains and exhibits variable U contents, consistent U-Pb ratios, and ID-TIMS U-Pb ages ranging from 523.5 2.1 Ma 29 to 524.5 3.7 Ma 17 , with low Cl contents around 200 g/g 25.28 .Analytical methods.Characterization of reference apatite included backscattered electron (BSE) imaging, major element analysis, U-Pb geochronology, and determination of trace elements and Cl contents.All analyses were performed at the State Key Laboratory of Lithospheric and Environmental Coevolution, University of Science and Technology of China (USTC).Scanning electron microscope ima ging.Crystal morphology, internal structure, and compositional zoning were examined using a TESCAN MIRA3 (Oxford, Czech Republic) scanning electron microscope operated at 15 kV accelerating voltage and 15 nA beam current.Electron probe microanalyzer analysis (EPMA).Analyses were performed on polished grain surfaces, avoiding impurities and surface imperfections.The homogeneity Cl contents of individual apatite was mapped initially using a EPMA (Shimadzu Electron Probe Micro Analyzer EPMA-1600; Shimadzu Corp) 1 interference ( 36 Ar H ) in Ar-based plasma.The