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Titanite and apatite can incorporate significant amounts of common Pb (204Pb) into their mineral structures, which leads to uncertainty when applying the U-Pb decay series for geochronology. The isobaric interference between 204Pb and 204Hg creates an additional complexity when calculating common lead corrections. Here we investigate the removal of 204Hg interferences during titanite U-Pb dating using reaction cell gas chemistry via triple quadrupole mass spectrometry compared with traditional methods that calculate U-Pb ages using a common lead correction. U-Pb dates for titanite natural reference materials MKED-1 and BLR-1 were determined using an ESI NWR193UC excimer laser coupled with an Agilent 8900 ‘triple quadrupole’ mass spectrometer. The 8900 is equipped with an octopole collision/reaction cell, which enables online interference removal. In order to compare traditional methods for U-Pb dating with interference removal methods, two experiments were run, one in which data was collected in NoGas mode, and one in which the 8900 was run in MS/MS mode, in order to assess the feasibility of determining U/Pb ratios with mass shifted isotopes. In MS/MS mode, NH3 was flowed through the reaction cell in order to enable a charge transfer reaction between NH3 and Hg+, effectively neutralizing Hg. During spot analyses in NoGas mode, masses 202Hg, 204Hg, 204Pb, 206Pb, 207Pb, 208Pb, 232Th, 235U, and 238U were monitored. For spot analyses in MS/MS mode, Th and U isotopes were measured on-mass at 232Th, 235U, 238U and mass-shifted to 247Th, 250U, and 253U. Pb isotopes were measured on-mass since Pb does not react with NH3. Ratios for 207Pb/235U, 206Pb/238U, and 207Pb/206Pb were calculated in Iolite (v.3.7.1) using the Geochron4 DRS using MKED-1 as the primary reference material and BLR-1 as a secondary reference material. Dates were calculated using IsoplotR. Weighted mean ages for titanite BLR-1 in MS/MS mode are 1043.8 ± 10.5 Ma (2σ, MSWD=1.08) for U isotopes measured on mass, and 1039.7 ± 8.3 Ma (2σ, MSWD=1.08) for mass-shifted U isotopes. These dates are both in agreement with the TIMS 206Pb/238U date for the BLR-1 titanite of 1047.1 ± 0.4 Ma. The use of NH3 for reaction cell chemistry has the potential to enable measurement of 204Pb without needing to correct for Hg interferences.
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A systematic evaluation of titanite reference materials for optimizing trace element and U Pb analysis by LA-ICP-MS
Titanite (CaTiSiO5) is a commonly occurring and versatile accessory mineral with broad applications in petrochronology. In situ U-Pb and trace element analyses via SIMS and LA-ICPMS are routinely performed using a matrix-matched reference material for U-Pb and standard glasses (non-matrix matched reference material) for elemental abundance determination. We report U-Pb isotopic ratios and major and trace element concentrations for three titanite samples (Ecstall, McClure and FCT) which are commonly used as reference materials in petrochronology studies. In addition, we characterize two new samples which can potentially serve as matrixmatched reference materials for titanite trace element geochemistry (BLR-2 and BRA-1). Based on electron microprobe analysis, samples BLR-1 and BLR-2 are homogeneous and suitable for use as a primary reference material for trace element concentrations. Whereas Ecstall, McClure, and FCT titanite reference materials show high intra-grain heterogeneity, yielding relative standard deviations for most trace elements between 5% and 40%, with higher standard deviations for U of 70% for Ecstall (n = 26), 265% for McClure (n = 22), and 202% for FCT (n = 26). Therefore, we suggest that these grains are unsuited to serve as reference materials for trace element quantification. The BRA-1 titanite has low trace element concentrations and is chemically heterogeneous (total REE abundances of 40 ppm for the rim and 95 ppm for the core of the grain), thus is not suitable for standardization of chemical composition using LA-ICPMS. It is commonly asserted that a matrix-matched standardization provides a more robust downhole fractionation correction compared to a non-matrix matched standardization. However, it remains unclear which standardization approach (matrix-matched vs non-matrix matched/glass) is more accurate for titanite trace element quantification. To resolve this, we tested several standardization approaches for trace element quantification, comparing matrix-matched (BLR-1) and nonmatrix- matched (NIST612) standardizations with different internal elemental standards (IES; Ca, Si and Ti) and without internal standardization (semi-quantitative). To provide an independent constraint on the accuracy of the various trace element standardization techniques we compared results to trace element concentrations obtained via solution Q-ICPMS on crushed BLR-2 and BRA-1 aliquots. The matrix-matched standardization using Si as the IES yields the best reproducibility of trace element concentrations followed by the matrix-matched reduction using Ti as the IES. Moreover, the matrix-matched semi-quantitative correction yielded the lowest weighted percentage of difference compared to reference trace composition quantified by solution ICPMS. Finally, in this contribution we also benchmark sampling-size for precise U-Pb dating of common-Pb rich phases like titanite.
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- Award ID(s):
- 2020935
- PAR ID:
- 10529755
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Chemical Geology
- Edition / Version:
- 1
- Volume:
- 636
- Issue:
- C
- ISSN:
- 0009-2541
- Page Range / eLocation ID:
- 121635
- Subject(s) / Keyword(s):
- Titanite Trace elements Matrix-matched reference materials LA-HR-ICP-MS Non-radiogenic Pb
- Format(s):
- Medium: X Size: 9.9 Other: pdf-A
- Size(s):
- 9.9
- Sponsoring Org:
- National Science Foundation
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Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mechanisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains.more » « less
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Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mecha- nisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1016/j.chemgeo.2023.121635
