Analytical Capabilities

Lu-Hf, Sm-Nd geochronology (by ID-MC-ICPMS)

The Radiogenic Isotope & Geochronology Laboratory (RIGL) at WSU specializes in garnet Lu-Hf and Sm-Nd geochronology. Garnet geochronology takes advantage of garnet’s high compatibility for Lu and Sm relative to Hf and Nd, which results in the elevated Lu/Hf and Sm/Nd ratios in the garnets, and thus can be utilized for use in the isochron technique. We determine an isochron age for each sample from the analyses of several garnet fractions and their co-genetic low Lu/Hf and Sm/Nd phases (typically a whole rock powder).

All garnet fractions are dissolved using a nonaggressive HF-HNO³ dissolution protocol designed to avoid the digestion of low Lu/Hf zircon. For some garnet fractions containing substantial phosphate inclusions with low Sm/Nd ratios (e.g., apatite, monazite), we apply the sulfuric acid (98% H₂SO₄) leaching method before dissolving the garnets. In terms of the whole rock fractions for Lu-Hf chemistry, it is noteworthy that powders should not be ground in a tungsten-carbide mortar in order to prevent ¹⁸⁰W contamination; otherwise, the procedure is similar to garnet digestion. After HF-HNO₃ dissolution, the sample solution is spiked with mixed ¹⁷⁶Lu-¹⁸⁰Hf and ¹⁴⁹Sm-¹⁵⁰Nd tracers. Our ¹⁷⁶Lu-¹⁸⁰Hf spikes cover a range of low Lu/Hf ratio for zircon, intermediate Lu/Hf ratio for felsic and mafic rocks, to high Lu/Hf ratio for garnet (Vervoort et al., 2004). After isotopic equilibrium of the sample and spike mixture, the elements of interest (Lu, Hf, Sm, and Nd) are isolated from the solution by ion-exchange chromatography.

All isolated elements are analyzed using a Thermo Scientific Neptune Plus MC-ICPMS at RIGL. The measured values are corrected for interference and mass bias; here, the exponential law is employed for mass bias correction (see Vervoort et al., 2004 for the detailed description of Lu-Yb correction protocol). The widely accepted values of the JMC475 Hf standard and the JNdi Nd standard are ¹⁷⁶Hf/¹⁷⁷Hf = 0.282160 (Vervoort & Blichert-Toft, 1999) and ¹⁴³Nd/¹⁴⁴Nd = 0.512115 (Tanaka et al., 2000). Our average determinations of these standards over hundreds of analyses at RIGL are 0.282147±12 (2SD) and 0.512096±20 (2SD), respectively.

References

• Vervoort, J. (2014) Lu-Hf Dating: The Lu-Hf Isotope System. In: Rink, W., Thompson, J. (eds) Encyclopedia of Scientific Dating Methods. Springer, Dordrecht. DOI: 10.1007/978-94-007-6326-5_46-1
• Appendix A. Detailed analytical methods. of Johnson, T. A., Vervoort, J. D., Ramsey, M. J., Aleinikoff, J. N., & Southworth, S. (2018). Constraints on the timing and duration of orogenic events by combined Lu–Hf and Sm–Nd geochronology: An example from the Grenville orogeny. Earth and Planetary Science Letters, 501, 152–164. DOI: 10.1016/j.epsl.2018.08.030
• Anczkiewicz, R., & Thirlwall, M. F. (2003). Improving precision of Sm-Nd garnet dating by H₂SO₄ leaching: A simple solution to the phosphate inclusion problem. DOI:10.1144/GSL.SP.2003.220.01.05

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U-Th-Pb geochronology (by LA-ICPMS)

Laser ablation inductively coupled mass spectrometry (LA-ICPMS), where a solid sample (e.g., minerals, glass, fused powders etc.) is volatilized and isotopes ionized in an inductively coupled plasma source followed by mass analysis in a mass spectrometer, is capable of in-situ highly sensitive elemental and isotopic analysis at scales of a few tens of microns.

This instrumentation combines the advantages of microsampling by laser and fast high-precision analysis (sub-ppt level) by HR-ICPMS to allow determination of the ages of uranium- and thorium-bearing minerals such as zircon, monazite, titanite, etc. U-Th-Pb geochronology, along with studies of trace element compositions, Hf, Nd and O isotopes, and melt inclusions, provides great understanding of Earth system processes.

At RIGL, we perform U-Th-Pb geochronology by laser ablation using a 193nm ArF excimer laser ablation system (Analyte Excite) coupled to the Element 2 HR-ICPMS (ThermoScientific).

The analytical runs are automated after the desired laser spots/lines/rasters are manually selected by users. The laser is equipped with a high-definition microscope camera capable of resolving features as small as 2 μm using transmitted, reflective and cross polarized light, enabling users to navigate through samples for laser spot selections based on the imaged morphology of mineral grains. For the internal structure, users can examine polished grain mounts by cathodoluminescence (CL), the images of which then can be overlaid onto the laser image to guide spot selections of zones in single mineral grain. For cases where users cannot be physically present, we offer remote laser spot selection via Teamview.

The U-Th-Pb analysis can be performed alone or simultaneously with the HFSE and/or REE concentration measurements acquired from a single laser shot. Each analysis consists of a few to a dozen seconds of background measurements prior to the spot analysis. For U-Th-Pb age analysis, the background subtracted ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U, and ²⁰⁸Pb/²³²Th ratios are corrected for downhole fractionation (a.k.a. Laser Induced Elemental Fractionation or LIEF).

We adopt the Paton et al., 2010 method included in the Iolite software where the downhole fractionation trends are modeled based on the analyses of carefully selected reference standard with matching matrix as the unknowns. The model is then applied to the unknowns to efficiently correct the downhole fractionation. Instrument drift during an analytical session is corrected using the sample-standard bracketing approach. A secondary reference standard is also analyzed at the same operating condition throughout the session to assess the accuracy of the data acquisition and processing.

References

• Paton, C., Woodhead, J. D., Hellstrom, J. C., Hergt, J. M., Greig, A., & Maas, R. (2010). Improved laser ablation U-Pb zircon geochronology through robust downhole fractionation correction. Geochemistry, Geophysics, Geosystems, 11(3). DOI: 10.1029/2009GC002618
• Fisher, C. M., Bauer, A. M., Luo, Y., Sarkar, C., Hanchar, J. M., Vervoort, J. D., Tapster, S. R., Horstwood, M., & Pearson, D. G. (2020). Laser ablation split-stream analysis of the Sm-Nd and U-Pb isotope compositions of monazite, titanite, and apatite – Improvements, potential reference materials, and application to the Archean Saglek Block gneisses. Chemical Geology, 539, 119493. DOI: 10.1016/j.chemgeo.2020.119493

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