In-situ geochronology is a pivotal tool that allows geoscientists to establish the timing of mineralising events. In addition, geochronology allows the wider context of metallogenic events, linking mineralisation to regional-scale magmatism, uplift and/or metamorphism. Currently, U-Pb geochronology dominates the scene with the ability to date zircon, titanite, apatite, xenotime, monazite and a range of other minerals commonly found within ore deposits. However, there are occasionally drawbacks associated with the U-Pb geochronology method. Drawbacks include the occasional lack of minerals suitable for U-Pb geochronology, the presence of common-Pb, or even modern-day Pb-loss. In any case, there are often circumstances where the U-Pb method cannot be used, cutscene to Lu-Hf geochronology.
What is Lu-Hf geochronology?
The Lu-Hf geochronology uses the decay scheme of 176Lu to 176Hf, which has a half-life of 37.12 Ga (Söderlund et al., 2004). By using this method, multiple analyses can be undertaken on a specific mineral (apatite, calcite, fluorite, xenotime or garnet), of which plot along an isochron. First, it is important to know a few assumptions that are made when using this geochronology technique: (1) All minerals used to construct the isochron have the same initial Hf composition; (2) All minerals used to construct the isochron are co-genetic; (3) the sample has been in a closed system since crystallisation (Cornell University, 2009; Vervoort, 2014). Often, the analysed mineral will have varying proportions of common-Hf, thus different Lu/Hf ratios. The Hf isotopic composition evolves as a function of their Lu/Hf ratio (Vervoort, 2014). Samples with high Lu-Hf components will evolve quicker to elevated 176Hf/177Hf ratios, and lower Lu/Hf components will evolve proportionally slower (Cornell University, 2009; Vervoort, 2014). Once the system is closed, all components will have varying Lu/Hf ratios that will plot along a line, of which the angle is a function of time. The age of the analysed sample can be calculated using the equation = ln(slope + 1)/λ176Lu (Vervoort, 2014). Occasionally, with samples that contain low initial 176Hf/177Hf ratio (as common in calcite), the inverse ratios may be plotted.
The Traditional Method & The New Method
The traditional method relies on whole-rock techniques. This involves crushing and sieving a sample, extracting the individual minerals of interest, dissolving the minerals, measuring the elemental composition of the solvent, chemically separating 176Lu from 176Hf via column chromatography, and then finally analysing the 176Lu and 176Hf separately from the solvent (Barfod et al., 2003; Blichert‐Toft, 2001). Drawbacks associated with the traditional include the removal of any petrological context of the minerals, the presence of inclusions within the minerals can be extremely difficult or impossible to detect, mixing of age domains cannot be resolved, it is very timely, often taking months to complete, and it is very expensive with analyses from a single sample often costing >$1000.
The new in-situ method solves many of these issues. A major hindrance to the in-situ application of this method is the presence of isobaric interference between 176Lu and 176Hf (due to these elements having the same mass), making it difficult/impossible for mass spectrometers tell them apart. However, recent technological advances have found a way of overcoming this isobaric interface via the use of a tandem inductively coupled mass spectrometer (LA-ICP-MS/MS). In essence, this allows for an additional cell in the mass spectrometer, which is filled with NH3 gas, reacting out the Lu from Hf, allowing the elements to be measured independently (Simpson et al., 2021, 2022). A detailed methodology can be found in Simpson et al (2021). One of the biggest advantages to this method is that it is quick and cheap, allowing for a variety of samples (>25) to be dated in a single say. The new Lu-Hf technique allows a variety of hydrothermal minerals to be dated including garnet, apatite, calcite, xenotime and fluorite (Glorie et al., 2023; Simpson et al., 2021, 2022).
Current Research & Future Uses
This method has already been deployed on a range of ore deposits and metamorphic terrains (Brown et al., 2022; Cave et al., 2023; Simpson et al., 2022; Tamblyn et al., 2022). This method has already shown that garnets, which sit adjacent to each other can have a difference in ages of >700 Ma, recording significantly different metamorphic events (Brown et al., 2022). Calcite has been dated from a range of hydrothermal ore deposits including Iron-Oxide-Copper-Gold prospects, VMS, sediment-hosted Zn-Pb-Ag deposits, as well as U-Th prospects (Cave et al., 2023; Simpson et al., 2022). As the Lu-Hf system is not affected by common-Pb, it opens up the ability to directly date the age of carbonate-hosted SEDEX deposits (Cave et al., 2023). In the Gawler Craton, this method has been deployed to date the age of coarse-grained fluorite veins (Glorie et al., 2023). The application of this method is essentially limitless, with minerals datable via Lu-Hf geochronology present in almost all types of ore deposits.
References
Barfod, G. H., Otero, O., & Albarède, F. (2003). Phosphate Lu–Hf geochronology. Chemical Geology, 200(3–4), 241–253.
Blichert‐Toft, J. (2001). On the Lu‐Hf isotope geochemistry of silicate rocks. Geostandards Newsletter, 25(1), 41–56.
Brown, D. A., Simpson, A., Hand, M., Morrissey, L. J., Gilbert, S., Tamblyn, R., & Glorie, S. (2022). Laser-ablation Lu-Hf dating reveals Laurentian garnet in subducted rocks from southern Australia. Geology, 50(7), 837–842.
Cave, B., Lilly, R., Simpson, A., & McGee, L. (2023). A revised deposit model for the George Fisher and Hilton Zn-Pb-Ag Deposits, NW Queensland: insights from the geology, age and alteration of the local dolerite dykes. Ore Geology Reviews, 105311.
Cornell University. (2009). Geol. 655 Isotope Geochemistry - Other Decay Schemes. http://www.geo.cornell.edu/geology/classes/Geo656/656notes09/656_09Lecture08.pdf
Glorie, S., Mulder, J., Hand, M., Fabris, A., Simpson, A., & Gilbert, S. (2023). Laser ablation (in situ) Lu-Hf dating of magmatic fluorite and hydrothermal fluorite-bearing veins. Geoscience Frontiers, 14(6), 101629.
Simpson, A., Gilbert, S., Tamblyn, R., Hand, M., Spandler, C., Gillespie, J., Nixon, A., & Glorie, S. (2021). In-situ LuHf geochronology of garnet, apatite and xenotime by LA ICP MS/MS. Chemical Geology, 577, 120299.
Simpson, A., Glorie, S., Hand, M., Spandler, C., Gilbert, S., & Cave, B. (2022). In-situ Lu–Hf geochronology of calcite. Geochronology Discussions, 1–18.
Söderlund, U., Patchett, P. J., Vervoort, J. D., & Isachsen, C. E. (2004). The 176Lu decay constant determined by Lu–Hf and U–Pb isotope systematics of Precambrian mafic intrusions. Earth and Planetary Science Letters, 219(3–4), 311–324.
Tamblyn, R., Hand, M., Simpson, A., Gilbert, S., Wade, B., & Glorie, S. (2022). In situ laser ablation Lu–Hf geochronology of garnet across the Western Gneiss Region: campaign-style dating of metamorphism. Journal of the Geological Society, 179(4), jgs2021-094.
Vervoort, J. (2014). Lu-Hf dating: the Lu-Hf isotope system. Encyclopedia of Scientific Dating Methods, 1–20.