In situ Lu-Hf geochronology of garnet, apatite and xenotime by LA ICP MS/MS

Abstract

Lu–Hf geochronology is a powerful method to constrain the temporal evolution of geological systems. Traditional application of this dating method requires time-consuming chemical separation of the parent (¹⁷⁶Lu) and daughter (¹⁷⁶Hf) isotopes that is commonly accompanied by loss of textural context of the analysed minerals. In contrast, In-situ (laser-ablation based) Lu–Hf geochronology offers a number of advantages including rapid analysis with high spatial resolution, as well as control on textural relationships of the analysed mineral. However, laser-ablation based Lu–Hf geochronology has been hindered by isobaric interferences of ¹⁷⁶Yb and ¹⁷⁶Lu on ¹⁷⁶Hf that have effectively masked reliable determination of ¹⁷⁶Lu and ¹⁷⁶Hf. We present a methodology that resolves these interferences using LA-ICP-MS/MS (laser ablation tandem inductively coupled mass spectrometry) and NH₃ gas to separate Hf from Lu. Both Lu, Yb, and Hf react with NH₃ to form a variety of product ions. By measuring high order reaction products (e.g. Hf(NH)(NH₂)(NH₃)₃⁺), we demonstrate that ¹⁷⁶Hf can be measured interference-free from ¹⁷⁶Lu and ¹⁷⁶Yb with sufficient sensitivity to yield useful geochronological age data. The novel in-situ Lu–Hf technique has been successfully applied to a variety of Palaeozoic and Precambrianaged garnet, apatite and xenotime samples, including published reference materials. The resulting age uncertainties are as low as ~0.5% (95% conf. interval). The technique has the potential to obtain spatially-resolved Lu–Hf ages in garnet-bearing samples that would be difficult to obtain by conventional techniques. The method also offers the opportunity for rapid “campaign style” geochronology in complex terrains that record polymetamorphic histories. In apatite, the expected higher closure temperature of the Lu–Hf system compared to the commonly used U–Pb system allows high-temperature thermal history reconstructions. In addition, Lu–Hf dating of apatite allows dating of samples with low U and high common Pb (e.g. mafic and low-grade metamorphic rocks and ore deposits). Furthermore, apatite tends to incorporate little to no common Hf, allowing single grain ages to be calculated, which opens new doors for detrital provenance studies. In situ Lu–Hf dating of xenotime offers an additional avenue to U–Pb dating, and may be particularly beneficial to dating of rare earth element ore deposits that often have complex temporal records of development.