Lu-Hf Geochronology

 

Geochronology is traditionally focused on the U-Pb isotopic systematics of the mineral zircon, which is extremely robust to isotopic resetting. However, zircon does not commonly crystallize in low SiO₂ (mafic) magmas or hydrothermal systems. U-Pb dating of other mineral phases, such as phosphates and carbonates that are more commonly associated with hydrothermal alteration or mafic rocks, can be very challenging. Such minerals incorporate a significant amount of initial Pb during crystallization and their U-Pb systematics are susceptible to (partial) isotopic resetting at relatively low temperatures, meaning that resulting dates are often cooling ages or meaningless mixing ages. The Lu-Hf system has the potential to overcome these challenges. Although isotopic closure temperatures are not fully understood, theoretical and empirical observations have shown that high temperatures (>650°C) are required to induce isotopic resetting by volume diffusion. Furthermore, initial Hf is often negligible or well constrained, meaning that resulting dates are not strongly influenced by assumptions about the initial Hf reservoir at the time of crystallization/precipitation.

Traditional Lu-Hf isotopic analysis, involving laborious sample digestion and column chemistry in specialized labs, can produce very precise dates but at very slow rates (with wait times often exceeding 6 months). The novel laser ablation approach can generate accurate dates, in petrogenetic context, directly from rock blocks, and at several orders of magnitude faster than the traditional method. It provides a rapid avenue for dating hydrothermal systems, mafic rocks and detrital phases (in cover sequences), when zircons are not available and U-Pb systems in other minerals fail to record primary age information.

Description of Analytical Method

The analytical method uses an excimer laser ablation system, coupled to an (Agilent 8900) inductively coupled plasma – tandem mass spectrometer (ICP-MS/MS). The Lu-Hf method uses a NH₃ – He gas mixture in the reaction-cell of the mass spectrometer to promote high-order reaction products of Hf, while minimizing HREE reactions (i.e. Hf reacts at a rate of 50-60% while Lu reaction is < 0.003%). Consequently, the resulting mass-shifted (+82 amu) reaction products of ¹⁷⁶⁺⁸²Hf and ¹⁷⁸⁺⁸²Hf can be measured free from isobaric interferences. ¹⁷⁷Hf is subsequently calculated from ¹⁷⁸⁺⁸²Hf, assuming natural abundances. ¹⁷⁵Lu does not react significantly and can be measured on-mass as a proxy for ¹⁷⁶Lu. In addition to Lu and Hf isotopes, other trace elements, including a selection of other REEs can be measured simultaneously to monitor for inclusions and/or to geochemically characterise the analysed materials.

Isotope ratios and trace element concentrations are calculated using NIST 610 as a primary standard. Matrix-matched reference materials are analysed repeatedly within each analytical session and used to correct the Lu-Hf isotope ratios for matrix-induced fractionation. Down-hole fractionation has not been observed at laser beam diameters > 30µm. Lu-Hf ages are subsequently calculated as inverse isochron ages, or as weighted mean `single spot` ages in the case that initial Hf is negligible. A series of in-house reference materials are monitored in each analytical session for accuracy checks.

The main method paper to be referred to here.

Type of Sample Material/Media (e.g. Minerals, TS/Mount)

The Lu-Hf method has currently been demonstrated on the following minerals:

Garnet, apatite, calcite, dolomite, fluorite, epidote

Ongoing research is underway for other minerals that might be amenable for Lu-Hf dating.

The success rate of the method (to obtain ages with sufficient precision) is largely a function of the concentration of Lu, the isotopic in-growth time (i.e. the anticipated age) and the amount of initial Hf during crystallization. For relatively high-Lu phases such as garnet and felsic apatite, precise (~1-2% uncertainty) results can be obtained for samples as young as Mesozoic in age. For low-Lu phases, such as carbonates, fluorite and mafic apatite, the method is best suited for Precambrian samples.

For garnet (~30µm ablation depth), ages can be obtained from thin sections. For other phases (deeper ablation, ~60µm in apatite, up to ~80µm in carbonates/fluorite), thick sections or polished rock blocks are required. Laser beam diameters are catered to Lu concentrations. As a general guide, laser spot sizes of ~100-170 µm are used in apatite and garnet and ~170-260 µm are used in carbonates and fluorite. Smaller craters can be ablated, at the expense of larger age uncertainties.

MinEx Research

Within MinEx CRC, this research has been progressed through various PhD students and applied in NDI regions.

Workflows in case study areas that integrate this work is currently being produced.

MinEx CRC Researcher Contact Details