Lutetium–hafnium dating

[1] Due to chemical properties of the two elements, namely their valences and ionic radii, Lu is usually found in trace amount in rare-earth element loving minerals, such as garnet and phosphates, while Hf is usually found in trace amount in zirconium-rich minerals, such as zircon, baddeleyite and zirkelite.

[3] The trace concentration of the Lu and Hf in earth materials posed some technological difficulties in using Lu–Hf dating extensively in the 1980s.

[1] The Lu–Hf system is now a common tool in geological studies such as igneous and metamorphic rock petrogenesis, early earth mantle-crust differentiation, and provenance.

[3] When 176Lu atoms are incorporated into earth materials, such as rocks and minerals, they began to be "trapped" while starting to decay.

[4] Radiometric dating makes use of the decay relationship to calculate how long the atoms have been "trapped", i.e. the time since the earth material was formed.

[2] An age equation is set up for every radiometric dating technique to describe the mathematical relationship of the number of parent and daughter nuclide.

[10] ɛHf is expressed in the following equation:[3][4] where: According to the Goldschmidt classification scheme, Lu and Hf are both lithophile (earth-loving) elements, meaning they are mainly found in the silicate fraction of Earth, i.e. the mantle and crust.

[1] However, Hf is more incompatible than Lu, and thus it is relatively enriched in the crust and in silicate melts.

[3] A positive ɛHf value means that 176Hf concentration in sample is larger than that of chondritic uniform reservoir.

[3] Positive value would be found in the residue solid after melt extraction, as the liquid would be enriched in Hf.

[3] Using the same logic, a negative ɛHf value would represent the extracted melt from reservoir, forming an evolved, juvenile material.

[3] The original figure 9 from Rehman et al. (2012) showed an intermedia, mixed ɛHf trend for the eclogites that was studied.

The experimental result indicate that the eclogites were formed from ocean-island basalt with contamination from sediments to produce the intermediate ɛHf values.

[3][4] where: The chondritic uniform reservoir model are tightly constrained in order to use Lu–Hf system for age determination.

[12] One later study focused on chondrites of petrological types 1 to 3, which are unequilibrated, show variation of 3% in

[2] In the earliest years, at around the 1980s, age acquisition based on Lu–Hf system make use of chemical dissolution of sample and thermal ionization mass spectrometry (TIMS).

[1] Generally, rock samples are powdered and treated with HF and HNO3 in a Teflon bomb.

[14] Different studies may use slightly different protocols and procedures, but all are trying to ensure complete dissolution of Lu and Hf bearing materials.

[1][3] Isotope dilution is done by adding materials of known concentration of Lu and Hf into the dissolved samples.

[1][2] The above sample preparation procedures prevent convenient analysis of Lu–Hf, thus limiting its usage in the 1980s.

[1] Also, the age determination using TIMS require samples of high Lu and Hf concentration to be successful.

[1] However, common mineral phases have low concentrations of Lu and Hf, which again limits Lu–Hf uses.

[1] The most common analytical methods for Lu–Hf determination nowadays is by inductively coupled plasma mass spectrometry (ICP–MS).

[1] ICP–MS, with multi-collector, allow precision determination with materials with low Hf concentration, such as apatite and garnet.

[1] The amount of sample needed for determination is also smaller, facilitating utilization of zircon for Lu–Hf ages.

[1] Selective dissolution, i.e. dissolving the garnet but leaving the refractory inclusions intact, is applied to the Lu–Hf system.

By applying Hf concentration determination to zircons from A-type granites in Laurentia, ɛHf values ranging from −31.9 to −21.9 were obtained, representing a crustal melt origin.

In cases where rocks are silica-poor, if more evolved rocks of the same magmatic origin can be identified, apatite could provide high Lu/Hf ratio data to produce accurate isochron, with an example from Smålands Taberg, southern Sweden, where apatite Lu/Hf age of 1204.3±1.8 million yr was identified as the lower boundary of a 1.2 billion yr magmatic event that caused the Fe–Ti mineralization at Smålands Taberg.

[20] Garnets play an important role in Lu/Hf applications, as they are common metamorphic minerals while having high affinity to rare-earth element.

[29] Hf ages determined from detrital zircon can help to identify major event of crustal growth.

Zircon, a common target for Lu–Hf analysis
Original figure 2 from Debaille et al. (2017); [ 6 ] An example of Lu/Hf isochron.
Schematic diagram showing elemental movement starting from planetesimal formation. Light blue particles represent volatile elements, which will not condense during early Earth formation. Dark brown and orange particles are both refractory elements which condense to form the solid Earth (indicated by the black circle). Dark brown particles represent siderophile elements that sink to the centre of Earth during core formation while the orange lithophile elements do not.
Original figure 9 from Rehman et al. (2012); [ 11 ] An example of ɛHf plot.
A schematic Hf evolution diagram.The black curve is plotted using 176 Hf/ 177 Hf values from Patchett and Tatsumoto (1980). All other curves and values are hypothetical. 4.55 billion year was assumed to be the time of Earth formation.
Garnet, a common metamorphic mineral target for Lu/Hf dating.
Oslo Rift, also known as Oslo Graben.