[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.