Zircon is a common accessory mineral in various genetic rocks, with high sealing temperature and strong weathering resistance. It is not only an important target for U-Pb isotopic dating, but also an ideal mineral for Hf isotopic analysis. Zircon has a high Hf content (usually between 0.5% and 2%) and a low Lu/Hf ratio (usually less than 0.002). 176Hf produced by 176Lu decay is very small, so the measured Hf ratio 176Hf/ 177Hf basically represents the Hf isotopic composition when the system was formed. Combined with the U-Pb age of zircon, the accurate initial ratio of Hf isotopes can be obtained, so as to better trace the source of materials and understand the growth and evolution of continental crust. In recent years, the emergence of multi-receiver plasma mass spectrometry (MC-ICP-MS) technology has greatly accelerated the development of Lu-Hf isotope system, and the obtained information provides a new way to solve a series of important geological problems related to rock genesis. At present, the Hf isotopic composition of zircon is mainly determined by ICP-MC-MS and La-MC-ICP-MS. Laser denudation sampling analysis technology has become the main means of Hf isotopic analysis of zircon because of its advantages of simple sample preparation, fast analysis speed and in-situ analysis in micro-area. At the same time, some China scholars have put forward new understandings on some major geological problems based on the high-quality hafnium isotope data obtained.

tool

Dual focus multi-receiver plasma mass spectrometer (FinniganNeptune model)

The laser ablation spot diameter of 2 13nm ultraviolet laser ablation system can be adjusted between 10 ~ 150 μ m, and the maximum actual laser output power can reach 35J/cm2.

Analytical method

See Table 90.6 for the working parameters of the instrument.

Table 90.6 Receiver configuration table and instrument working parameters for isotope ratio determination (solution sampling and laser ablation sampling)

Note: Take FinniganNeptune dual-focus multi-receiver plasma mass spectrometer as an example.

The stability of the instrument and the reproducibility of analytical data were tested with 200ng/mL JMC475Hf standard solution. The experiment adopts static signal acquisition method and uses NeptuneMC-ICP-MS virtual amplifier technology. After the analyzer collects a set of data at the same time, the software automatically replaces the subsequent amplification circuits in turn. After collecting 9 groups of data, each amplifier circuit is consistent with the original analyzer. This technology can effectively eliminate the isotope ratio error caused by the different gain of Faraday cup receiver and improve the accuracy of isotope ratio determination. The ratio of 176Hf/ 177Hf of JMC475 is adjusted exponentially by179hf/177hfn = 240.7325, and the test result for six consecutive months is (0.28217/. The measured values are completely consistent with those reported in the literature within the error range. This solution test method takes a long time and is not suitable for zircon laser ablation sampling test.

Figure 90. 10 JMC475 solution test within six months

Xu et al. (2004) compared and measured JMC475 solution with integration time of 0. 13 1s without Neptune virtual amplifier. Although the analytical accuracy is decreased, the Hf isotopic composition obtained by two different experimental methods is very consistent. Therefore, the laser ablation test adopts the integration time of 0. 13 1s, without using the virtual amplifier.

176Hf has two allotropes, 176Lu and 176Yb. When measuring the ratio of 176Hf/ 177Hf, these two allotropes must be deducted accurately. The following equation (Chuetal. , 2002) is usually used to deduct the interference of Yb and Lu:

Investigation and analysis technology of resources and environment in the fourth volume of rock mineral analysis

Where: β is the correction coefficient of quality discrimination, β = ln (RM/RT)/ln (Ma/MB); Rm is the ratio actually measured; Rt is the true proportion; M is the isotope mass number.

Using179hf/177hf = 0.7325, the Hf isotope ratio was corrected by exponential normalized mass identification. Using173 Yb/172 Yb =1.35274, the quality discrimination correction of Yb isotope ratio is exponentially normalized. Since the ratio of 176Lu/ 177Hf in zircon is usually less than 0.002, the interference of 176Hf in zircon mainly comes from 176Yb. In the process of zircon laser ablation, the Yb signal is directly measured to calculate the βYb value, and then the average value of βYb during ablation is used for interference correction (Iizuka&Hirata, 2005; Wuetal。 ,2006)。

Due to the rapid erosion of zircon by laser, the stable duration of most zircon analysis signals does not exceed 65438±0min. Therefore, the laser in-situ sampling analysis adopts static measurement mode without Neptune virtual amplifier. Interference correction adopts the above method. In order to compare with the literature values, all the standard zircon test results are standardized and corrected by the Hf isotope ratio of JMC475176 HF/177 HF = 0.282160, and the international standard zircon GJ 1 is used as the external standard for sample determination.

Analysis results of standard zircon

1) temora zircon. Temora zircon is produced in the town of Temora in the Lackland fold belt in southeastern Australia. It is the laboratory standard of Australian National University, and its original rock age is 4 17Ma. Zircon fragments selected from original rocks range from tens to hundreds of μ m. Different zircon fragments are measured by 40μm erosion diameter, and the ratio of 176Hf/ 177Hf is 0.282700 64(2SD, N=22), which varies greatly. The ratio of three measurements of the same zircon fragment is 0.282683±24(2SD). It shows that the ratio of 176Hf/ 177Hf between different zircon fragments varies greatly and is uneven (Figure 90.6438+0 1).

Fig. 90. Test results of11standard zircon Hf isotope La-MC-ICP-MS.

2)GJ 1 zircon. GJ 1 zircon is purchased from jewelers in GEMOC Center of Macquarie University in Australia. It is believed that it is produced in African crystalline granite, and its U-Pb age measured by TIMS is 608Ma. The analysis of major elements and trace elements by CL, BSE, EMP and LA-ICP-MS confirmed that the composition of these zircons was homogeneous (Elhlouetal ., 2006). In this experiment, the standard of GJ 1 zircon was 5mm×5mm×6mm (GEMOC center number GJ 1/53). When the erosion diameter is 55μm, the ratio of 176Hf/ 177Hf is 0.282008 25(2SD, N=26) (see Figure 90. 1 1), which is different from that of Gerdes et al. (2006) (0. Elhlou et al. (2006) reported the value measured by LA-MC-ICP-MS method (0.28201519,2sd) and Gerdes et al. (2006) reported the value measured by separating Hf solution with resin by MC-ICP-MS method (0.28/kloc).

3)FM02 zircon. Zircon FM02, collected from Mingxi, Fujian Province, is a Cenozoic zircon megacryst with a size of 3mm×2mm×3mm, which was formed about 3Ma (Liu Ruoxin, 1992). The ratio of 176Hf/ 177Hf is 0.282967 44(2SD, N=27) when the erosion diameter is 55μm m.. The ratio of 176Hf/ 177Hf obtained under the condition of 30μm diameter is 0.28294 1 6 1 (2sd, N= 12) (see table 90.3 and figure 90.12). The Hf isotope ratios obtained under the two radii are consistent within the error range, but the measurement accuracy decreases under the smaller erosion diameter. This may be due to the increase of erosion depth under high laser energy and small erosion diameter, which leads to greater separation in the erosion process. The measurement results are in the same range as those of Qiu Zhili et al. (2005) on other zircon megacrysts in this area.

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This chapter was written by Zhou Jianxiong (Institute of Mineral Resources, Chinese Academy of Geological Sciences).