Hf isotopic compositions of the standard zircons and baddeleyites used in U–Pb geochronology
Introduction
It has been recognized that the zircon (ZrSiO4) Lu–Hf isotopic system is a powerful tool to decipher crust and mantle evolution of the Earth (Kinny and Maas, 2003, Harrison et al., 2005, Hawkesworth and Kemp, 2006). Zircon normally contains 0.5–2 wt.% Hf, which can be analyzed relatively easily and results in an extremely low Lu/Hf ratio (normally 176Lu/177Hf ratio < 0.002) and, subsequently, negligible radiogenic growth of 176Hf by beta decay of 176Lu. Therefore, the 176Hf/177Hf ratio of zircon can be regarded as the initial value when it crystallized. Zircon also has a high resistance to later thermal events, for example, the oldest rock so far identified is about 3.8 Ga, but the most ancient zircon preserved is 4.4 Ga (Wilde et al., 2001). Experimental calibration has shown that the zircon Hf closure temperature is ∼ 200 °C higher than that of Pb (Cherniak et al., 1997a, Cherniak et al., 1997b, Cherniak and Watson, 2000), indicating that Hf has remained isotopically closed during most of the later thermal events, even during high-grade metamorphism. Furthermore, zircon is a common accessory mineral in a wide range of rocks, and commonly preserves a very complex growth history. Its inherited cores and metamorphic overgrowths can be used to identify and distinguish various protoliths and geological processes from which the zircon was formed or later modified. Lastly, zircon is also an important mineral for U–Pb geochronology, O isotope and trace elemental analyses; all of these data combined together can provide invaluable information on crust and mantle evolution.
As with zircon, baddeleyite (ZrO2) also has high Hf concentrations and is an important mineral for U–Pb and Hf isotopic analyses (Heaman and LeCheminant, 1993, Scharer et al., 1997, Bodet and Scharer, 2000, Scherer et al., 2001, Soderlund et al., 2004, Soderlund et al., 2005). However, being different from zircon, baddeleyite occurs mostly in mafic–ultramafic rocks (such as kimberlite and dolerite), mantle xenoliths and rarely in metamorphic rocks (Heaman and LeCheminant, 1993). Considering its crystallization under SiO2 undersaturated conditions, baddeleyite cannot be formed by contamination of felsic crust, and hence can provide the initial Hf isotopic ratio of the mafic–ultramafic magma or mantle protolith. In addition, baddeleyite is unstable and would transform into zircon during later thermal/metamorphic events (Davidson and van Breemen, 1988). In this case, the baddeleyite–zircon association could provide information on both magmatism and metamorphism. Rarely, metamorphic baddeleyite can provide information about the metamorphic process (Rubatto and Scambelluri, 2003).
However, the difficulty of separating Zr from Hf by chemistry and the high ionization potential in mass spectrometers hampers the analysis of Hf. Generally, Hf isotope measurements can be carried out by TIMS (Thermal Ionization Mass Spectrometry) and hot-SIMS (Secondary Ion Mass Spectrometry) through dissolving the bulk grain and separating the pure Hf element from other interfering and matrix elements (Salters, 1994, Blichert-Toft et al., 1997). But this purification process is laborious and time-consuming, and requires special skills, although much progress has occurred recently (Bizzarro et al., 2003). Moreover, any spatial information of Hf isotope variation in zircon cannot be achieved because the whole grain is destroyed during this process.
Kinny et al. (1991) firstly attempted zircon in-situ Hf analysis using the SHRIMP (Sensitive High Resolution Ion Micro Probe) technique. With a ∼ 25 μm spot size, the small amount of analytical material, low ionization efficiency and single-collector ion counting meant the precision of the obtained Hf isotopic ratio was one order of magnitude less than that obtained by the TIMS technique of bulk analyses (Kinny et al., 1991). Consequently, this method is not widely adopted (Kinny and Maas, 2003). In addition, the expensive equipment, maintenance, and limited availability of machine time are also big obstacles for its general use. The advent of LA-MC-ICPMS (Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry) technique in recent years is proving to be an ideal tool for in-situ Hf isotopic ratio measurement on single grains of zircon (Thirlwall and Walder, 1995, Griffin et al., 2000, Griffin et al., 2002, Machado and Simonetti, 2001, Woodhead et al., 2004, Iizuka and Hirata, 2005). The aerosol of samples ablated by the high energy laser is carried by Ar or He carrier gas into the ICP torch where almost all elements in the plasma are ionized at temperature higher than 8000°C. The precision and accuracy of Hf isotope ratios determined by LA-MC-ICPMS can not only compare with those of TIMS, but the sample preparation is much easier and efficiency of analyses is much higher than those of TIMS as well. By the use of Back Scattered Electron (BSE) or Cathodoluminecence (CL) images of zircon, the Hf isotope ratios of zoned zircon can also be obtained.
However, standard materials are generally needed to monitor machine drift and are used for external calibration during in-situ analyses by LA-MC-ICPMS and SIMS. Undoubtedly, standard zircons and baddeleyites used in U–Pb geochronology, such as SHRIMP, TIMS and LA-ICPMS techniques, are an ideal choice to meet this requirement. However, their suitability for standard Hf material has been little investigated. To date, although there are numerous papers on the Hf compositions of standard zircon 91500 (Wiedenbeck et al., 1995, Amelin et al., 2000, Xu et al., 2004, Woodhead et al., 2004, Iizuka and Hirata, 2005, Nebel-Jacobsen et al., 2005, Harrison et al., 2005, Qi et al., 2005, Hawkesworth and Kemp, 2006) and TEMORA (Xu et al., 2004, Woodhead et al., 2004, Harrison et al., 2005, Qi et al., 2005, Hawkesworth and Kemp, 2006), others are poorly known, although CZ3 and CN92-1 have been reported by Xu et al. (2004) and Kemp et al. (2006), and an additional 5 standards (266, FC-1, Mud Tank, Monastery and QGNG) have been reported by Woodhead and Hergt (2005) in order to test their suitability as standards. As suggested by Woodhead and Hergt (2005), more data and inter-laboratory comparisons are necessary to evaluate the suitability of the above zircons as standard Hf material. In this paper, we present new Hf isotopic analyses for zircon 91500, TEMORA, CZ3 and CN92-1 based on one-year data accumulation in our laboratory, combined with some new analyses of Phalaborwa zircons and baddeleyite. In addition, the suitabilities of a zircon megacryst (FM0411) from basalt and baddeleyites from gabbro are also evaluated for use as in-house Hf standards. To achieve this, various protocols were evaluated for the isobaric interference correction of 176Yb and 176Lu on 176Hf and an updated correction method has been developed for data acquisition.
Section snippets
Instrumentation
All our measurements were carried out using a Neptune MC-ICPMS hosted at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. This machine is a double focusing multi-collector ICP-MS and has the capability of high mass resolution measurements in multiple collector modes. Detailed descriptions for this instrument can be found in Weyer and Schwieters (2003). Due to the zoom design of ion optical magnification, a large mass dispersion of 810 mm and an increased focal
Measurement of standard solutions
The JMC 475 standard solution with 200 ppb Hf was used for evaluating the reproducibility and accuracy of the instrument during Hf isotope measurement. Signals of Hf isotopes were measured in static mode. One run of Hf isotope ratio for JMC 475 standard solution consists of a baseline measurement at half mass positions and 90 cycles of sample signal collection. The 90 cycles of signals are divided into 9 blocks so as to complete the 9 rotations of the amplifiers connected to the Faraday cups,
Standard zircon measurements
Our previous experiments demonstrated that the obtained 176Hf/177Hf ratios are identical, within analytical uncertainty, under in-situ or line scan conditions (Xu et al., 2004). Therefore, in our following studies, in-situ analyses with spot sizes of 32, 63 and 94 μm were obtained. Normally, a signal intensity of ∼ 10 V at 180Hf mass can be obtained using the laser repetition rate of 10 Hz and a 63-μm laser beam with energy density of 15 J/cm2. The data are listed in Table 2. It is noted that we
Discussion
During in-situ zircon Hf analyses by LA-MC-ICPMS, standard material is needed to calibrate the machine and monitor for mass drift. It is generally acceptable to use a standard solution to check the machine, but this method is not applicable during practical analyses, since the change between the solution and solid samples would take too much time, and would also change the machine conditions. Presently, standard 91500 is widely used as a standard for in-situ zircon Hf analyses since it is
Conclusions
- (1)
During MC–ICPMS analyses, mass biases of Yb (βYb) and Hf (βHf) change with time and behave differently for material in solution or in the solid state. Precise Hf isotopic compositions can be obtained when the mean βYb value of the individual analytical spot is applied for in-situ analysis correction.
- (2)
During zircon in-situ analyses, isobaric interference of 176Yb on 176Hf is significant, although the different methods of obtaining the βYb value have little effect on the accuracy of the Hf
Acknowledgments
We are grateful to Shan Gao, Dun-Yi Liu, S. A. Wilde, Min Sun, Zhi-Li Qiu, A. Kroner and E. Scherer for providing 91500, TEMORA, CZ3, CN92-1, FM0411 and Phalaborwa standards, respectively. Thoughtful and constructive comments by two anonymous referees have substantially improved the manuscript. We thank Simon Wilde for proof reading the final version. This study was supported by the Natural Science Foundation of China (Grant 40325006) and Chinese Academy of Sciences. [RLR]
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