Lu-Hf Isotopic Geochemistry: Methods And Case Studies For Proterozoic Metamorphic Rocks And Mesozoic Granitoids From South China

This thesis reviews the progress of Lu-Hf isotopic system and its applications, and establishes a rapid chemical separation method of Hf from whole-rock samples and zircon minerals and ¹⁷⁶Hf/¹⁷⁷Hf ratios determination by MC-ICPMS. This new isotopic systems has been applied to investigations of Paleoproterozoic metamorphic rocks and Mesozoic granitoids from South China. The contents include:1. The progress of Lu-Hf isotopic study. Widespread application of Lu-Hf isotopic systems has long been hampered due to the very high ionization potential of hafnium until the advent of MC-ICP-MS technique since 1990s. Meanwhile, a number of methods of separation and purification of Hf have been developed, which significantly reduce the sample mass required for Hf isotopic analysis. This part reviews the geochemical characteristic of Lu-Hf isotopic system, and development of separation and purification methods of Lu and Hf, Hf isotope ratio determination using MC-ICPMS, and the applications of this Lu-Hf isotopic system as a new tool of geochronological dating technique and a geochemical tracer in geological studies.2. A modified single-column Ln-Spec extraction chromatography method has been developed for separation of Hf from rock samples. Using this method, hafnium can be effectively separated from the major and rare earth elements as well as the interference elements in rock samples, with recovery of >90% and whole procedure blank of <5×10¹¹ g for Hf. A number of international and national rock standards were analyzed for Hf isotopic compositions using both single- and double-focus multiple collector-inductively coupled mass spectrometers. While the analytical results for the international rock standards are identical with those reference values within analytical errors, the external precision for the national rock standards is in good agreement.3. A method that Hf in zircon was purified from other interferential elements was developed and the Hf isotope ratios were determined by using a Micromass IsoProbe MC-ICPMS for two international zircon standards 91500 and Temora and one in-house zircon standard We-1. Zircons were digested by HF in high-pressure bombs. For the reason that Zr does not affect Hf isotopic determinations in the simulative experiment, Hf was separated using a modified one-step ion-exchange chromatography. The overall recovery of Hf is >90%. Calibrated by an external standard solution JMC-475 Hf, ¹⁷⁶Hf/¹⁷⁷Hf ratios of these standard zircons were precisely determined. The average ¹⁷⁶Hf/¹⁷⁷Hf values of 91500 and Temora and we-1 were 0.282310±0.000034,0.282706±0.000020 and 0.281534±0.000009, respectively. It is concluded that the ¹⁷⁶Hf/¹⁷⁷Hf ratios of 91500, Temora and We-1 were homogeneous, because the aforementioned Hf isotopic results consist of at least seven separated performances of aliquots for each zircon sample. These results have significant implications for zircon Hf isotopic determination by using LA-MC-ICPMS.4. New Lu-Hf isotopic data were reported for the 1.77 Ga amphibolites in the Cathaysia Block of South China. These amphibolite samples have very high initial eHf(t) values of+15.2 to +23.3 and the eHf(t) values are tightly correlated with eNd(t) values. This suggests that the amphibolites were derived from a depleted mantle source with minor crustal contamination, and their Sm-Nd isotope system was not disturbed during later metamorphism. The least-contaminated samples possess exceptionally high eNd(t) value of ca.+8.5 andεHf(t) value of ca.+22, which are the highest values ever reported for the same-aged rocks worldwide, indicating there once existed a ultra-depleted mantle underlying the Cathaysia Block in the late Paleoproterozoic. This mantle source was likely formed by large scale crust/mantle differentiation in the Neoarchean. Lu-Hf and Sm-Nd isotopic investigations demonstrate that this ultra-depleted mantle is spinel peridotite with minor amount of (～1%) garnet. Garnets in spinel-garnet phase transition removed Lu from peridotites vicinity to their nucleations during cooling of the newly-formed lithospheric mantle, resulting in very high Lu/Hf ratio and strongly radiogenic Hf isotopic compositions found in the studied Cathaysian amphibolites. There existed great isotopic heterogeneity of depleted mantle in the late Paleoproterozoic.5. Detailed petrological, Sr-Nd isotope and in-situ Zircon Hf isotope data are reported for Pubei intrusive, Jiuzhou intrusive and Taima intrusive, which consist of main body of Darongshan-Shiwandashan granitoid belt. Petrological and element geochemistry show that these intrusives are typical S-type granites. High I_Sr (>0.721) and low eNd(t) (-9.9 to -13.0) indicate they originated from remelting of old crust. After corrected by their each own SHRIMP Zircon U-Pb age (Deng et al., 2004), most of zircons which age are～230 Ma have focused eHf(t) value of -11 to -9 and the corresponding T_dm2 age between～1.9 and～1.8 Ga while the rest have higher eHf(t) value of-4.5 and corresponding to T_dm2 age of～1.5 Ga. Inherited zircons in age of 1681～384 Ma haveεHf(t) of-17.1 to +3.4 and their T_DM2 a]ge focused at～2.4 Ga,～1.9 Ga and～1.5 Ga. Considering that most～230 Ma magmatic zircons have eHf(t) values consistent with the estimated values in terms of Hf-Nd isotopic array, we interpret that the crust with average residential time of 1.9Ga is the dominant material source for the studied granites. Some magmatic and inherited zircons have nearly the same T_dm2 ages of ¹.5Ga, indicating that the crust with average residential time of 1.5Ga is also involved in formation of these granitoids. Given that no igneous zircon with T_dm2 older than 2.0Ga have been detected, minor amount of recycled crustal materials with average residential time of 2.4Ga could also be involved. Because new mantle inputs have not been identified in terms of zircon Hf isotopes, mantle-derived magmas might not have been involved in formation of this S-type granitoid belt.