| In recent years, with the improvement of analytical techniques and the promotion of modern analysis instrument, isotope analysis method (including both radiogenic and stable isotope analysis) have become a very popular analytical technique in geoscience research with rapid development. In crust-mantle system, radiogenic isotope Hafnium has been used to trace tracing reservoirs and their long term evolution as a perspective of observation, compared with the traditional strontium-neodymium-lead (Sr-Nd-Pb) isotope system; metal stable isotope iron (Fe) has advantages in tracing petrogenetic process and has been a new geochemistry tracer, in virtue of its isotope fractionation principle. Based on the booming of isotope analysis method and combining special tracing advantage between radiogenic and metal stable isotope, we have carried out a series of study in Hf-Fe isotope conjoint analysis method and its geological application, aimed at revealing accurately the source characteristics and crust-mantle interaction and opening up a new research approach in isotope geochemical field. This thesis has mainly launched in four parts as followings:(1) The study progress of Hf, Fe isotope analysis methodSince the development of multi-collector inductively coupled plasma mass spectrometers (MC-ICP-MS), isotope analytical techniques has a great breakthrough. This part reviews the development process of Hf, Fe isotope analysis, the analysis process chemical separation process and the influence factors in the determination of mass spectrometry. Throughout history, Hf and Fe independent chemical separation method have been increasingly mature, and are widely used in domestic and abroad laboratory. In addition, the MC-ICP-MS has become the current major method for the determination of Hf, Fe isotopic composition.(2) To establish "Hf-Fe chemical separation method from a single rock dissolution"In previous studies, the major elements Fe can seriously affect the isotope analyses of the trace element Hf. How to remove Fe from Hf completely during chemical separation is a problem need to be solved. In more recent years, However, stable Fe isotope analyses has further been proven useful for high-temperature rocks, also in combination with Hf isotope analyses. Thus, we put forward an idea that whether or not obtain simultaneously pure Hf and Fe from a single rock dissolution? It could not only eliminate Fe interference in Hf isotope analysis, but also get Hf and Fe isotope information at the same time, then realize Hf-Fe isotope conjoint analysis. Based on the previous way of Hf, Fe separation, here we present a simple but efficient method of Hf-Fe chemical separation. This is a two-stage chemical separation process with the combination of ion exchange chromatography and extract chromatography. A purified Fe fraction is first quantitatively separated from the rock matrix using AG-MP-1M resin in HCl. Hafnium is subsequently isolated using a modified version of a commonly applied method using Eichrom LN-Spec resin. Noting that, this separation process except to thoroughly separate Hf and Fe, which also can conduct the further purification of Lu, Rb, Sr, Sm, Nd, Pb as required, to achieve multi-isotope information from the single rock dissolution.(3) To assess "the appropriate Hf, Fe isotope standard material"Accurate and reliable isotope standard material is an important guarantee to promote the development of isotopic analytical techniques. However, as to Hf, Fe isotope analyses, international standard material in short supply to a large extent hindered the promotion of this technology. In addition, the establishment of a new international standard is not only difficult but time consuming. So, a common practice by many laboratories is the development of secondary, so-called "in-house" standards, with routine analyses in addition to international SRM. In this contribution, we investigate the suitability of three igneous rock standards (GSR-1 (granite), GSR-2 (andesite), and GSR-3 (basalt)) distributed by the Chinese Academy of Geological Sciences as international standard reference materials for Hf-Fe isotope determinations. Experiments using spiked (isotope dilution) and non-spiked samples reveal an overall homogeneous Hf isotope composition for CNS GSR-2 and GSR-3 with 176Hf/177Hf=0.282640±06 (MSWD=1.5,2σ, n=10) and 0.282985±04 (MSWD=0.48,2a, n=10), respectively. However, GSR-1 Granite may have inherited zircon and show the isotopic composition of inhomogeneity. Therefore, GSR-1 should not be used for the Hf isotopic analysis. Besides, GSR 1-3 appear suitable standards for Fe isotope analyses and Fe isotope determinations on a subset of samples yields values of δ57Fe=+0.35±0.02%o,+0.22±0.05%o,+0.43±0.02%o (relative to IRMM-014, 2a, n=3) for GSR-1, GSR-2, and GSR-3, respectively.(4) Hf-Fe-(Sr-Nd-Pb) isotopic tracing the source of late Cenozoic Tengchong volcanicsThe late Cenozoic Tengchong Volcanic Field (TVF) in Southwest China is a cluster of mafic continental intraplate lavas, represented by 68 volcanic cones spanning an area of ca.800 km2. We present major and trace element data and Hf-Fe-Sr-Nd-Pb isotope ratios of 18 newly sampled rocks from the area and, in combination with published data, aim to constrain modes of emplacement and the geochemical character of their mantle source(s).It shows that most of the Tengchong volcanic rocks experienced assimilation-fractional-crystallization (AFC) processes during the genesis of these rocks. Few of the most serious contamination of samples, show a very light Fe isotope characteristics of anomaly, speculated that could be the result of strong crustal contamination. Three samples, identified as the most primitive samples based on major and trace elements, AFC evolution trend and Hf -Nd isotopic features, are interpreted to be derived from the source of the rocks without major modifications and represent the isotope signature (176Hf/177Hf=0.282962,δ56Fe=0.13‰, 87Sr/86Sr=0.705861, 143Nd/144Nd=0.512675,206Pb/204Pb=18.305,207Pb/204Pb=15.642, 208Pb/204Pb=38.948) to the present-day enriched part of the SCLM underneath SW China. Modelling of a best-fit scenario indicates that the isotope and trace element character of these samples can be reproduced by ca.1-4% partial melting of a metasomatised mantle source, composed of 1-5% of a global subducting sediment (GLOSS) component added to a depleted MORB mantle (95-99%). We ascribe these isotopic signatures to ancient modifications of parent-daughter ratios in the SW Chinese SCLM by past subduction-related metasomatism. Accordingly, we cannot identify the previously proposed enriched geochemical reservoirs (EM-1, EM-2) or so-called DUPAL mantle in the source of the Tengchong volcanics, but only processes that lead to similar isotopic signatures. In the absence of an active plume component, we infer that the Tengchong volcanism is triggered by passive upwelling of hot, re-enriched lithospheric mantle in an extensional setting associated with the rotation of the Indian subcontinent. Because fertile (metasomatised) mantle sections will preferentially melt, the TVF lavas are a product of decompressional melting of re-enriched sub-continental lithospheric mantle. |
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