Re-Os Isotopic Measurement By MC-ICPMS And Its Application To Muli Picrites From Emeishan LIPs In Southwest China

This Ph.D thesis firstly reviews the progresses of Re-Os isotopic system in past years and its applications on geochemical field, and summarizes the techniques used for Re-Os analysis including chemical separation procedures and mass spectrometer measurements. Then the thesis reports a new analytical method of Re-Os concentration and ¹⁸⁷Os/¹⁸⁸Os ratio in a natural rock sample by multi-collector inductively coupled plasma mass spectrometry. Moreover, this Re-Os isotopic measurement technique has been used to study the Permian picrites from Emeishan LIPs. The main conclusions of this thesis are as following:1. The progresses of Re-Os isotopic system in instrumental measurement and chemical separation method. With the development of N-TIMS and application of MC-ICPMS techniques, both TIMS and MC-ICPMS can provide higher and higher sensitivity and precision in Re-Os measurements at present. On the other hand, Re-Os isotopic system has unique geochemical behavior, i.e., Os is preferentially retained in a mantle mineral facies during a partial melting process in contrast to all other radiogenic isotopic systems in common use (e.g. Rb-Sr, Sm-Nd, Lu-Hf, U-Th-Pb) where both parent and daughter elements are strongly concentrated in a melt. As a result, the Re-Os isotopic system has been recognized of particularly useful in tracing the geochemical evolution of mantle rocks and in defining the chronology of mantle differentiation. In addition, our works also indicate that MC-ICPMS technique shows a good potential on Re-Os isotopic measurement.2. A new method for the determination of low Re-Os abundances and ¹⁸⁷Os/¹⁸⁸Os in a natural rock sample by MC-ICP-MS has been established in our laboratory. Rock samples were digested in inverse aqua regia in sealed Carius tube for 24 hours at 230°C, then Os was separated from the matrix by CCl₄ solvent extraction, and further purification by microdistillation. For Re chemical procedure, Re was separated through anion exchange chromatography. Re separation with single ion exchange column appears to suffice for measurement therefore it is not necessary for the twice separation, which shows relatively less preparative chemistry to be required. During analyzing Re by MC-ICPMS, a sample solution is introduced to the plasma through an Aridus desolvating introduction system. This system can remove water and acid in the sample solutions by passing them through a desolating membrane in 160°C, thus providing a "dry" sample to the plasma, increasing ionization efficiency by a factor of 15～20 times over conventional sample introduction system. Therefore a Re sample with very low concentration can be measured; meanwhile element Ir is added into Re sample solutions for in-run Re isotopes mass fractionation correction. For Os measurement by MC-ICPMS, an Os sample solution is introduced into ICP through a conventional introduction system, Os memory effect may be eliminated by using 5% HCl-EtOH mixture solution and 5% HC1 to clean inlet system alternately. ¹⁸⁷Os, ¹⁸⁸Os, ¹⁹⁰Os and ¹⁹²Os ions were detected simultaneously by four ion-counters in order to increase the sensitivity and precision. Using this Re-Os measurement method, Re-Os abundances and ¹⁸⁷Os/¹⁸⁸Os of the picrite samples from Emeishan have been measured. The results were consistent with those measured by N-TIMS at JAMSTEC Os isotopic laboratory in Japan.3. Re-Os and Sr, Nd isotopic data have been reported for Permian picrites which were collected from Muli area in Emeishan LIPs. The picrites have low Re and high Os concentrations, which are probably controlled by the degree of partial melting and crystallization fractionation. The low Re ,Yb and Re/Os ratios of the picritic lavas support models of high degree partial melting with residual garnet in their mantle source during the early stages of partial melting, and without residual sulphide. Based on Ti/Y and Os isotope data, the picrites in Muli area can be classified into two subgroups: (1) a low Ti/Y type that exhibits low Ti/Y (<520),γOs (-4.1～-0.2) andεNd_t (+3.2～+6.6) ;(2) a high Ti/Y type that has Ti/Y (>520),γOs (+7.6～+11.5) andεNd_t (+5.9～+7.7) . The initial Os and Nd isotopes of high-Ti/Y type picrites are consistent with those of magmatic rocks related to plume, indicating that they were derived from a mantle plume and experienced little contamination with a continental crust and SCLM, therefore, most probably representing Re-Os composition of a primary melt. On the other hand, the low-Ti/Y type picrites and low-Ti basalts were also probably derived from the same plume source, but magmas of the low-Ti/Y type picrites most likely experienced a mixture with more than 15% bulk SCLM material; and magams of the low-Ti basalts were probably contaminated with over 15% melt from SCLM.