Isotope plays an important role in both tracing and dating in earth science, especially 87Rb-86 Sr system. With the development of earth science, whole-rock analysis can＇t sufficiently meet the requirements for scientific research and the micro-analysis becomes more and more significant. Laser ablation multi-collector inductively-coupled plasma mass-spectrometry（LA-MC-ICP-MS） has been extensively applied in micro-zone analysis due to its low sample-consumption, high accuracy, in situ and low requirements on matrix, but it is still difficult to accurately measure Sr isotope compositions especially for the samples with high Rb/Sr ratios and low Sr contents as it is restricted by severe quality discrimination and various types of mass spectrum interferences. Consequently, thermal ionization mass-spectrometry（TIMS）, as the most accurate and precise method to analyze isotopic ratios, is still the most popular method of analyzing Sr ratios, especially for the samples with low Sr contents. This paper makes a systematic review on the high-precision Sr isotope analyses of low-Sr geological samples, including the micro-sampling technique, ultra-low procedural blank chemical method and TIMS measurement technique. The combination of ultra-low procedural blank and TIMS can be used to perform high-precision micro-analysis of the samples with ng magnitude, which will be undoubtedly an important direction for Rb-Sr geochronology, geochemistry and environmental studies.
Jie LinYongsheng LiuHaihong ChenLian ZhouZhaochu HuShan Gao
Garnet-rich granulite xenoliths collected from the Hannuoba basalts, the North China craton （NCC）, were studied to reveal the Mesozoic crnst-mantle interaction. These xenoliths are characterized by low SiO2 （37.7 wt.%-46.0 wt.%） and high Al2O3 （10.8 wt.%-17.9 wt.%） contents. Their Mg# （60-75, Mg#=100×Mg/（Mg＋Fe）, atomic number） are relatively low for their low SiO2 contents. They have low rare-earth element （REE） contents and LREE-rich REE patterns, and show remarkable enrichments in Sr relative to the adjacent REE. Some of them exhibit convex RISE patterns with a maximum at Nd and remarkably positive Eu anomalies. Taking into account their high garnet mode （generally 〉30%）, these features suggest that they are high-pressure metamorphic products of lowpressure cumulates （e.g., gabbro） after it had been depressed into the garnet stability field. They have evolved Nd and Sr isotopic compositions （143Nd/144Nd=0.511 763-0.512 173, STSr/86Sr=0.705 34-0.706 99） and fall in the trend defined by the 〉110 Ma Mesozoic basalts and high-Mg# andesites from the NCC. Zircon U-Pb dating by LA-ICP-MS shows a wide age range from 83 to 2 581 Ma, most of which cluster in 83-134 Ma. CL images of some Mesozoic zircons from the granulites show typical features of igneous zircons, providing direct evidence for the Mesozoic underplating event in this area. Neither peridotite-derived basaltic underplating model nor residue model of ancient lower crust after lithospheric thinning alone can reasonably explain the above features of the garnet-Hch granulite xenoliths. Combined with the previous research, we propose that most of the granulite xenoliths from the Hannuoba basalts are products of the Mesozoic magmatie underplating and mixing with the pre-existing lower crust （i.e., AFC process）. However, the melts could be mostly derived from partial melting of basaltic layers that were previously subducted （a fossil oceanic slab） or underplated into the base of the lithospheric mantle, or from pa
