Nd and Sr isotopic compositions of basalts from the Alatao Pass, Xinjiang

THE studied region is near the Alatao Pass in the northwestern Xinjiang Uygur AutonomousRegion.It is near the northeastern margin of the Gongneis Plate tectonically.The Aibihufault,situated in the northeastern border of the plate,striking NW,is a component of theBoloholo suture zone which separates the Gongneis Plate from the Junggar Plate.The Pre-cambrian basement crops out in the Wenquan County,about 100 km west of the studied re-

Sr AND Nd ISOTOPIC COMPOSITION OF THE ULTRAMAFIC XENOLITHS FROM EASTERM CHINA:INFERENCES ABOUT THE STRUCTURE OF THE SUBCONTINENTAL LITHOSPHERE MANTLE AND THE ORIGIN OF BASALTIC MAGMAS

Uliramafie xenoliths provide an important constraint on the composition,strueture and evolution of the lithosphere mantle beneath eastern China. Most of the xenoliths entrained by the Cenzoic basalts have de pleted Sr and Nd isotopie composition. Interestingly evidences have been found for old ehriehed lithosphere mantle beneath thick Arehaean and post-Arehaean erust (＞35 km ) and for relatively homogeneous and depleted lithosphere mantle bencath thin erust (＜ 35 km ). Furthermore the chemieal composition of the lithosphere mantle is overall fertile regardless of the age of the overlying erust. The spatial variability in ehemieal and Sr and Nd isotopie eompositions of the ultramafie xenoliths in relation to the lithospherie ages and geometry implies a mixture strueture of the lithosphere mantle beheath easiern China. which consists of recently aecreted asthenospherie diapirs and old modified remnants. Isotopie eomposition of the Cenozoie basalts show obvious similarities with ocein island basalt (OIH ). We argue that enriehed lithosphere mantle delamineted during Mesozoic-Cenozoie tectonie-thermal events was generally involved in the Cenozoie basaltic voleanism.

Review of High-Precision Sr Isotope Analyses of Low-Sr Geological Samples

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.

Geochemical provenancing and direct dating of the Harbin archaic human cranium

As one of the most complete archaic human fossils,the Harbin cranium provides critical evidence for studying the diversification of the Homo genus and the origin of Homo sapiens.However,the unsystematic recovery of this cranium and a long and confused history since the discovery impede its accurate dating.Here,we carried out a series of geochemical analyses,including non-destructive X-ray fluorescence(XRF),rare earth elements(REE),and the Sr isotopes,to test the reported provenance of the Harbin cranium and get better stratigraphic constraints.The results show that the Harbin cranium has very similar XRF element distribution patterns,REE concentration patterns,and Sr isotopic compositions to those of the Middle Pleistocene-Holocene mammalian and human fossils recently recovered from the Harbin area.The sediments adhered in the nasal cavity of the Harbin cranium have a 87Sr/86Sr ratio of 0.711898,falling in the variation range measured in a core drilled near the Dongjiang Bridge,where the cranium was discovered during its reconstruction.The regional stratigraphic correlations indicate that the Harbin cranium was probably from the upper part of the Upper Huangshan Formation of the Harbin area,which has an optically stimulated luminescence dating constraint between 138 and 309 ka.U-series disequilibrium dating(n=10)directly on the cranium suggests that the cranium is older than 146 ka.The multiple lines of evidence from our experiments consistently support the theory that the Harbin cranium is from the late Middle Pleistocene of the Harbin area.Our study also shows that geochemical approaches can provide reliable evidence for locating and dating unsystematically recovered human fossils,and potentially can be applied to other human fossils without clear provenance and stratigraphy records.