Ⅰ. High-Precision Measurement Of Boron Isotope Of Silicate Materials And Its Application Ⅱ. Geochronological And Geochemical Studies On Cenozoic Basalts From South China Sea Seamounts

This dissertation contains two parts:(1) set-up of high-precision measurement of boron isotope of silicate materials and its application;(2) Geochronological and geochemical studies on Cenozoic basalts from South China Sea seamounts.Boron(B) is one of the elements with great mass difference between isotopes, so large isotopic differentiations are observed in nature. As a highly mobile, highly incompatible, and highly soluble element, great difference in B concentration is also present in various geochemical reservoirs. Therefore, B has great potential in geochemical researches. However, the purification of B from silicate materials is much more complicated than from others. A method with simplified chemical treatment has been developed to determine high-precision δ11B in silicate rocks by multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS) using sample-standard-bracketing(SSB) mode. Boron in silicate rocks is extracted by digestion with HF, and purified by a single ion-exchange chromatographic step through a column filled with AG MP-1 anion resin. Only one dryness process is involved in the entire chemical treatment procedure. This simplified procedure diminishes the risk of boron contamination and losses during chemical treatment. An introduction system with a PFA spray chamber and a sapphire injector is adopted to decrease boron memory during the measurement on MC-ICP-MS, which provides high-precision δ11B results. The internal precision for δ11B is better than +0.05‰(2σ standard error), and the external precision for δ11B is better than +0.30‰(2σ standard error) estimated by the long-term analyses of SRM 951. The δ11B of a series of international rock standard references are measured by this method. Some of these standards have δ11B reference values, and our results agree well with them within analytical errors. This indicates that our method is feasible for high-precision δ11B measurement for various silicate rocks, and our results may provide δ11B reference values for these rock standards. This new technique is applied to Cenozoic continental basalts from Shuangliao, Northeast China to trace the “recycled material” in its mantle source. Although the boron concentrations are comparatively low in the Shuangliao basalts, they exceed those of the primitive mantle. The early basanite and alkali olivine basalts have relatively high and disperse δ11B values(-9.7‰ to +1.5‰, average value is-3.0‰), including two with positive values(BBT-1 and BLS-4), indicating the involvement of seawater-altered materials. All the late transitional basalts and diabases show negative δ11B values(-11.3‰ to-5.8‰, average value is-8.2‰), close to the primitive mantle values(-10±2‰). As the δ11B values are not affected by crustal assimilation and contamination, and are not sensitive to fractional crystallization and partial melting processes, the boron isotopic data of the samples could reflect the signature of the mantle source. Therefore, the positive δ11B values may indicate the involvement of oceanic materials in the mantle source, in agreement with previous studies.Late Cenozoic basaltic magmatism is widespread in South China Sea(SCS) and its surrounding areas, including Macclesfield, Reed Banks, Indonesia, Thailand, Vietnam, Hainan Island and Leizhou Peninsula. The petrology, isotopic geochronology and geochemistry research in this area is important not only for understanding the evolution of SCS and related deep processes, and the global plate tectonic evolution, but also for the SCS deepwater oil and gas exploration. We select fourteen late Cenozoic basalts from Daimao, Zhenbei and Zhongnan seamounts in SCS to conduct whole-rock petrologic, geochronological, geochemical and isotopic studies. The 40Ar-39 Ar dating results indicate that Daimao seamount was erupted at 21.62 Ma during the spreading of the SCS, however, Zhenbei and Zhongnan seamounts were formed after the sea-floor spreading ceased(7.35-8.42 Ma and 6.79-8.18 Ma, respectively). The geochemical data of SCS seamounts are compared with those from the surrounding areas formed in the same period. Such a comparison leads to the two main observations:(1) Great overlaps are observed for most major and trace elements composition, as well as all the isotopic ratios, which reflect that the basalts of SCS area shared the same mantle source;(2) Some discrepancies are observed between the seamount basalts and their surrounding area counterparts in terms of their major and trace element compositions. The differences in Mg O, Mg# and Ni may be due to different extent of crystalline differentiation of olivine and/or clinopyroxene. According to the lithospheric lid effect, the different lithosphere thickness at the time of basaltic eruption is likely to be responsible for the differences in HREE patterns,(Sm/Yb)N,(Nb/Ta)N,(Zr/Hf)N, Ca72/Al72 between seamounts and basalts in the surrounding areas and the varying LREE,(Zr/Hf)N and(Sm/Yb)N among the seamounts. La/Yb and Dy/Yb ratios suggest that the basalts from the SCS are mixture of melts derived from both the garnet- and spinel-facies mantle, whereas the seamount samples have a greater contribution of melts from the the spinel-stability field mantle. The cooling model is employed to quantitatively constrain the thickness of the oceanic lithosphere when the seamounts formed. The results show that the lithosphere under Daimao seamount is reasonably thinner than that beneath the other two seamounts; the lithosphere in the SCS is thinner than that beneath its surrounding continents. The isotopic compositions of basalts from SCS seamount and surrounding areas witness a binary mixing model between DMM and EM2 end-members with Dupal lead isotopic anomaly. Along with geophysical data, enriched end-member EM2 may be related to recycled oceanic crust from Indo-Australian and Pacific plates and the mantle peridotite is considered to be the depleted component. The Hainan plume provided ample energy and materials for the Late Cenozoic basalts in the SCS area.