用非传统稳定同位素探索全球大洋玄武岩、深海橄榄岩成因和地球动力学的几个重要问题

The applicant proposes to use his seafloor petrology collections of mid-ocean ridge basalts (MORB), gabbros and abyssal peridotites from the Pacific, Atlantic and Indian ocean ridge systems to study several fundamental geochemical and geodynamical problems of global significance. These include: (1) using Ti-Zr-Hf stable isotope methods to test the hypothesis that the observed huge Nb-Ta and Zr-Hf fractionations (Nb/Ta and Zr/Hf ratios show over 2 orders of magnitude correlated variation) result from mass-dependent fractionation under mantle magmatic conditions; (2) using a MORB sample suite with uniform ratios of incompatible elements and Sr-Nd-Pb isotopes with large major element compositional variation to test the common hypothesis of iron isotope fractionation, i.e., the affinity of heavy Fe with ferric Fe (Fe3+), and both heavy Fe and ferric Fe (Fe3+) being more incompatible than light Fe and ferrous Fe (Fe2+) during magma.evolution; while using an incompatible trace element and Sr-Nd-Pb isotope highly variable MORB suite to test the same hypothesis during low-degree mantle melting (i.e., the effect of mantle metasomatism); (3) proposing and testing the hypothesis that the high oxygen fugacity of the Earth’s mantle is a consequence of plate tectonics by subducting partially serpentinized oceanic mantle lithosphere with abundant ferric Fe (e.g.. Fe3+/total Fe > 2); (4) the recent work by Andersen et al. (Nature, 2015) is a milestone contribution by using U isotope variation in oceanic basalts to hypothesize that the O2-rich atmosphere since the late Archean (~ 2.4 Ga) mobilized the water soluble U (6+ vs. 4+) from continents, transported to the ocean and subducted with sediments to the upper mantle, which.explains the low Th/U in MORB (< 2.5) and the high Th/U (> 3.5) in ocean island basalts (OIB) do not see such U addition effect probably because OIB source materials are all ancient (> ~ 2.4 Ga) if there were subducted component. The applicant emphasizes that the Cenozoic alkali basalts from eastern China are ideal materials to evaluate the significance of the subducted seafloor materials for the.petrogenesis of OIB and enriched MORB by a combined method of U and Tl isotopes, which is expected to revise and improve the Andersen et al (2015) hypothesis.

申请人用自己过去20多年积累的采自太平洋、大西洋和印度洋洋中脊玄武岩（MORB）、辉长岩和深海橄榄岩研究地球化学和地幔动力学的几个基本科学问题：1.用Ti-Zr-Hf稳定同位素验证“Nb-Ta、Zr-Hf元素对质量分异的假说”；2.从MORB演化过程和地幔熔融过程检验目前铁同位素分馏的基本假说，即重Fe同位素与Fe3+有亲和性，且比轻Fe同位素和Fe2+更不相容；3. 提出并试图检验地幔高氧逸度是板块构造的结果，即含有大量Fe3+的蛇纹石化大洋岩石圈地幔橄榄岩俯冲到深部地幔所致；4.Andersen等（2015）用大洋玄武岩铀同位素变化探讨了在地球演化过程中其内动力与外动力过程的相互关系，具有里程碑意义（申请人参加了这项工作），因其主要结论意义深远，必须用更具有代表性的MORB样品验证，并用铀和铊同位素研究中国东部新生代碱性玄武岩（最理想岩石），修正、改进、完善Andersen等的假说。

本项目探究高温地质过程中非传统稳定同位素的行为和分馏机制、大洋玄武岩和深海橄榄岩等岩石的成因以及大洋上地幔物质成分的不均一性。在项目执行期间，项目组成员在中科院海洋所“大洋岩石圈与地幔动力学”实验室建立了Fe-Cu-Zn-Mo元素分离和同位素分析测试方法，分析了采自太平洋、大西洋和印度洋的洋中脊玄武岩（MORB）、辉长岩和深海橄榄岩等样品的Fe-Mo同位素组成，取得了以下认识：（1）东太平洋洋隆（EPR）10°30'N MORB和西南印度洋脊ODP 735B钻孔的辉长岩和酸性岩脉样品的δ56Fe变化反映了结晶分异作用对玄武质熔体Fe同位素组成的重要影响；（2）EPR 5-15°N洋脊两侧海山玄武岩的Fe-Mo同位素组成反映了软流圈地幔源区中Fe-Mo同位素的高度不均一性，Fe-Mo同位素与元素、Sr-Nd-Pb-Hf同位素高度相关揭示了低熔融程度熔体的地幔交代作用是导致大洋上地幔物质成分不均一性的重要原因；（3）马里亚纳海槽17-23°N玄武岩的Fe同位素组成呈现了从南向北逐渐变轻的趋势，其主微量元素、Sr-Nd-Pb-Hf放射性同位素也有系统性变化，揭示了玄武岩从南向北受到亏损弧下岩石圈地幔物质的贡献程度逐渐增大，验证了“板块内部的密度差导致俯冲带起始”的假说；（4）金川铜镍硫化物矿床海绵陨铁结构的矿石样品中硅酸盐矿物的δ56Fe值接近球粒陨石，而硫化物矿物的Fe同位素组成变化较大，说明在成矿作用过程中Fe同位素在硅酸盐熔体和硫化物熔体之间发生了显著的分馏，因此推测核-幔分异过程中一定发生了显著的Fe同位素分馏；（5）铜绿山-安庆矽卡岩矿床样品的单矿物Fe同位素组成说明了矽卡岩型矿床中黄铁矿形成过程对Fe同位素分馏具有重要的控制作用；（6）大庙铁矿中富铁矿物的Fe同位素组成说明磁铁矿与钛铁矿之间的Fe同位素分馏是出溶作用的结果，氧逸度不是导致矿物间Fe同位素分馏的唯一因素；（7）Cabo Ortegal Complex及Raspas Complex地区的榴辉岩及蓝片岩样品的Mo同位素组成显示在俯冲过程中的流体作用导致了Mo同位素分馏。上述研究结果对深刻理解非传统稳定同位素的分馏机制、研究大洋玄武岩、辉长岩和深海橄榄岩的成因和地幔过程具有重要的意义。

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