Pt-Os同位素体系在地球科学研究中的应用

1 90 Pt通过α衰变生成1 86Os ,目前采用的衰变常数λ =1 .5 4 2× 1 0 -1 2 a-1 是由诺里尔斯克矿床的Pt Os等时线确定的。IIAB群铁陨石的1 90 Pt 1 86Os等时线年龄为 (4 2 90± 2 84 )Ma(2σ) ,说明Pt Os定年方法可能主要适合于古老样品。Pt Os体系的示踪研究以夏威夷现代地幔柱和诺里尔斯克矿床 (与西伯利亚地幔柱有关 )等为例 ,夏威夷苦橄岩的N(1 86Os) /N(1 88Os)值为 0 .1 1 98339～ 0 .1 1 985 2 6 ,γ(Os)为 1 .2 3～ 9.0 6 ,诺里尔斯克矿石的N(1 86Os) /N(1 88Os)为 0 .1 1 984 9,γ(Os)为 8,这种1 86Os/ 1 88Os 1 87Os/ 1 88Os双异常现象揭示了地幔柱物质中有来自地球外核的Os的贡献 ,这是迄今为止Pt Os体系对地球科学研究的最大贡献。Pt Os体系在壳源岩石方面的研究以肖德贝里Cu Ni硫化物矿床为例。研究支持肖德贝里的矿石主要来自地壳的结论 ,并可用二元混合模型 (太古宙苏必利尔省和元古宙Huronian省的沉积变质岩 )来解释其1 87Os/ 1 88Os同位素组成。但是该矿区的某些矿床的1 86Os/ 1 88Os比值偏高 ,需要有古老的Pt/Os比高的镁铁质岩的加入来解释

APPLICATION OF Pt-Os ISOTOPIC SYSTEM TO EARTH SCIENCES

Pt produces ~(186) Os through α-decay with a decay constant of λ=1.542×10~(-12) a~(-1) determined by a Pt-Os isochron of Noril’sk ore. The IIAB iron meteorites are of a ~(190) Pt-~(186) Os isochron of ((4 290)±284) Ma (2σ), which indicates that the Pt-Os system is capable of dating paleo-samples. The ~(186) Os/(~(188) Os) of the Hawaii picrites related to a recent mantle plume ranges from 0.119 833 9 to 0.119 852 6 and (γ(Os)) from 1.23 to 9.06. The ~(186) Os/~(188) Os of the Noril’sk Ni-Cu-PGE deposit associated with the Permian-Triassic Siberian plume is 0.119 849 and γ(Os) is 8. The coupled ~(186) Os/~(188) Os-~(187) Os/~(188) Os enrichments suggest that the outer core material has been added to the plume source. Os isotopic compositions of the Sudbury Ni-Cu sulfide deposit supports the previous conclusion that the deposit and the igneous complex are derived from crustal materials, the ~(187) Os/~(188) Os data can be explained by a two end-member mixing model (Archean Superior Province and Proterozoic Huronian metasedimentary rocks). However, a third end-member that is old mafic rocks with high Pt/Os ratio is required to explain the enrichment of ~(186) Os for the McCreedy West(MW) and Falconbridge(F) deposits.