Platinum-group elemental and Re–Os isotopic geochemistry of Permian Emeishan flood basalts in Guizhou Province, SW China
Platinum-group elemental and Re–Os isotopic geochemistry of Permian Emeishan flood basalts in Guizhou Province, SW China
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DOI：
10.1016/j.gca.2006.06.939
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发表时间：
2006-08
影响因子：
5
通讯作者：
L. Qi;Mei‐Fu Zhou
中科院分区：
文献类型：
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作者：
L. Qi;Mei‐Fu Zhou
峨眉山中二叠世晚期陆相溢流玄武岩（ECFB）位于下二叠统茅口组和上二叠统宣威组之间，是峨眉山大火成岩省（ELIP）的一部分。在ELIP [2]的最东端贵州省的一个剖面中系统地采集了玄武岩样品，分析了铂族元素（PGE）和Re-Os同位素，以及主元素氧化物和微量元素。剖面上550 m厚的层序包括12个玄武岩流，其厚度从10到140 m不等。大部分玄武岩为高钛型（TiO_2 =1.9- 4.4wt%，Ti/Y=400-700）。与流9-12和流1-8的样品相比，序列下部在原始地幔标准化微量元素蛛网图上显示Nb、Ta、P和Sr亏损，Zr、La和Th相对富集，与较强的地壳混染相一致。除流9外，Zr/Nb、Th/Nb和Th/Ta比值从层序底部到顶部逐渐降低，表明污染程度降低。Os、Ir和Ru的范围为0.01至0.23 ppb，并且相对于Rh、Pt和Pd较低，Rh、Pt和Pd的范围分别为0.2至1.1、6至17和5至18 ppb。不同的熔岩流有不同的原始地幔标准化的铂族元素模式，虽然样品从相同的流量显示类似的铂族元素模式。Pd/Ir与Pt/Pd、Ni/Pd与Pd/Cr、Ni/Pd与Cu/Ir比值呈负相关，表明是结晶分馏的结果。来自岩浆流1-8的岩石显示出比流9- 12的岩石更低的部分熔融程度。剖面上大部分流（流4和流9除外）的原始地幔标准化模式都具有明显的负Ru异常，这可以解释为原生岩浆中的月桂石或Os-Ir-Ru合金分馏，然后与铬尖晶石、橄榄石或硫化物分馏共沉淀。因此，金属的变化反映了不同程度的晶体分馏和地壳污染。
The Late Middle-Permian Emeishan continental flood basalts (ECFB) lie between the Lower Permian Maokou Formation and Upper Permian Xuanwei Formation, being part of the ~260 Ma Emeishan Large Igneous Province (ELIP) in SW China [1]. Basaltic samples systematically collected in a cross section in Guizhou Province, the easternmost part of the ELIP [2], were analyzed for platinum-group elements (PGE) and Re-Os isotopes, in addition to major element oxides and trace elements. The 550-m thick sequence in the cross section includes 12 basaltic flows whose thicknesses range from 10 to 140 m. The majority of the basalts are high-Ti variety (TiO2=1.9-4.4 wt% and Ti/Y=400-700). In contrast to samples from Flows 9-12, those from Flows 1-8, the lower part of the sequence show depletion of Nb, Ta, P and Sr in the primitive mantle normalized trace elemental spider diagram and are relatively rich in Zr, La and Th, consistent with stronger crustal contamination. Gradual decrease of Zr/Nb, Th/Nb and Th/Ta ratios from the bottom to the top of the sequence, except Flow 9, indicate decreasing degrees of contamination. Os, Ir and Ru range from 0.01 to 0.23 ppb and are low relative to Rh, Pt and Pd which range from 0.2 to 1.1, 6 to 17 and 5 to 18 ppb, respectively. Different lava flows have different primitive mantle-normalized PGE patterns, although samples from the same flow show similar PGE patterns. Pd/Ir and Pt/Pd, Ni/Pd and Pd/Cr and Ni/Pd and Cu/Ir ratios show negative correlation, suggest the result of crystal fractionation. Rocks from Flows 1-8 derived from magma show lower degrees of partial melting than rocks of Flows 9- 12. The primitive mantle normalized pattern with clearly negative Ru anomaly for most of the Flows (except Flows 4 and 9) in the cross section can be interpreted by the fractionation of laurite or Os-Ir-Ru alloys in the primary magma and then coprecipitation with chromian spinel, olivine or sulfide fractionation. Thus the metal variations reflect variable extent of crystal fractionation and crustal contamination.