Partial re-equilibration of highly siderophile elements and the chalcogens in the mantle: A case study on the Baldissero and Balmuccia peridotite massifs (Ivrea Zone, Italian Alps)
Partial re-equilibration of highly siderophile elements and the chalcogens in the mantle: A case study on the Baldissero and Balmuccia peridotite massifs (Ivrea Zone, Italian Alps)
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地幔中高亲铁元素和硫属元素的部分重新平衡：Baldissero 和 Balmuccia 橄榄岩地块（意大利阿尔卑斯山伊夫雷亚区）的案例研究
DOI：
10.1016/j.gca.2013.01.021
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发表时间：
2013
影响因子：
5
通讯作者：
Gawronski
中科院分区：
文献类型：
--
作者：
Becker;Gawronski
贫化地幔橄榄岩的熔体渗滤和反应渗透分馏高亲铁元素（HSE）并引起地幔岩石中 187 Os/ 188 Os 重新平衡的条件仍然缺乏约束。在古生代 Balmuccia (BM) 和 Baldissero (BD) 橄榄岩构造岩（意大利北部伊夫雷亚-韦尔巴诺带）的对比研究中，研究了部分熔融和熔融渗透对 HSE、硫属元素（S、Se 和 Te）和 187Os/188Os 丰度的影响。在 BM 处，二辉橄榄岩和置换沙丘的 Re 贫乏年龄 (TRD) 显示出均匀分布，最大值接近 400–500Ma。 BD 橄榄岩也显示出古生代分布峰，但大量样品产生了元古代 TRD。两个天体中古生代 Re 耗尽年龄的优势与 Sm-Nd 年龄以及伊夫雷亚-韦尔巴诺带的晚古生代岩浆和地球动力学演化一致。 BM和BD橄榄岩中古187Os/188Os保存的不同程度被解释为反映了古生代不相容贫元素渗透熔体不同程度的同位素均一化和化学再平衡。两个物体之间的差异还反映在 HSE 和硫族元素丰度的差异上，BD 显示出 HSE 模式的大分散性，Re 相对于 Au 略有再富集，Pd、Se 和 Te 与 Al2O3 呈线性趋势。模型年龄分布的差异和HSE丰度的异质性支持了这样的观点，即不同组二辉橄榄岩的亲石元素、HSE和硫属元素的变化可能反映了地幔橄榄岩中不同程度的反应性熔体渗透，以及开放系统环境中的部分再平衡和熔体萃取。然而，BM和BD二辉橄榄岩的变量再平衡显然没有对Os/Ir、Ru/Ir、Rh/Ir和Pd/Pt等HSE比值产生显着差异，这些比值处于原始地幔值范围内，而Au、Re和S则有所贫化。两个套件中 S 与 Al2O3 的良好线性相关性反映了硫化物的去除受硫在硅酸盐熔体中的溶解度或与辉石和尖晶石的共沉淀控制，并表明 S 和 Al 的体积分配系数非常相似。二辉橄榄岩中的S/Se和Se/Te随Al2O3的减少变化不大。 BM二辉橄榄岩的结果与硫化物-硅酸盐熔体分配相一致，这是在低至中等熔体萃取过程中对 HSE、S、Se 和 Te 丰度的主要控制（DPt>DPd>DTe⩾DSe⩾DS≈DRe）。 Balmuccia 的替代沙丘岩具有低丰度的 Re、Au、Pd 和硫属元素，但 Os、Ir 和 Ru 的丰度可变且较高，S/Se 和 Se/Te 较高，但 187Os/188Os 与 BM 二辉橄榄岩相似。残余的 HSE 和硫属元素成分与俯冲相关蛇绿岩的沙丘中的成分不同。 BM纯岩与主岩的组成和接触关系可能反映了开放系统熔融和二辉橄榄岩重新平衡后硫不饱和熔体的集中流动。因此，Balmuccia 地块的成分记录反映了不同类型熔体的成分及其在不同 P-T 条件下与橄榄岩的相互作用。
The conditions at which melt percolation and reactive infiltration of depleted mantle peridotites fractionate highly siderophile elements (HSE) and cause re-equilibration of187Os/188Os in mantle rocks are still poorly constrained. In a comparative study of the Paleozoic Balmuccia (BM) and Baldissero (BD) peridotite tectonites (Ivrea-Verbano Zone, Northern Italy), the influence of partial melting and melt infiltration on abundances of HSE, chalcogens (S, Se and Te) and187Os/188Os have been studied. At BM, Re depletion ages (TRD) of lherzolites and replacive dunites display a uniform distribution with a maximum near 400–500Ma. BD peridotites also display a Paleozoic distribution peak but a significant number of samples yielded Proterozoic TRD. The predominance of Paleozoic Re depletion ages in both bodies is consistent with Sm–Nd ages and the late Paleozoic magmatic and geodynamic evolution of the Ivrea-Verbano Zone. The different extents of preservation of ancient187Os/188Os in BM and BD peridotites are interpreted to reflect different degrees of isotopic homogenization and chemical re-equilibration with incompatible element-depleted infiltrating melt during the Paleozoic. The differences between the two bodies are also reflected by differences in HSE and chalcogen abundances, with BD displaying large scatter among HSE patterns, slight re-enrichment of Re relative to Au, and linear trends of Pd, Se and Te with Al2O3. The differences in distributions of model ages and heterogeneity in HSE abundances support the view that the lithophile element, HSE and chalcogen variations of different suites of lherzolites likely reflect different extents of reactive melt infiltration in mantle peridotites, with partial re-equilibration and melt extraction in open system environments. However, the variable re-equilibration of BM and BD lherzolites apparently did not produce significant differences in HSE ratios such as Os/Ir, Ru/Ir, Rh/Ir, and Pd/Pt, which are in the range of primitive mantle values, whereas Au, Re and S are somewhat depleted. The good linear correlation of S with Al2O3in both suites reflects sulfide removal controlled by sulfur solubility in silicate melt, or co-precipitation with pyroxenes and spinel, and indicates very similar bulk partition coefficients for S and Al. S/Se and Se/Te in the lherzolites change little with decreasing Al2O3. Results for BM lherzolites are consistent with sulfide–silicate melt partitioning as the dominant control on abundances of the HSE, S, Se and Te during low to moderate degrees of melt extraction (DPt>DPd>DTe⩾DSe⩾DS≈DRe). Replacive dunites at Balmuccia have low abundances of Re, Au, Pd and chalcogens, but variable and higher abundances of Os, Ir and Ru, high S/Se and Se/Te, yet187Os/188Os similar to BM lherzolites. The residual HSE and chalcogen compositions differ from those in dunites of subduction-related ophiolites. The composition and contact relations of the BM dunites with the host rocks likely reflect focused flow of sulfur-undersaturated melt after open system melting and re-equilibration of the lherzolites. The compositional record of the Balmuccia massif thus reflects the composition of different types of melts and their interaction with the peridotites at different P–T conditions.
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