Mantle Pb paradoxes: the sulfide solution
Mantle Pb paradoxes: the sulfide solution
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DOI：
10.1007/s00410-006-0108-1
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发表时间：
2006-09-01
影响因子：
3.5
通讯作者：
Gaetani, G. A.
中科院分区：
文献类型：
--
作者：
Hart, S. R.;Gaetani, G. A.
越来越多的证据表明，地幔橄榄岩中的铅预算主要包含在硫化物中，并且相对于硅酸盐熔体，铅强烈地分配到硫化物中。此外，有证据表明铅在硫化物（固体或熔体）中的扩散速率非常快。鉴于硫化物熔体有可能“润湿”亚固相线地幔硅酸盐，并且具有非常低的粘度，因此对地幔熔融过程中 Pb 行为的影响是深远的。关于铅在硫化物和硅酸盐之间分配的实验数据很少，并且没有关于硫化物中铅扩散速率的数据。充分了解铅在硫化物中的行为可能是解开几个长期存在的重要铅悖论和谜团的关键。经典的铅同位素悖论源于这样一个事实：所有已知的地幔储层都位于地球年代的右侧，而对于“平衡”储层的身份尚未达成共识。我们建议硫化物（含铅）长期偏析到核心可能会解决这一悖论。另一个 Pb 悖论源于这样一个事实：OIB 和 MORB 的 Ce/Pb 比率都大于土体，并且恒定在 25 的值。这种“规范比率”的恒定意味着 Ce 和 Pb 在岩浆过程中具有相似的分配系数（Hofmann 等人，Earth Planet Sci Lett 79:33-45, 1986），而大多数实验研究表明，Pb 在硅酸盐中比 Ce 更不相容。如果 Pb 的硫化物熔体/硅酸盐熔体分配系数的值接近 14，则熔融过程中残余地幔硫化物中 Pb 的保留有可能使 Ce 的整体分配与 Pb 相等。模拟表明，此类熔体的 Ce/Pb（或 Nd/Pb）仍将准确地反映源的 Ce/Pb（或 Nd/Pb），从而强化了 OIB 和 MORB 地幔具有明显更高的 Ce/Pb（和Nd/Pb) 优于块状硅酸盐土。这意味着这些玄武岩的地幔源中铅的大量缺乏。硫化物在岩浆形成过程中可能发挥其他重要作用：（1）平流/扩散硫化物网络可能形成有效的交代剂（在地幔中引入和消除Pb同位素异质性）； （2）硅酸盐熔体网络可以很容易地与环境地幔硫化物交换Pb（通过扩散或同化），因此与硅酸盐控制的同位素示踪系统（Sr、Nd、Hf）不同，在同位素异质地幔域中“采样”Pb，这些系统具有明显的“解耦”作用。
There is growing evidence that the budget of Pb in mantle peridotites is largely contained in sulfide, and that Pb partitions strongly into sulfide relative to silicate melt. In addition, there is evidence to suggest that diffusion rates of Pb in sulfide (solid or melt) are very fast. Given the possibility that sulfide melt "wets" sub-solidus mantle silicates, and has very low viscosity, the implications for Pb behavior during mantle melting are profound. There is only sparse experimental data relating to Pb partitioning between sulfide and silicate, and no data on Pb diffusion rates in sulfides. A full understanding of Pb behavior in sulfide may hold the key to several long-standing and important Pb paradoxes and enigmas. The classical Pb isotope paradox arises from the fact that all known mantle reservoirs lie to the right of the Geochron, with no consensus as to the identity of the "balancing" reservoir. We propose that long-term segregation of sulfide (containing Pb) to the core may resolve this paradox. Another Pb paradox arises from the fact that the Ce/Pb ratio of both OIB and MORB is greater than bulk earth, and constant at a value of 25. The constancy of this "canonical ratio" implies similar partition coefficients for Ce and Pb during magmatic processes (Hofmann et al. in Earth Planet Sci Lett 79:33-45, 1986), whereas most experimental studies show that Pb is more incompatible in silicates than Ce. Retention of Pb in residual mantle sulfide during melting has the potential to bring the bulk partitioning of Ce into equality with Pb if the sulfide melt/silicate melt partition coefficient for Pb has a value of similar to 14. Modeling shows that the Ce/Pb (or Nd/Pb) of such melts will still accurately reflect that of the source, thus enforcing the paradox that OIB and MORB mantles have markedly higher Ce/Pb (and Nd/Pb) than the bulk silicate earth. This implies large deficiencies of Pb in the mantle sources for these basalts. Sulfide may play other important roles during magmagenesis: (1) advective/diffusive sulfide networks may form potent metasomatic agents (in both introducing and obliterating Pb isotopic heterogeneities in the mantle); (2) silicate melt networks may easily exchange Pb with ambient mantle sulfides (by diffusion or assimilation), thus "sampling" Pb in isotopically heterogeneous mantle domains differently from the silicate-controlled isotope tracer systems (Sr, Nd, Hf), with an apparent "de-coupling" of these systems.