The Role of Sulfide in Mantle Pb Geochemistry

理论价值：间接证据表明，地球深部硅酸盐地幔中的[1]大部分Pb包含在硫化物相中，[2]Pb相对于硅酸盐熔体强烈倾向于硫化物，[3]Pb在硫化物（固体或熔体）中的迁移速率可能非常快，并且考虑到低粘度硫化物熔体可能会在地幔硅酸盐中形成相互连接的网络。如果这是真的，硫化物相可能会深刻地影响铅在地幔熔融过程中的行为，并可能是解决几个长期存在的重要铅悖论和谜题的关键。例如，洋脊玄武岩和热点玄武岩的Ce/Pb值均接近恒定（接近25），表明岩浆作用过程中Ce和Pb的分配相似，但硅酸盐矿物/熔体Ce和Pb的分配系数存在显著差异。此外，脊状玄武岩和热点玄武岩的微量元素模式均表现出较大的负Pb异常，这表明在熔融过程中存在较大的Ce/Pb分异，或者这些玄武岩的地幔源中缺乏Pb。初步实验表明，相对于硅酸盐熔体，铅确实优先由硫化物承载（约50倍），并且在硫化物存在下的熔体的详细模拟表明，在大范围的熔体中可以产生恒定的Ce/Pb比率，熔体直接反映了地幔源的Ce/Pb。因此，地幔中的铅确实可能被耗尽，硫化物（和铅）可能长期封存在地幔深部或地核中。为了更全面地研究地幔中铅的行为，提出了一个为期三年的计划，测量铅在硫化物熔体和硅酸盐熔体之间以及固体硫化物和熔融硫化物之间的分配。在高P-T条件下（地幔深度~100 km），确定了Fe-Ni-Cu硫化物的固相和液相以及Pb在固体硫化物（磁黄铁矿）和Fe-Ni-Cu硫化物熔体中的扩散速率，并确定了与地幔熔融有关的温度和氧、硫活性范围。这些实验将有助于限制以下内容：[1]海洋玄武岩中Ce/Pb悖论的起源，[2]陆地储层中Pb同位素演化，[3]硫化物网络作为交代介质的潜力，[4]硫化物熔体在同位素不均匀的地幔域中（通过平流或扩散）重新分配Pb的作用，[5]对地幔演化和深部地球过程的更广泛理解。更广泛的影响：这项研究的结果将通过出版路线和一个面向更外行读者的网站传播；它们将被纳入WHOI/MIT联合计划的课程和公共领域的研讨会。两所学院都积极参与各级学生和博士后的实验室培养，本研究将为实验岩石学和微量元素地球化学的教学技术提供许多机会。

Intellectual merit: Circumstantial evidence suggests that [1] most Pb in the silicate mantle of the deep Earth is contained in sulfide phases, [2] Pb strongly prefers to be in sulfide relative to silicate melt, and [3] rates of Pb mobility in sulfide (solid or melt) are likely to be very fast, and given the likelihood that low viscosity sulfide melt will form interconnected networks in mantle silicates. If true, sulfide phases may profoundly influence Pb behavior during mantle melting, and may hold the key to several long-standing and important Pb paradoxes and enigmas. For example, both ocean ridge and hotspot basalts have nearly constant Ce/Pb (near 25), implying similar partitioning for Ce and Pb during magmatic processes, yet silicate mineral/melt partition coefficients for Ce and Pb differ significantly. Also, both ridge and hotspot basalt trace element patterns exhibit large negative Pb anomalies relative to the bulk silicate earth, implying large Ce/Pb fractionations during melting, and/or deficiencies of Pb in the mantle sources for these basalts. Preliminary experiments suggest that Pb is indeed preferentially hosted by sulfide relative to the silicate melt (by a factor of ~50), and detailed modeling of melting in the presence of sulfide shows that constant Ce/Pb ratios can be produced over a wide range of extent of melting, with the melts directly reflecting the Ce/Pb of the mantle source. Thus the mantle may indeed be depleted in Pb, and long-term sequestering of sulfide (and Pb) into the deep mantle or core is probable.To more fully investigate the behavior of Pb in the mantle, a three year program is proposed to measure the partitioning of Pb between sulfide melt and silicate melt, and between solid sulfide and molten sulfide. The solidus and liquidus of Fe-Ni-Cu sulfide and the rate of Pb diffusion in solid sulfide (pyrrhotite) and in Fe-Ni-Cu sulfide melts will also be determined under high P-T conditions corresponding to mantle depths to ~100 km, and a range of temperatures and activities of oxygen and sulfur relevant to mantle melting. These experiments will help constrain the following: [1] origin of the Ce/Pb paradox in oceanic basalts, [2] Pb isotope evolution in terrestrial reservoirs, [3] the potential of sulfide networks as metasomatic agents, [4] the role of sulfide melts in redistributing Pb (by advection or diffusion) across otherwise isotopically heterogeneous mantle domains, and [5] broader understanding of mantle evolution and deep earth processes. Broader impacts: Results of this study will be disseminated through the publication routes, and via a web site aimed at a more lay audience; they will be incorporated in courses taught by both in the WHOI/MIT Joint Program, and seminars in the public domain. Both PIs are active in the lab training of students and postdocs of all levels, and this research will provide many opportunities for teaching techniques of experimental petrology and trace element geochemistry.