The Role of Sulfide in Mantle Pb Geochemistry

智力优势：环境证据表明，[1]地球深处硅酸盐地幔中的大部分铅都包含在硫化物中，[2]相对于硅酸盐熔体，[2]铅强烈地倾向于在硫化物中，[3]铅在硫化物(固体或熔体)中的迁移速度可能非常快，并考虑到低粘度的硫化物熔体在地幔硅酸盐中形成相互连接的网络的可能性。如果是真的，硫化物相可能深刻地影响地幔熔融过程中的铅行为，并可能成为几个长期存在的重要铅悖论和谜团的关键。例如，洋脊玄武岩和热点玄武岩的Ce/Pb值几乎不变(接近25)，表明在岩浆作用过程中Ce和Pb值相似，但硅酸盐矿物/熔体对Ce和Pb值的分配系数存在显著差异。此外，山脊玄武岩和热点玄武岩的微量元素模式相对于块体硅酸盐地球都表现出较大的负铅异常，这意味着熔融过程中Ce/Pb分馏较大，和/或这些玄武岩的地幔来源中存在铅的亏损。初步实验表明，与硅酸盐熔体相比，铅确实优先赋存于硫化物中(~50倍)，在硫化物存在下熔融的详细模拟表明，在很大的熔融范围内可以产生恒定的Ce/Pb值，熔体直接反映地幔来源的Ce/Pb值。为了更全面地研究铅在地幔中的行为，提出了一项为期三年的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分配的计划。Fe-Ni-Cu硫化物的固相线和液相线以及铅在固体硫化物(磁黄铁矿)和Fe-Ni-Cu硫化物熔体中的扩散速率，也将在相当于地幔深度~100公里的高P-T条件下，以及与地幔熔融有关的氧和硫的温度和活度范围内测定。这些实验将有助于约束下列问题：[1)大洋玄武岩中Ce/Pb悖论的起源，[2]陆地储集层中的铅同位素演化，[3]硫化物网络作为交代剂的潜力，[4]硫化物熔体在跨其他同位素不均匀的地幔区域重新分配铅(通过平流或扩散)的作用，以及[5]更广泛地理解地幔演化和地球深部过程。更广泛的影响：这项研究的结果将通过出版途径和针对更多非专业受众的网站传播；它们将被纳入世卫组织/麻省理工学院联合计划的课程和公共领域的研讨会。这两个PI都活跃在各级学生和博士后的实验室培训中，这项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.