The Role of Sulfide in Mantle Pb Geochemistry

智力优点：间接证据表明[1]地球深部硅酸盐地幔中的大多数铅都包含在硫化物相中，[2]相对于硅酸盐熔体，铅强烈倾向于处于硫化物中，[3]硫化物（固体或熔体）中的铅流动速率可能非常快，并且考虑到低粘度硫化物熔体将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，硫化物相可能会深刻地影响铅的行为在地幔熔融过程中，并可能持有的关键，几个长期存在的和重要的铅悖论和谜。例如，洋中脊和热点玄武岩几乎恒定的Ce/Pb（近25），这意味着类似的分配在岩浆过程中的Ce和Pb，但硅酸盐矿物/熔体分配系数Ce和Pb显着不同。此外，无论是脊和热点玄武岩微量元素模式表现出大的负铅异常相对于散装硅酸盐地球，这意味着大的Ce/Pb分馏在熔融过程中，和/或这些玄武岩的地幔来源的铅的不足。初步的实验表明，铅确实是优先托管硫化物相对于硅酸盐熔体（由一个因素的50），和详细的建模熔融硫化物的存在下，可以产生恒定的Ce/Pb比在很宽的范围内的熔融，熔体直接反映了Ce/Pb的地幔源。为了更全面地研究Pb在地幔中的行为，提出了一个为期三年的计划，测量Pb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。Fe-Ni-Cu硫化物的固相线和液相线以及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也将在对应于地幔深度~100 km的高P-T条件下以及与地幔熔融相关的温度和氧、硫的活动范围内确定。这些实验将有助于限制以下方面：[1]大洋玄武岩中Ce/Pb矛盾的起源，[2]陆地储层中Pb同位素的演化，[3]硫化物网络作为交代剂的潜力，[4]硫化物熔体在Pb再分配中的作用（通过平流或扩散）穿过同位素不均匀的地幔域，[5]对地幔演化和地球深部过程有了更广泛的认识。更广泛的影响：这项研究的结果将通过出版途径传播，并通过一个针对更多非专业观众的网站传播;它们将被纳入WHOI/MIT联合方案和公共领域研讨会的课程。这两个PI都活跃在学生和博士后的实验室培训的所有级别，这项研究将提供实验岩石学和微量元素地球化学的教学技术的机会。

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.