The Role of Sulfide in Mantle Pb Geochemistry

硫化物在地幔铅地球化学中的作用

• 批准号: 0635530
• 负责人: Stanley Hart
• 金额: $ 35万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-01-15 至 2011-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0635530&HistoricalAwards=false
• 关键词: Role; Sulfide; Mantle; Pb; Geochemistry

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.

智力优点：间接证据表明，在硫化物阶段中，大多数PB中的大多数PB都包含在硫化物相中，[2] PB强烈喜欢相对于硫化物熔体，而[3]硫化物中的PB迁移率在硫化物（固体或熔体）中的速度很快，并且可能是非常快的速度，并且可能是固定的，并且可能是固定的。硅酸盐。如果是真的，硫化物阶段可能会深刻影响地幔融化过程中的PB行为，并可能拥有几个长期且重要的PB悖论和谜团的关键。例如，海脊和热点玄武岩的CE/PB几乎恒定（接近25），这意味着在岩浆过程中对CE和PB进行了类似的分区，但是CE和PB的硅酸盐矿物/熔体分区系数差异很大。同样，脊和热点玄武岩痕量元件模式都相对于散装硅酸盐呈较大的负Pb异常，这意味着在熔化过程中，大量的CE/PB分馏和/或PB缺陷在这些盆地的地幔源中。初步实验表明，PB确实是通过硫化物相对于硅酸盐熔体（约50倍）托管的，并且在硫化物存在下熔化的详细建模表明，恒定CE/PB比可以在熔融范围内产生恒定的CE/PB比，并直接反映了熔体直接反映MELT的CE/PB。因此，实际上可能会在PB中耗尽地幔，并可能将硫化物（和PB）的长期隔离到深幔或核心中。更可能是为了更全面地研究PB在地幔中的行为，提出了为期三年的计划，以测量PB在硫化物融合和硅酸盐材料之间的分配，以及固体硫化物和硫化物硫化物之间的分配。 Fe-Ni-Cu硫化物的固态和液体以及固体硫化物（吡ot石）和Fe-Ni-Cu硫化物熔体中的PB扩散速率也将在与左右的高度深度至〜100 km的高P-T条件下确定，以及氧气和氧气和硫相关的温度和活性。这些实验将有助于限制以下内容：[1] CE/PB悖论在海洋基础中的起源，[2] PB在陆地储层中的同位素进化，[3]硫化物网络作为代质剂的潜力，[4]硫化物在硫化物中的作用（否则）跨硫化物的作用，否则跨度PB（否则）否则互动率是跨性别的，否则依次依次依次依次依次依次依次依赖eNATERISERATION） [5]对地幔进化和深层过程的更广泛理解。更广泛的影响：这项研究的结果将通过出版路线和针对更多专业受众的网站传播；他们将在WHOI/MIT联合计划中教授的课程中，并在公共领域的研讨会中纳入。这两个PI都活跃于各个级别的学生和博士后的实验室培训中，这项研究将为实验质学和痕量元素地球化学的教学技术提供许多机会。