Nb/Ta Fractionations as Tracer of Subduction

Nb/Ta 分数作为俯冲示踪剂

• 批准号: 0635864
• 负责人: Vincent Salters
• 金额: $ 29.91万
• 依托单位: Florida State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-12-01 至 2010-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0635864&HistoricalAwards=false
• 关键词: Nb; Ta; Fractionations; Tracer; Subduction

Intellectual merit. Using current estimates for the composition of the Earth's principal reservoirs, mass balance for niobium (Nb) and hafnium (Hf) does not add up and some Nb and Hf is unaccounted for. This proposal is aimed at testing the hypothesis of the existence of a mantle reservoir that contains the balance of the Nb and Hf. This "hidden" reservoir would be high in Nb/Ta and/or low in Lu/Hf. The recognition of such a reservoir would have significant implications for understanding mantle evolution and the recycling of crustal components back into the mantle. Subducted oceanic crust may be the key to solving the apparent mass balance problem. However, for oceanic crust to be the postulated hidden reservoir, it needs to lose Ta compared to Nb in the subduction process. Nb/Ta is an especially useful petrogenetic tracer because very few processes can fractionate these elements (given their similar chemical properties), and thus it can become a very reliable indicator of recycled material. However, because the total variation is limited, high precision analyses are required to recognize the variations. Low-precision analyses of island arc volcanics indicate that there is a significant fractionation in Nb/Ta during subduction. However, the "when" and "where" of the Nb/Ta fractionation is unknown. We propose to determine the behavior of these and other elements in subducted material from various stages of subduction to determine the reaction(s) that result in the fractionation of Nb from Ta and to characterize the material that is subducted. We will work on natural materials as the experimental evidence on elemental partitioning is inconclusive. Once the fractionation process is identified the Nb/Ta can be used as a fingerprint of that process. We complement the analyses of the subducted material with analyses of island arc samples - with the aim to understand where in the subduction process Nb and Ta fractionate. Broader impacts. One outcome will be development of "new" high precision trace element analytical methods that will have wide applicability. This project will support a young scientist and a graduate student and it will provide core support for the operation of the laboratory, which the PI tries to operate as an open use facility, especially for students. In combination with existing programs at the National High Magnetic Field Laboratory (NHMFL) the proposed research will train and educate undergraduate students as well as teachers. The presence of an active research program is an essential component in being able to expose students and teachers to scientific discovery.

智力上的优点。根据目前对地球主要储层成分的估计，铌 (Nb) 和铪 (Hf) 的质量平衡无法相加，并且一些 Nb 和 Hf 下落不明。 该提议旨在检验地幔储层是否存在且含有 Nb 和 Hf 平衡的假设。 该“隐藏”储层的 Nb/Ta 含量较高且/或 Lu/Hf 含量较低。 认识到这种储层对于理解地幔演化和地壳成分再循环回地幔具有重要意义。 俯冲洋壳可能是解决表观质量平衡问题的关键。 然而，为了使洋壳成为假定的隐藏储层，在俯冲过程中，与铌相比，它需要失去钽。 Nb/Ta 是一种特别有用的成岩示踪剂，因为很少有过程可以分馏这些元素（鉴于它们相似的化学性质），因此它可以成为回收材料的非常可靠的指示剂。 然而，由于总变异是有限的，因此需要高精度分析来识别变异。岛弧火山岩的低精度分析表明，俯冲过程中 Nb/Ta 存在显着的分馏。 然而，Nb/Ta 分馏的“时间”和“地点”尚不清楚。 我们建议从俯冲的各个阶段确定俯冲材料中这些元素和其他元素的行为，以确定导致 Nb 从 Ta 中分馏的反应，并表征俯冲材料。 我们将研究天然材料，因为元素分配的实验证据尚无定论。 一旦分馏过程被识别，Nb/Ta 就可以用作该过程的指纹。 我们通过岛弧样本分析来补充俯冲物质的分析，目的是了解俯冲过程中 Nb 和 Ta 分馏的位置。 更广泛的影响。一项成果将是开发具有广泛适用性的“新”高精度痕量元素分析方法。 该项目将支持一名年轻科学家和一名研究生，并将为实验室的运营提供核心支持，PI 试图将其作为一个开放使用的设施进行运营，特别是对学生而言。 结合国家强磁场实验室（NHMFL）的现有项目，拟议的研究将对本科生和教师进行培训和教育。 积极的研究计划是让学生和教师接触科学发现的重要组成部分。