Nb/Ta Fractionations as Tracer of Subduction

智力上的优点。使用目前对地球主要储层组成的估计，铌（Nb）和铪（Hf）的质量平衡不加起来，一些Nb和Hf是未知的。 这一建议旨在验证存在一个地幔储层的假设，其中包含的Nb和Hf的平衡。 这种“隐藏”储层可能是Nb/Ta高和/或Lu/Hf低。 认识到这样的储集层将对了解地幔演化和地壳成分再循环回到地幔产生重大影响。 俯冲洋壳可能是解决表观物质平衡问题的关键。 然而，洋壳是假定的隐藏的水库，它需要失去Ta相比，Nb在俯冲过程中。 Nb/Ta是一种特别有用的岩石成因示踪剂，因为很少有过程可以压裂这些元素（考虑到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标。 然而，由于总的变化是有限的，需要高精度的分析来识别变化。低精度的岛弧火山岩分析表明，在俯冲过程中，Nb/Ta存在明显的分馏作用。 然而，Nb/Ta分馏的“时间”和“地点”是未知的。 我们建议从俯冲的各个阶段确定俯冲物质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，并表征俯冲的物质。 我们将研究自然材料，因为关于元素分配的实验证据是不确定的。 一旦分馏过程被识别，Nb/Ta可以用作该过程的指纹。 我们补充俯冲物质的分析与岛弧样品的分析-目的是了解在俯冲过程中的Nb和Ta断裂。 更广泛的影响。其成果之一将是开发具有广泛适用性的“新的”高精度微量元素分析方法。 该项目将支持一名年轻科学家和一名研究生，并将为实验室的运作提供核心支持，PI试图将实验室作为一个开放使用的设施，特别是对学生。 结合国家高磁场实验室（NHMFL）的现有项目，拟议的研究将培训和教育本科生和教师。 积极的研究计划的存在是能够使学生和教师接触科学发现的重要组成部分。

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.