Nb/Ta Fractionations as Tracer of Subduction

知识价值。根据目前对地球主要储层组成的估计，铌（Nb）和铪（Hf）的质量平衡不合理，一些铌和铪也没有计算在内。这一建议旨在测试地幔储层存在的假设，其中包含Nb和Hf的平衡。这个“隐藏”的储层可能是高Nb/Ta和/或低Lu/Hf。认识到这样一个储层将对理解地幔演化和地壳成分再循环到地幔中具有重要意义。洋壳俯冲可能是解决表观物质平衡问题的关键。然而，洋壳要成为假定的隐伏储层，就需要在俯冲过程中失去Ta而不是Nb。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分离出这些元素（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，由于总变化是有限的，因此需要高精度的分析来识别这些变化。岛弧火山岩的低精度分析表明，在俯冲过程中Nb/Ta存在明显分馏。然而，铌/钽分馏的“何时”和“何地”是未知的。我们建议从俯冲的不同阶段确定这些元素和其他元素在俯冲物质中的行为，以确定导致Nb和Ta分馏的反应，并描述俯冲物质的特征。由于元素分配的实验证据尚无定论，我们将研究天然材料。一旦确定了分馏过程，Nb/Ta就可以作为该过程的指纹。我们用岛弧样品的分析来补充对俯冲物质的分析，目的是了解在俯冲过程中铌和钽的分馏位置。更广泛的影响。一个成果将是发展出具有广泛适用性的“新型”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。与国家高磁场实验室（NHMFL）现有的项目相结合，拟议的研究将培训和教育本科生和教师。积极的研究项目的存在是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.