The Fate of Zircon and Other Accessories in Deeply Subducted Terrigeneous Sediments

深俯冲陆相沉积物中锆石和其他附件的命运

• 批准号: 2103552
• 负责人: Robert Rapp
• 金额: $ 24.46万
• 依托单位: University of Hawaii
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2103552&HistoricalAwards=false
• 关键词: Fate; Zircon; Other; Accessories; Deeply

Although volumetrically insignificant, the mineral zircon and its fellow accessory minerals monazite and rutile exert strong control over the distribution of several groups of geochemically- and strategically-important elements that are concentrated in the Earth’s crust: the high-field strength elements (Zr and Hf in zircon; Ti, Nb and Ta in rutile), the radiogenic and heat-producing elements U and Th (in zircon and monazite), and the rare-earth element group, La-Lu (in zircon and monazite). Isotopic analyses of these accessory minerals have been extensively used for decades to date rocks of the continental crust, but it has only been with experimental study of the behavior of these accessory minerals in the laboratory, under conditions where high-temperature fluids or partial melts (magmas) are present, that geochemists have been able to make more broad-ranging insights into the nature of crustal tectonics and the extent of oceans and emergent continents on the early Earth. The low solubility of these minerals in crustal fluids and melts suggests that they are likely to survive ultra-metamorphism in continent-continent collision zones (e.g., the Himalayas), or in any continental-derived sediments that are thought to be carried down deeper into the mantle in subduction zones, into the slab graveyards of the lower mantle at depths of 700 km or more. These are where mantle plumes that feed volcanism on ocean island chains found in the middle of the world’s oceans may originate. A geochemical signature attributable to this deeply recycled (subducted) continental component is seen in the lavas erupted on the Pitcairn and Samoan archipelagos in the middle of the SW Pacific Ocean. How that continental signature comes to be in the source region for these lavas deep in the Earth’s lower mantle, and the role that the accessory minerals zircon, monazite, and rutile play in conveying that signature, is the subject of this experimental study. The experiments may also lead to the discovery of new crystal structures and novel ceramics that may be of practical use in the long-term storage of high-level radioactive waste. An experimental study of the behavior of zircon, monazite and rutile in a lithologic context appropriate to subduction of hydrous continental sediments from the base of the lithosphere (~5- 7 GPa) to the top of the lower mantle (~23-25 GPa) is proposed. The stability of these minerals with respect to dehydration reactions involving major mineral phases (e.g., phengite and hydrous aluminous minerals), and their solubility in the fluids these reactions give rise to, will be ascertained under conditions appropriate to variable slab geotherms extending from the crust to the lower mantle. The concentrations of the high field strength (HFSE), heat-producing (HPE), and rare Earth elements (REE) in the accessory minerals and in the sediment-derived fluids with which they are in equilibrium will be determined by electron probe microanalysis (EPMA) and secondary ion mass spectrometry (SIMS). These analyses will be used to assess the effects of pressure on the partitioning systematics of the HFSEs, HPEs, and REEs between hydrous fluids and zircon, monazite and rutile (or their high- pressure polymorphs) in subducted terrigenous sediments. The potential for additional fractionation of the geochemical pairs Zr-Hf and Nb-Ta, and fractionation of the light REEs from the heavy REEs, will also be assessed. The research would provide empirical constraints on the nature of the “continental” isotopic and geochemical signature in the plume source for ‘enriched-mantle’ ocean island basalts. A better understanding of the role that the minerals zircon, monazite and rutile play in conveying a crustal signature into the lower mantle, in terms of the HFSEs, HPEs and REEs, is envisioned. New crystal structures appropriate for the storage of HLRW may emerge from the proposed experimental studies, and the development of new experimental techniques that increase the efficiency of multi-anvil experiments and new analytical techniques for measuring trace elements in coexisting fluids, melts and minerals are additional benefits expected to come from this research.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

矿物锆石及其附件矿物质矿物质和金红石矿物质矿物质及其同胞对几组在地球地壳中的地球壳的分布施加了强烈的控制：高场强度元素（zircon e in kird和the emprodipe）锆石和独居石），以及稀土元素群，la-lu（锆石和独居石）。这些附件矿物质的同位素分析已被广泛使用数十年来，直到迄今为止连续地壳的岩石都进行，但它仅在实验室中对这些附件在实验室的行为进行的实验研究，在存在高温流体或部分融化的条件下，存在着更广阔的范围，并且能够使大洋界变得更宽。在地球早期继续出现。 The low solubility of these minerals in crustal fluids and melts suggests that they are likely to survive ultra-metamorphism in continuous-continent collision zones (e.g., the Himalayas), or in any continuous-derived sediments that are thought to be carried down deeper into the mantle in subduction zones, into the slab graveyards of the lower mantle at depths of 700 km or more.这些是在世界海洋中部发现的海洋岛链中供电火山链的地幔羽流的地方。在SW太平洋中间的Pitcairn和Samoan Archivelagos爆发的熔岩中，可以看到这种归因于这种深层回收（俯冲）连续分量的地球化学标志。这种连续的签名如何出现在地球下层深处的这些熔岩的源区域，以及辅助矿物锆石，独居石和金红石在传达该签名中的作用，是这项实验研究的主题。这些实验还可能导致发现新的晶体结构和新型陶瓷，这些晶体可能在长期存储高级放射性废物中实际使用。在岩性环境下，提出了适合于从岩石圈（〜5-7 GPA）到下层（〜23-25 GPA）的含水大陆沉积物（〜23-25 GPA）的含水大陆沉积物俯冲的实验研究。这些矿物质在脱水反应方面的稳定性涉及主要矿物相（例如，铝铝矿物和含水铝矿物），在这些反应中产生的氟中心的溶解度将在适用于适合于从甲壳延伸至下层壁炉的可变平板地面的条件下确定。在附件矿物质中以及与等效探针微分析（EPMA）和二级离子质谱（SIMS）中，高场强度（HFSE），热产生（HPE）和稀土元素（REE）的浓度，稀土元素（REE）。这些分析将用于评估压力对HFSES，HPE和REE分配系统的影响，以及在俯冲的Terrigense累积沉积物中的含水液和锆石，石钠石和金红石（或其高压多晶型物）之间的影响。也将评估地球化学对ZR-HF和NB-TA的额外分馏，并将评估光REE的分馏。这项研究将提供有关羽流源中“大陆”同位素和地球化学特征的性质的经验约束，以供“富集的甲壳虫”玄武岩。可以更好地理解矿物锆石，独居石和金红石在将地壳签名传达到下层地幔中，就HFSES，HPES和REE的作用而言。适用于HLRW存储的新晶体结构可能会来自提出的实验研究以及新的实验技术的开发，这些技术提高了多动能实验的效率和新的分析技术，以衡量共存的烟道，熔体和矿物质的痕量痕迹的痕量要素，并通过此研究获得了诚实的效果。智力优点和更广泛的影响审查标准。