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

锆石及其伴生矿物独居石和金红石虽然在体积上不显著，但对集中在地壳中的几组具有地球化学意义和战略意义的元素的分布具有很强的控制作用：高场强元素（锆石中的Zr和Hf；金红石中的Ti、Nb和Ta），放射成因和产热元素U和Th（锆石和独居石），以及稀土元素组La-Lu（锆石和独居石）。几十年来，这些辅助矿物的同位素分析已被广泛用于确定大陆地壳岩石的年代，但只有在实验室中，在高温流体或部分熔融（岩浆）存在的条件下，对这些辅助矿物的行为进行实验研究，地球化学家才能对地壳构造的本质以及海洋和早期地球上出现的大陆的范围有更广泛的了解。这些矿物在地壳流体和熔体中的低溶解度表明，它们很可能在大陆-大陆碰撞带（例如喜马拉雅山脉）的超变质作用中幸存下来，或者在任何大陆衍生的沉积物中幸存下来，这些沉积物被认为是在俯冲带中被带到更深的地幔中，进入700公里或更深的下地幔的板块墓地。这些地方可能是地幔柱的发源地，这些地幔柱为在世界海洋中心发现的海洋岛链上的火山活动提供了动力。在西南太平洋中部皮特凯恩群岛和萨摩亚群岛喷发的熔岩中，可以看到这种深度再循环（俯冲）大陆成分的地球化学特征。这个大陆特征是如何在地球下地幔深处的熔岩源区出现的，以及辅助矿物锆石、独居石和金红石在传递这个特征中所起的作用，是本实验研究的主题。这些实验还可能导致新的晶体结构和新型陶瓷的发现，这可能在高放射性废物的长期储存中有实际用途。本文提出了锆石、独居石和金红石在适合于从岩石圈底部（~5 ~ 7 GPa）到下地幔顶部（~23 ~ 25 GPa）的含水大陆沉积物俯冲的岩性背景下的行为的实验研究。这些矿物在涉及主要矿物相（如白云石和含水铝矿物）的脱水反应中的稳定性，以及它们在这些反应产生的流体中的溶解度，将在适合于从地壳延伸到下地幔的可变板块地热的条件下确定。利用电子探针微量分析（EPMA）和二次离子质谱法（SIMS）测定了副矿物和与之平衡的沉积物衍生流体中高场强（HFSE）、产热（HPE）和稀土元素（REE）的浓度。这些分析将用于评估压力对俯冲陆源沉积物中含水流体与锆石、独居石和金红石（或它们的高压多晶型）之间的高通量、高通量和稀土分配系统的影响。还将评估地球化学对Zr-Hf和Nb-Ta的额外分馏潜力，以及轻稀土和重稀土的分馏潜力。该研究将为“富地幔”洋岛玄武岩羽源的“大陆”同位素性质和地球化学特征提供经验约束。展望了矿物锆石、独居石和金红石在向下地幔传递地壳特征方面的作用，即hfse、hpe和ree。拟议的实验研究可能会产生适合储存高锰矿水的新晶体结构，开发新的实验技术，提高多砧实验的效率，以及开发新的分析技术，测量共存流体、熔体和矿物中的微量元素，这些都是本研究预计将带来的额外好处。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.