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

虽然矿物锆石及其副矿物独居石和金红石的体积很小，但它强烈控制着几组地球化学和战略上重要元素的分布，这些元素集中在地壳中：高场强元素(锆石中的Zr和Hf；金红石中的Ti、Nb和Ta)，放射成因和生热元素U和Th(在锆石和独居石中)，以及稀土元素La-Lu(在锆石和独居石中)。几十年来，对这些辅助矿物的同位素分析一直被广泛用于确定大陆地壳岩石的年龄，但只有在实验室对这些辅助矿物在高温流体或部分熔体(岩浆)存在的条件下的行为进行实验研究后，地球化学家才能够对地壳构造的性质以及早期地球上海洋和新兴大陆的范围做出更广泛的见解。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，或者在任何陆源沉积物中幸存下来，这些沉积物被认为在俯冲带中更深地幔中，在700公里或更深的深度进入下地幔的板岩墓地。这就是在世界海洋中央发现的海洋岛链上为火山活动提供动力的地幔热柱可能起源的地方。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，可以看到这种深度循环(俯冲)大陆成分的地球化学特征。大陆特征是如何在地球下地幔深处的熔岩源区形成的，以及辅助矿物锆石、独居石和金红石在传递这种特征中所起的作用，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的陶瓷，这些可能在长期储存高放射性废物方面具有实际用途。对锆石、独居石和金红石在岩石圈底部(~5~7 Gpa)向下地幔顶部(~23~25 Gpa)俯冲的陆相含水沉积物的岩性背景下的行为进行了实验研究。这些矿物相对于主要矿物相(如多硅白云母和含水铝矿物)的脱水反应的稳定性以及它们在这些反应产生的流体中的溶解度，将在从地壳延伸到下地幔的可变板条地温的适当条件下确定。副矿物中高场强(HFSE)、生热(HPE)和稀土元素(REE)的含量将通过电子探针微区分析(EPMA)和二次离子质谱仪(SIMS)测定。这些分析将用于评估压力对水合流体与俯冲陆源沉积物中的锆石、独居石和金红石(或其高压晶型)之间的HFSE、HPES和REE分配体系的影响。此外，还将评估进一步分馏Rb-Hf和Nb-Ta地球化学组的可能性，以及从重稀土中分馏轻稀土的可能性。这项研究将对“富集型地幔”洋岛玄武岩的烟柱来源中的“大陆”同位素和地球化学特征的性质提供经验约束。预计将更好地了解矿物锆石、独居石和金红石在向下地幔传递地壳特征方面所起的作用，即高场强元素、高能元素和稀土元素。拟议的实验研究可能会出现适合储存高放废物的新晶体结构，开发新的实验技术以提高多砧板实验的效率，以及用于测量共存流体、熔体和矿物中的微量元素的新分析技术的开发，预计将从这项研究中获得额外的好处。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.