Are zircons forever? On the problem of survival of inherited zircons in juvenile crust and lithospheric mantle

自从离子微探针技术（如SHRIMP）和高分辨率ICP-MS的发展，以及含铀矿物（如锆石）中常规的原位U-Th-Pb分析成为可能以来，地球科学经历了一场影响深远的革命。使用SHRIMP的主要发现之一是偶尔存在继承的（异晶）锆石，或锆石的岩心，比宿主岩石古老得多。这种遗传颗粒甚至在远离任何大陆的洋内环境中形成和沉积的年轻火成岩中也发现了。这些环境中继承锆石的存在挑战了我们对地幔地球动力学的看法，并表明存在未知的再循环过程，甚至可能涉及深部地幔。尽管锆石的遗传问题对地壳演化和地幔地球动力学具有重要的意义，但它尚未得到充分的认识，也没有得到认真的处理。因此，在看似年轻的幔源岩中确定外来锆石的成因和保存条件是本项目的目标。我们将通过考虑不同时代的各种地球动力学环境（例如，海洋内的弧前、弧后、洋中脊和洋岛），并通过使用来自加勒比海（古巴和格林纳达群岛）、Galápagos和中亚（蒙古和吉尔吉斯斯坦）的岩石样本，来解决这个长期存在的地质问题。我们采用了一种新颖的方法，将U/Pb年龄定年、锆石中的Lu/Hf和o同位素、地幔源硫化物中的Re/Os同位素、化学分析（如使用SHRIMP II、LA/ICP/MS和EPMA进行锆石中的元素/同位素分布）和锆石中矿物包裹体的化学组成结合起来。对shrimp定年锆石颗粒进行Hf、O同位素测定。整个岩石地球化学（主要元素和微量元素）和矿物分析将定期对岩石样品进行锆石定年。我们还将研究古巴和中亚洋内弧沉积物的碎屑锆石种群，以便将物源年龄与继承颗粒年龄进行比较。这些分析方法将分别由美因茨大学和法兰克福大学的活塞缸和多砧压力机进行实验补充。本文将首次研究高压ZrSiO4多晶体（reidite）中pb的扩散率。该项目将展示如何利用先进的小域地质年代学、地球化学和实验岩石学来了解地壳的形成、演化和活动板块边缘的大规模再循环。

Earth sciences experienced a far-reaching revolution since ion microprobe technology (e.g. SHRIMP) and high-resolution ICP-MS were developed and routine in-situ U-Th-Pb analysis of domains in U-bearing minerals (e.g. zircon) became available. One of the mayor discoveries using SHRIMP was the occasional presence of inherited (xenocrystic) zircons, or cores of zircons, much older than the host rock. Such inherited grains were even found in juvenile igneous rocks generated and emplaced in intra-oceanic environments, far away from any continent. The presence of inherited zircons in these environments challenges our view of mantle geodynamics and suggests the existence of yet unknown recycling processes, possibly even involving the deep mantle. The problem of zircon inheritance is not yet fully understood and has not been seriously addressed despite its considerable implications for crustal evolution and mantle geodynamics. Therefore, resolving the origin and conditions of preservation of exotic zircon in seemingly juvenile, mantle-derived rocks is the target of this project. We will address this long-standing geological problem by taking into consideration various geodynamic settings (e.g., intra-oceanic fore-arc, arc, back-arc, mid-ocean ridge and ocean island) of different ages and by using rock samples from the Caribbean (Cuba and Grenada Islands), Galápagos, and Central Asia (Mongolia and Kyrgyzstan). We follow a novel approach in combining U/Pb age dating, Lu/Hf and O-isotopes in zircons, Re/Os isotopes in mantle-derived sulfides as well as chemical analyses (e.g. element/isotope distribution in zircons using SHRIMP II, LA/ICP/MS and EPMA) and the study of the chemical composition of mineral inclusions in zircon. The Hf and O isotope information will be obtained on SHRIMP-dated zircon grains. Whole rock geochemistry (major and trace elements) and mineral analysis will be routinely performed on rock samples from which the zircons are dated. We shall also study the detrital zircon population of intra-oceanic arc sediments from Cuba and Central Asia in order to compare the provenance ages with the age of inherited grains. These analytical methods will be complemented by experiments using piston cylinder and multi-anvil presses at Mainz and Frankfurt Universities, respectively. The Pb-diffusivity in reidite (high-pressure ZrSiO4 polymorph) will be investigated for the first time. The project will demonstrate how advanced small-domain geochronology, geochemistry and experimental petrology can be used to understand crust formation and evolution and mass-recycling at active plate margins.