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

随着离子微探针技术(如SHRIMP)和高分辨率电感耦合等离子体质谱仪的发展，以及对含铀矿物(如锆石)中的磁区进行常规的原位U-Th-Pb分析，地球科学经历了一场深远的革命。市长使用SHRIMP的发现之一是偶尔发现继承的(捕捉晶)锆石，即锆石的核心，比主岩古老得多。这种遗传的颗粒甚至在远离任何大陆的洋内环境中生成和侵位的年轻火成岩中也发现了。这些环境中继承的锆石的存在挑战了我们对地幔地球动力学的看法，并表明存在未知的循环过程，甚至可能涉及地幔深处。尽管它对地壳演化和地幔地球动力学有相当大的影响，但对锆石继承的问题还没有完全了解，也没有得到认真解决。因此，解决看似年轻的地幔岩石中奇异的锆石的成因和保存条件是本项目的目标。我们将考虑到不同时代的各种地球动力学背景(例如，大洋前弧、弧后、大洋中脊和洋岛)，并使用加勒比(古巴和格林纳达群岛)、加拉帕戈斯和中亚(蒙古和吉尔吉斯斯坦)的岩石样本，来解决这一长期存在的地质问题。我们采用了一种新的方法，将U/Pb测年、锆石的Lu/Hf和O同位素、地幔硫化物的Re/Os同位素和化学分析(如SHRIMP II、LA/ICP/MS和EPMA)相结合，并研究了锆石中矿物包裹体的化学成分。Hf和O同位素信息将在SHRIMP年龄的锆石颗粒上获得。将对确定锆石年代的岩石样品进行常规的全岩地球化学(主要和微量元素)和矿物分析。我们还将研究古巴和中亚大洋内弧状沉积物的碎屑锆石群体，以便将其源区年龄与继承颗粒的年龄进行比较。这些分析方法将得到美因茨大学和法兰克福大学分别使用活塞式气缸和多座压力机进行的实验的补充。本文首次研究了辉绿岩(高压硅酸锆晶型)中的铅扩散系数。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和演化以及活动板块边缘的物质再循环。

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.