Lu-Hf地质年代学及其在低温高压变质岩中的应用

The lutetium (Lu) and hafnium (Hf) isotopic system has become increasingly valuable as a tool to understanding the timescales of metamorphic processes in high-pressure metamorphic orogens. This has occurred over the past decade as technology has improved the sensitivity of the instruments required to measure the isotopes. Increasing number of Lu-Hf dates of metamorphic rocks in various orogens worldwide have successfully unraveled the timing of specific metamorphic stages along the overall P-T path, and provided valuable information about the protracted timescale of subduction, multiply cycles of subduction to mantle depths and polyphases metamorphism in diverse orogenic belts. However, little is known about the promise and pitfalls of applying Lu-Hf geochronology to low-temperature and high-pressure rocks. The continuously improved precision of the Lu-Hf radiometric dates requires a refining decay constant of 176Lu, and the precise dates have to take the uncertainty of the decay constant into account in order to interpreting the dates correctly. The lack of matured Lu-Hf geochronology laboratory in our homeland has highly been hindering innovative contributions in this field from domestic scientists. This project aims to establish a state-of-art high-resolution Lu-Hf dating platform attached with micro-sampling facilities to offer the opportunity of original contributions in the Lu-Hf geochronology as well of its applications in solid earth studies for domestic scientists. Using this facility, we will perform a detailed Lu-Hf study of those typical low-temperature and high-pressure rocks that outcropped in domestic orogens in order to unravel the promise and pitfalls of Lu-Hf chronology in dating these LT-HP rocks. We also aim to constrain a more precise decay constant of 176Lu using combined analyses of zircon U-Pb isotope with Lu-Hf dating for the zircon standards as well as several well studied and carefully selected natural samples.

经过十多年的迅猛发展，Lu-Hf地质年代学已经成为研究高压/超高压造山带演化的重要利器。越来越多的Lu-Hf地质年代学工作不仅有效地约束了板片深俯冲各阶段的时限，而且揭露出板片长时间俯冲和多期俯冲旋回等以前未曾发现的重要地球动力学信息。然而，Lu-Hf地质年代学应用到低温高压变质岩尚存太多未知；随着分析精度的提高，衰变常数的不确定性所引起的系统误差对定年结果的影响已经不容忽视；国内严重滞后的Lu-Hf年代学实验平台正在阻碍和制约着我国相关研究的发展。因此，本项目拟通过联合微区取样装置，搭建国际承认的高空间分辨率的高精度Lu-Hf年代学测年平台；以国内几个典型造山带出露的蓝片岩和低温榴辉岩为主要研究对象，厘清Lu-Hf年代学应用到低温高压变质岩中的优势和局限性；同时，通过对几个锆石标样及合适的样品进行联合U-Pb和Lu-Hf定年，力图给出更高精度和准确度的176Lu衰变常数。

项目以国内几个典型造山带出露的低温高压榴辉岩为主要研究对象，通过搭建微区取样Lu-Hf年代学分析平台，对这些典型的低温高压变质岩为主的岩石，进行了以Lu-Hf方法为主其它定年体系为辅的系统研究。同时结合岩矿观测，相平衡模拟，探寻同位素平衡的机制和尺度，同时尝试确定更高精度及准确度的Lu衰变常数。项目执行取得如下新认识和成果：1)搭建了微区取样Lu-Hf年代学分析平台；2)首次给出单颗粒石榴石耦合Lu-Hf和Sm-Nd的实例研究。所获得的年龄从核部往边部变年轻，Sm-Nd年龄比对应的Lu-Hf年龄要小，记录了从前进变质到峰期变质的时间跨度；3)首次通过微钻对榴辉岩中常规毫米级的石榴石进行了微区Lu−Hf年代学工作，分别获得石榴石核部和边部的年龄，限定了石榴石生长的时间跨度，从而有效地厘定了早期到晚期变质的时限。该方法具有极其广阔的应用前景，突破了以往对普通不含异常大颗粒石榴石的变质岩只能获得混合等时线年龄的限制和无法确切制约早期变质时限的局限；4)对一个石榴角闪岩进行了石榴石Lu-Hf和Sm-Nd，锆石和独居石U-Pb等多同位素定年研究。发现石榴石的微裂隙导致有效扩散半径变小，Lu-Hf和Sm-Nd年龄解耦约70Myr，展示了一个协和从高温到低温的多同位素定年体系的研究范例；5)从拉萨地块旁那的低温石榴石片岩中分选出不同颗粒大小的石榴石，进行地Lu-Hf年代学研究，成功地把Lu-Hf体系运用到峰期变质温度低于500摄氏度的变质岩。尝试更新176Lu衰变常数的相关工作由于技术原因而有些滞后，尚处于撰写论文阶段，后续将持续补充正式发表的成果。