Trace elements and Hf isotopic compositions of four ancient zircon megacrysts from Yakutian kimberlite, Siberia: Implications for identifying mantle and old zircon megacrysts from Kimberlites

Abstract

金伯利岩中的锆石按照颗粒大小可以分为细粒锆石（一般小于200μm）和巨晶锆石（一般大于500μm）。前人的研究结果显示在金伯利岩中粒径较大的巨晶锆石的U-Pb体系在高温的地幔中一直保持着开放状态，直到寄主金伯利岩浆的喷发才使地幔锆石的U-Pb体系封闭，因此这些巨晶锆石是确定金伯利岩年龄的重要矿物之一。然而，近年来的研究表明，金伯利岩中还存在一些时代远老于金伯利岩年龄的锆石，也具有较大的粒径（以下称古老锆石巨晶）。它们的存在无疑影响了利用锆石U-Pb方法确定金伯利岩年龄的准确性。本文以西伯利亚雅库特（Yakutia）金伯利岩省中的四颗古老锆石巨晶为研究对象，通过形态学、年代学、微量元素和Hf同位素组成，讨论古老锆石巨晶的来源。同时，我们统计和对比了全球多个金伯利岩中能够确定金伯利岩年龄的锆石和古老锆石巨晶的形态学、U、Th含量和微量元素组成、Hf-O同位素等特征。研究结果显示，金伯利岩中的古老锆石巨晶的t_DM年龄和O同位素组成与可以用来确定金伯利岩年龄的锆石巨晶具有明显的差别。这些手段在未来的研究中可以用来区分可确定金伯利岩年龄的锆石巨晶和古老的锆石巨晶。;Zircon from kimberlite can be divided into fine-grained zircon (<200μm) and megacrystal zircon (>500μm) according to its grain size. Previous studies have shown that the U-Pb system of mantle zircon is an open state in the high temperature mantle until the eruption of host kimberlite. Therefore, mantle zircon is one of the important minerals for dating the age of kimberlite. However, recent studies have shown that some zircon megacrysts from kimberlite with ages much older than kimberlite also have large grain sizes. Their presence undoubtedly influences the quasi-dating of kimberlite ages. In this study, four ancient zircon megacrysts from the Siberian kimberlite were studied,and the origin of the ancient zircon megacrysts was discussed through morphology, chronology, trace elements and Hf isotopic compositions. Moreover, the morphology, U and Th contents, trace element composition, and Hf-O isotopes of mantle zircon and old zircon from cratonic kimberlite rocks in the world are collected and compared. The results show obvious differences in t_DM ages and O isotopic compositions between the large old zircon and mantle zircons. These methods can be used to distinguish old zircon from mantle zircon megacrysts in kimberlite in future studies.