副矿物地球化学研究进展

副矿物广泛分布于各类火成岩、变质岩和沉积岩中,对寄主岩石的形成环境有很高的敏感度。以锆石、金红石和磷灰石为例,分别从结构特征、微量元素地球化学特征和同位素年代学等方面介绍副矿物地球化学的主要研究进展。锆石中U和Th的含量、Th/U值、Zr/Hf值等均与锆石成因类型有关,但共生矿物的存在和蜕晶化等因素可能会对这些微量元素产生影响,也可能导致不准确的同位素年龄结果。金红石以高Nb和Ta为特征,不同类型岩浆岩中Nb/Ta值差异较大,Cr-Nb二元判别图解也可以用来判断变质原岩类型。Mn2+、Eu2+、Sm3+、Dy3+、Er3+等微量元素会使磷灰石在受到阴极射线的激发下产生黄、绿或蓝色的光,因此磷灰石的发光特征可以反映其化学组成。不同类型岩石中的副矿物REEs的组成特征明显不同,说明受后期地质活动影响较弱的副矿物仍能精确指示其形成环境。由于锆石中的Ti和金红石中的Zr均是温度的函数,因此是重要的地质温度计,而榍石中的Zr对压力较敏感,常被用作地质压力计。锆石、金红石和磷灰石均为高铀矿物,均可进行U-Pb同位素定年,副矿物Sm-Nd、Lu-Hf也已逐渐成为常规定年法。热年代学得到快速发展以来,高-中-低温同位素体系与地质温度计和压力计的配套使用,不仅能够获取更准确的年龄值,还能提供更完整的地质体演化p-T-t曲线。副矿物地球化学将会更注重于尚未深入研究的其他副矿物在岩浆和高温变质过程中、在油气成藏演化的不同阶段中、在微生物活动影响下的内部结构和成分变化,新的同位素体系、纳米技术和精度更高的分析仪器等的发展会为副矿物研究带来更多突破。

Geochemical research progress of accessory minerals

Accessory minerals are very sensitive to diagenetic environment of their host rocks, which will be inevitable to be embodied in the structural and geochemical characteristics of accessory minerals. Zircon, rutile, apatite were frequently reported on their crystal structure, trace element characteristics and isotopic chronology. This review will provide the previous outstanding works and try to predict the future of geochemical researches of accessory minerals. Content of U, Th, Zr, Hf and their ratios are relevant to genetic types of zircon, but paragenetic minerals of zircon and metamictization may have influence on these parameters, as well as the precision of U-Pb age. Rutile is characterized by high Nb and Ta, and the ratio of Nb/Ta of rutile varies in different igneous rocks. Cr-Nb diagram is useful in recognizing protoliths. The existence of Mn²⁺, Eu²⁺, Sm³⁺, Dy³⁺, Er³⁺ is the reason why apatite luminescence displays yellow, green or blue. Trace elements, such as REEs, in accessory minerals which were not or slightly affected by geological activities reserve substantial information from the diagenetic environment of their host rocks. Ti in zircon and Zr in rutile are function of temperature, and these make Ti-in-zircon and Zr-in-rutile thermometers. Inclusions in accessory minerals contain original materials from magma source and wall rocks, so they are significant for researches of rock formation and magma evolution. With high content of uranium, zircon, rutile and apatite are used in U-Pb geochronology; in addition, Sm-Nd and Lu-Hf become regular isotopic methods of geochronology. After the rapid development of thermochronology, different isotopic systems of high-, mid- and low-temperature can supplement each other. This application will present not only more accurate ages, but also completer p-T-t paths. In the future, changes of structure and chemical composition of less investigated accessory minerals in high-temperature metamorphism, in each stages of hydrocarbon accumulation and under microbiological activities may become research focuses. New isotopic systems, nanotechnology and the development of analytical instrument will bring vitality to researches of accessory minerals with no doubt.