俯冲带蛇纹岩的变质过程

Metamorphism of subduction zone serpentinite

俯冲带蛇纹岩是俯冲带流体的重要来源,特别是其深部脱水作用对地幔动力学影响深远,是研究俯冲带约80-200km深度范围的地球动力学的关键,因此研究蛇纹岩的变质作用过程及其相关特征矿物（组合）的温压稳定范围具有重要意义。蛇纹岩具有简单的矿物（组合）：蛇纹石类、硅镁石类、磁铁矿、氢氧镁石、绿泥石、橄榄石、透辉石、角闪石、滑石等,并且这些矿物（组合）对温压变化不敏感从而很难用来判定蛇纹岩所经历的变质演化轨迹。近几十年来,研究者通过实验岩石学和野外地质观察,主要研究了蛇纹石类矿物和硅镁石类矿物的温压稳定范围,并且试图使用这些特征矿物（组合）来判定俯冲带蛇纹岩的峰期变质条件。本文总结了蛇纹岩中这些主要矿物的温压稳定范围和相关变质反应,并且以中国西南天山蛇纹岩为例,展示使用特征矿物（组合）和叶蛇纹石Al等值线判定蛇纹岩峰期温压条件在实际岩石中的应用。另外,早期对叶蛇纹石的研究表明：随着温压条件的变化,叶蛇纹石的晶体结构会发生相应的调整。表现为单位晶胞内硅氧四面体的个数（m值）发生变化：温度升高,m值变小;压力升高,m值变大,这个发现在高压实验和天然样品中得到了一定程度的验证。本文利用已知峰期温压范围的叶蛇纹石样品分别采用粉末制样法和离子减薄制样法,进行透射电镜测试（TEM）样品的m值,并通过统计的方法获得叶蛇纹石的m值的峰值。结果显示叶蛇纹石的m值的峰值在一定程度上可以用以指示温压条件。本文提出可以用矿物组合、叶蛇纹石Al等值线和叶蛇纹石m值峰值相结合的方法确定蛇纹岩的变质温压条件和P-T轨迹。

Serpentinite of subduction zones is an important source of subduction zone fluid. Especially fluid liberated by dehydration of serpentinite at mantle depth plays a crucial role in the Earth dynamics. It is a key phenomenon for understanding the geodynamics of subduction zones in the ca. 80 ~ 200 km depth range. Therefore,studying the metamorphic processes of serpentinite and the stability of its minerals or mineral assemblages is of great significance. The minerals or mineral assemblages of serpentinite are simple. It contains serpentine-group minerals,humite-group minerals,magnetite,brucite,chlorite,olivine,diopside,amphibole,talc,etc. These minerals or mineral assemblages are usually insensitive to the change of temperature or pressure and they hardly could be used to constrain the metamorphic P-T path of serpentinite. In recent decades,researchers have mainly studied the stability of serpentine-group and humite-group minerals based on experiments and field observations,and tried to determine the peak P-T conditions of subduction zone serpentinite using diagnostic minerals or mineral assemblages. In this paper we review the stability fields of these major minerals in serpentinite and the metamorphic reactions involved. Moreover,taking Chinese southwestern Tianshan serpentinite as an example,we show how the peak P-T conditions of a natural serpentinite can be constrained using diagnostic mineral assemblage and antigorite Alisopleth. In addition,early studies have shown that the crystal structure of antigorite changes with variations of temperature and pressure. The number of tetrahedra along an entire wavelength（ m value） decreases with increasing temperature,but increases with the rise of pressure,a relation proven to some extent by experimental and natural sample studies. Here,we study the m value of antigorite samples from Chinese southwestern Tianshan,the peak P-T ranges of which are known. Powder samples and ion beam thinning samples are both prepared for transmission electron microscope measurements（ TEM）. Peak m values of different samples are obtained based on a statistical method. The results show that the peak m value of antigorite can be used to constrain the pressure-temperature conditions of the serpentinite. We suggest that the metamorphic P-T conditions and the P-T path of serpentinite can be determined by combining the mineral assemblage information,the Al isopleths of antigorite and peak m value of antigorite.