视剖面图温压计研究进展评述

视剖面图温压计是目前定量估算变质温度和压力的主要方法之一。其理论基础为热力学平衡原理与质量守恒定律。利用内恰的热力学数据库和相应的成分-活度模型对特定成分体系进行视剖面图计算,可以正演模拟给定温度和压力条件下的矿物组合、矿物丰度及其成分,与实测岩石对比,可以准确获取岩石的形成温度和压力。在应用视剖面图温压计时,应选择受全岩成分影响小、且受退变质作用影响微弱的矿物成分;对于有成分变化的体系,应结合多种方法恢复有效全岩成分。文中通过视剖面图对KFMASH(K2O-Fe O-Mg O-Al2O3-Si O2-H2O)体系下不同矿物组合内多硅白云母硅含量及其等值线斜率的研究,认为多硅白云母硅含量及其等值线斜率主要由不同矿物组合内多硅白云母参与的契尔马克替换及纯转换端元反应中的主导反应决定,进一步深化了对变质过程中控制矿物成分变化的内部缓冲反应机制的理解,从而也可为选择和应用矿物温压计提供指导。因此,视剖面图方法是目前研究变质岩石形成条件及变质作用精细过程的最佳方法之一。

A brief review of the advance of pseudosection thermobarometry

Pseudosection thermobarometry is one of the most popular approaches for quantitative determination of metamorphic pressure-temperature (P-T) conditions. It is on the basis of equilibrium thermodynamics and mass conservation principles. Pseudosections, which are calculated from internally consistent thermodynamic datasets and corresponding activity-composition relationships, not only involve a forward modeling of mineral stabilities, modal proportions and mineral composition with given bulk composition and P-T conditions, but also can acquire the metamorphic conditions when compared with natural rocks. It is best to choose those minerals whose composition is rarely affected by retrograde metamorphism and diffusion, and those with a flatter slope of the mineral composition isopleths in P(T)-X pseudosections; in addition, we should try our best to recover the effective bulk composition for those system with varying composition. P-T pseudosection and Si content isopleths of phengite in the system KFMASH (K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O) are calculated, and the Si content and slope of isopleths are highly dependent on the dominant reactions of tschermak and net transfer reactions between phengite and other minerals' end members, which help us knowing the mechanism of buffering reactions controlling changes of minerals composition, furthermore it also can help us to choose and apply suitable minerals thermobarometries. So pseudosection approach is one of the best ways to recover the metamorphic conditions and study the elaborate metamorphic processes.