超高压变质岩矿物中的显微构造缺陷:来自TEM和FTIR研究的证据

超高压变质矿物的某些显微构造缺陷可能指示了岩石短暂和快速抬升过程。文中报道了中国大别山双河地区超高压硬玉石英岩矿物显微构造缺陷的透射电镜(TEM)和Fourier变换红外光谱(FTIR)的研究结果。用TEM研究方法,在硬玉石英岩中硬玉包裹体内发现了亚微米级天然蒙钠长石(MA,C2/m)、高钠长石(HA,C)和低钠长石(LA,C)三种多形变体。表明岩石在折返过程经历过高温变质作用(>930℃),以及退变质过程的快速冷却;在石英包裹体内发现了纳米级柯石英和石盐子矿物,提供了柯石英转变为石英以及峰期变质条件下高盐流体存在的证据。"名义上无水矿物"(NAMs)的结构水(OH/H2O)是以缺陷形式赋存于矿物结构中。FTIR分析结果显示硬玉、石榴石、金红石和石英中结构水的平均含量分别为1000×10-6、(900~1600)×10-6、>2000×10-6和<4×10-6,硬玉石英岩全岩含水量为(490~600)×10-6,表明在高压-超高压变质作用过程中,地壳或原岩中的水可以通过这些NAMs携带到地球深部。该类显微构造缺陷可能是大陆碰撞造山带在高应变速率下的局部弱化和深部断层作用的结果。

Microstructural defects in the UHP metamorphic rock: evidence from TEM and FTIR Study.

Defect microstructures of minerals as a potential indicator of extremely rapid and episodic exhumation of ultrahigh-pressure metamorphic rock. The microstructure defects in the minerals from UHP jadeite quartzite at Shuanghe, Dabie Mountains, China, were investigated using transmission electron microscope (TEM) and Fourier transform infrared spectroscopy (FTIR). TEM observations indicate that three polymorphs, namely submicron-scale nature monalbite (MA, C2/m) (about 0.8 μm), high albite (HA, C) and low albite (LA, C) occur in jadeite inclusions from jadeite quartzite. The existence of MA implies that high-temperature metamorphism (>930 ℃) during exhumation and rapid cooling during retrogression have taken place. The occurrence of nano-scale coesite (about 80 nm) and halite daughter minerals in quartz inclusions provides evidence for the transformation from the original coesite into quartz and the existence of high-salty fluids at peak metamorphic conditions. Structural water (OH/H2O) in nominally anhydrous minerals (NAMs) exists in structures of these minerals as defect. The FTIR results show that the average contents of structural water in jadeite, garnet, rutile and quartz are 1000×10-6, (900-1600) ×10-6, >2000×10-6 and <4×10-6 respectively, and the estimated whole-rock water content is (490-600) ×10-6. The results reveal that the water in crust or protolith could transport into the earth's depth through NAMs during HP-UHP metamorphism. These unique defect microstructures can be rationalized by a high strain rate at local weakening and deep faulting of continental collision orogens.