大陆深俯冲过程中的水:"名义上无水矿物(NAMs)"的信息

Water in the Deep Subducted Continental Plate: Message from NAMs

对大别山典型产地榴辉岩中石榴子石和绿辉石的傅立叶变换红外光谱(FTIR)观察表明,石榴子石和绿辉石中都含育一定量的结构水,以OH-形式赋存。石榴子石中的水含量(H2O,wt)多为(200-1000)×10-6,可高达2200×10-6;绿辉石中的水含量相对高一些,多在500×10-6以上,最高可达3000×10-6;UHP榴辉岩的全岩水含量可高达10000×10-6。绿辉石的δD值为-67‰--110‰。矿物中的水含量和δD值都存在小尺度(同一样品不同颗粒间以及不同样品之间)的不均一性,这不仅表明超高压变质过程中自由流体的活动范围是有限的,而且说明板块的俯冲和折返都是十分快速的。"名义上无水矿物(NAMs)"的结构水含量和分布特征不仅可以示踪大陆深俯冲过程中流体的演化,还可以提供有意义的深俯冲过程的动力学信息。

Garnets and omphacites in eclogites from Dabieshan were investigated by Fourier transform infrared spectroscopy (FT IR). The results indicate that garnet and omphacite could contain structural water which occurs as hydroxyl (OH). The structural hydroxyl contents of the Dabieshan garnets are varied from 20×10-6 to 2200×10-6 ppm (H2O,wt) with majority ranging from 200×10-6 to 1000×10-6 , whereas those of the Dabieshan omphacites are varied from 100×10-6 to 3000×10-6 with majority of over 500×10-6. The UPH eclogite could contain up to 1000×10-6 water. The δD values of omphacites are varied form -67‰ to-110‰. Both the H2 O contents and SD values show uneven distribution among various samples from the same locality and even among various grains within the same sample. This suggests that the mobility of fluids was very limited during UHP metamorphism. It also indicates that both the plate subduction and the exhumation processes of UHP rocks were occurred in a short time period. The structural water contents and the distribution characteristics of NAMs not only can trace the evolution of fluids during UHP metamorphism, but also can provide some geodynamic information of deep continental subduction.