大别山双河超高压榴辉岩中的水: 微区红外光谱分析

大陆深俯冲板块到一定深度后(约90~110km), 几乎没有含水矿物存在, 超高压岩石中名义上无水矿物(NAMs) 成为俯冲板块中水的主要载体, 是示踪超高压变质流体的重要途径.对大别山双河地区超高压榴辉岩中的石榴石和绿辉石进行了详细的微区傅立叶变换红外光谱(MicroFTIR) 分析.FTIR结果显示所有石榴石和绿辉石颗粒都含有结构水, 以OH的形式存在, 其含量范围分别为(30~1860)×10-6和(360~620)×10-6.榴辉岩全岩水含量为(300~750)×10-6, 表明即使是在超高压变质作用的温压条件下, 榴辉岩也可以至少携带数百10-6的水进入深部地球.对石榴石颗粒内部的多点观察发现, 结构水含量的分布出现2种情况: (1) 颗粒内部的均一分布; (2) 核部水含量高而边部低.石榴石颗粒边部的低水含量可能是抬升过程中由于压力降低引起的H扩散所致, 扩散出来的H可能构成了早期退变质流体的重要来源.对于同一样品来说, 结构水含量在绿辉石和石榴石之间的比值为0.5~3.5. 

Water in UHP Eclogites at Shuanghe, Dabieshan: Micro-FTIR Analysis

When the continental crust subducted to a certain depth (~90-110 km), almost no hydrous minerals existed in the plate, and nominally anhydrous minerals (NAMs, such as garnet, pyroxene, quartz) in UHP rocks became the main carriers transporting water into the mantle. Garnet and omphacite in UHP eclogites at Shuanghe, Dabieshan have been investigated by Micro-FTIR. The results demonstrate that all garnet and omphacite grains contain structural water occurring as hydroxyl (OH) with the content from 30×10-6 to 1860×10-6 (H ₂O wt.) and from 360×10-6 to 620×10-6, respectively. The water content (300×10-6 to 750×10-6) of whole rocks suggests that UHP rocks can recycle at least several hundred ×10-6 water into the mantle by deep continental subduction. There are two types of water distribution characteristics within a single garnet grain, one is homogeneous and the other has higher water content in the core and lower in the rim. The lower water content in the rim possibly arises from hydrogen diffusion in response to a sharp decrease in pressure during the exhumation, and the expelled hydrogen may be the source of early-stage retrograde fluid. The ratio of water content in omphacite and garnet is between 0.5 and 3.5.