CCSD超高压变质岩中金红石结构水(OH^-)的傅里叶变换红外光谱测定及其地球动力学意义

Structural Hydroxyl of Rutile in UHP Metamorphic Rocks from Chinese Continental Scientific Drilling Project (CCSD) and Areas Nearby and Its Implication for Geodynamics

名义上无水矿物(NAMs)的结构水研究是认识超高压变质作用(UHPM)过程中流体活动规律的重要手段,并对板块汇聚边缘大陆动力学及水的深部地球循环有重要意义。本文采用傅立叶变换红外光谱(非偏振光)研究了中国大陆科学钻探(CCSD)主孔岩心及其周围地表榴辉岩中副矿物—金红石中的结构水。所有测试样品都在3285 cm^(-1)和3295 cm^(-1)附近产生尖锐的吸收峰,证实“水”以结构羟基(OH^-)的形式赋存于金红石晶格之中。采用最新标定的摩尔吸收系数[38000 L/(mol·cm^2)]计算表明,金红石结构水含量在324×10^(-6)～523×10^(-6)范围内,远较前人有关金红石结构水含量的计算结果为低。同时,结构水分布不均一性在不同样品之间和颗粒内部都有明显表现,指示超高压变质过程中有限的流体活动和快速的板块俯冲—折返过程。

The research of structural water in nominally anhydrous minerals （NAMs） is significant for understand of the fluid flow activity in ultra-high pressure metamorphism, the geodynamics in convergent continental margins and the water recycling in mantle. This study focus on the structural hydroxyl concentrations of rutile in eclogites from main-hole of the Chinese Continental Scientific Drilling project （CCSD） and its areas nearby emp light. The rutile grains all exhib oying microscope Fourier infrared spectroscopy（FTIR） with unpolarized t a single sharp band at near 3285 cm^-1 or 3295 cm^-1, which is the structural hydroxyl （OH^- ） absorption band, indicating water is structurally incorporated in rutiles. Calculating with the molar absorption coefficient [38000 L/（mol. cm^2）] of Maldener et al. （2001） shows that the ＂water＂ contents in rutiles are 324 to 523 × 10^- 6, which is greatly lower than the previous researcher＇s result calculated with the molar absorption coefficient[3270 L/（mol · cm^2）]of Hammer et al. , （1991）. Theinhomogenous distribution of OH^- in rutile is shown not only in different samples but also in the different micro-areas in the same grain, indicating that the fluid flow activity in ultra-high pressure metamorphism is limited and slabe subduction and exhumation are quick.