Water solubility in pyrope to 100 kbar
Water solubility in pyrope to 100 kbar
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DOI：
10.1007/s004100050321
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发表时间：
1997-09
影响因子：
3.5
通讯作者：
R. Lu;H. Keppler
中科院分区：
文献类型：
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作者：
R. Lu;H. Keppler
研究了西阿尔卑斯山Dora Maira天然镁铝榴石中水的溶解度和掺入机理。未经处理的天然样品（58 ppm水）的红外光谱在OH伸缩区域显示出几个异常尖锐的谱带，包括3601.9 cm− 1处的单一谱带和3640.5、3650.8和3660.6 cm−1处的主要组分谱带系统。高温和高压的红外光谱表明，这两种吸收特征来自于不同压缩性和热膨胀性的位置上几乎自由的OH基团，导致3601.9 cm− 1谱带的位置更硬。在过量水、过量SiO2和过量Al 2SiO 5的存在下，将镁铝榴石样品在活塞-圆筒或多砧装置中退火数天，以确定水在镁铝榴石中至100 kbar的平衡溶解度。总溶解度随压力增加，然而，这完全是由于高频带系统，而低频带的强度随压力降低。在1000 °C和Ni-NiO缓冲液的氧逸度下，体相溶解度可以用方程cOH =AfH2O0.5exp（−PΔV/RT）描述，其中A = 0.679 ppm/bar0.5，ΔV= 5.71 cm 3/mol。 该方程意味着水作为孤立的OH基团结合在晶体中。随着温度的升高，溶解度下降，ΔH= − 14 kJ/mol。在Fe-FeO缓冲液条件下，溶解度比Ni-NiO缓冲液低30%至50%，这表明OH的掺入与Fe 3+的还原无关。3601.9 cm− 1的谱带可能与四面体OH B缺陷有关，而高频谱带可能与十二面体OH Li缺陷有关。根据实验建立的溶解度模型，估计石榴石在一个热俯冲板将超过角闪石的击穿极限170 ppm的水输送到地幔。在冷板中，在多硅白云母的击穿极限下，470 ppm的水可以结合到石榴石中。这些数字意味着，自元古代以来，水圈中总水量的很大一部分已经再循环到地幔中。
The solubility and incorporation mechanism of water in natural, almost pure pyrope from Dora Maira, Western Alps was investigated. The infrared spectrum of the natural, untreated sample (58 ppm water) shows several exceptionally sharp bands in the OH-stretching region, including a single band at 3601.9 cm−1and a band system with main components at 3640.5, 3650.8 and 3660.6 cm−1. High-temperature and high-pressure infrared spectra suggest that the two absorption features arise from almost free OH groups in sites with different compressibility and thermal expansivity, with the site causing the 3601.9 cm−1band being much stiffer. Pyrope samples were annealed in a piston-cylinder or multi-anvil apparatus for several days in the presence of excess water, excess SiO2and excess Al2SiO5to determine the equilibrium solubility of water in pyrope to 100 kbar. Total solubility increases with pressure, however, this is exclusively due to the high-frequency band system, while the intensity of the low-frequency band decreases with pressure. At 1000 °C and the oxygen fugacity of the Ni-NiO buffer, the bulk solubility can be described by the equationcOH=AfH2O0.5exp(−PΔV/RT)withA= 0.679 ppm/bar0.5and ΔV= 5.71 cm3/mol. This equation implies the incorporation of water in the crystal as isolated OH groups. With increasing temperature, solubility appears to decrease with ΔH= − 14 kJ/mol. At Fe-FeO buffer conditions, solubility is 30 to 50% lower than with the Ni-NiO buffer, suggesting that the incorporation of OH is not coupled to the reduction of Fe3+. Possibly, the 3601.9 cm−1band is associated with the tetrahedral OH B defect and the high-frequency system with the dodecahedral OH Li defect. Based on the experimentally established solubility model, it is estimated that garnet in a hot subducted slab will transport 170 ppm of water into the mantle beyond the breakdown limit of amphibole. In a cold slab, 470 ppm of water can be incorporated into garnet at the breakdown limit of phengite. These numbers imply that a significant fraction of the total water in the hydrosphere has been recycled into the mantle since the Proterozoic.