|
|||||
|
|
Low-temperature evolution of OH bands in synthetic forsterite, implication for the nature of H defects at high pressure |
|
| Authors: | J Ingrin J Liu C Depecker S C Kohn E Balan K J Grant |
| Institution: | 1. UMET, UMR CNRS 8207, Université de Lille1, Bat. C6, 59655, Villeneuve d’Ascq, France 2. CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China 3. School of Earth Sciences, University of Bristol, Queens Rd., Bristol, BS8 1RJ, UK 4. Institut de Minéralogie et Physique des Milieux Condensés (IMPMC), UMR CNRS 7590, UMR IRD 206, Université Paris VI, case 115, 4 Place Jussieu, 75252, Paris Cedex 05, France 5. GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, 2109, Australia |
| Abstract: | We performed in situ infrared spectroscopic measurements of OH bands in a forsterite single crystal between ?194 and 200 °C. The crystal was synthesized at 2 GPa from a cooling experiment performed between 1,400 and 1,275 °C at a rate of 1 °C per hour under high silica-activity conditions. Twenty-four individual bands were identified at low temperature. Three different groups can be distinguished: (1) Most of the OH bands between 3,300 and 3,650 cm?1 display a small frequency lowering (<4 cm?1) and a moderate broadening (<10 cm?1) as temperature is increased from ?194 to 200 °C. The behaviour of these bands is compatible with weakly H-bonded OH groups associated with hydrogen substitution into silicon tetrahedra; (2) In the same frequency range, two bands at 3,617 and 3,566 cm?1 display a significantly anharmonic behaviour with stronger frequency lowering (42 and 27 cm?1 respectively) and broadening (~30 cm?1) with increasing temperature. It is tentatively proposed that the defects responsible for these OH bands correspond to H atoms in interstitial position; (3) In the frequency region between 3,300 and 3,000 cm?1, three broad bands are identified at 3,151, 3,178 and 3,217 cm?1, at ?194 °C. They exhibit significant frequency increase (~20 cm?1) and broadening (~70 cm?1) with increasing temperature, indicating moderate H bonding. These bands are compatible with (2H)Mg defects. A survey of published spectra of forsterite samples synthesized above 5 GPa shows that about 75 % of the incorporated hydrogen belongs to type (1) OH bands associated with Si substitution and 25 % to the broad band at 3,566 cm?1 (type (2); 3,550 cm?1 at room temperature). The contribution of OH bands of type (3), associated to (2H)Mg defects, is negligible. Therefore, solubility of hydrogen in forsterite (and natural olivine compositions) cannot be described by a single solubility law, but by the combination of at least two laws, with different activation volumes and water fugacity exponents. |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ | |