Pressure dependence of hydroxyl solubility in coesite |
| |
Authors: | J L Mosenfelder |
| |
Institution: | Bayerisches Geoinstitut, Universit?t Bayreuth, 95440 Bayreuth, Germany, DE
|
| |
Abstract: | The solubility of hydroxyl in coesite was investigated in multianvil experiments performed at 1200 °C over the nominal pressure
range 5–10 GPa, at an f
O2 close to the Ni-NiO buffer. The starting material for each experiment was a cylinder of pure silica glass plus talc, which
dehydrates at high P and T to provide a source of water and hydrogen (plus enstatite and excess SiO2). Fourier-transform infrared (FTIR) spectra of the recovered coesite crystals show five sharp bands at 3606, 3573, 3523,
3459, and 3299 cm−1, indicative of structurally bonded hydrogen (hydroxyl). The concentration of hydrogen increases with pressure from 285 H/106 Si (at 5 GPa) to 1415 H/106 Si (at 10 GPa). Assuming a model of incorporation by (4H)Si defects, the data are fit well by the equation C
OH=Af
2
H2<\INF>Oexp(−PΔV/RT), with A=4.38 H/106 Si/GPa, and ΔV=20.6 × 10−6 m3 mol−1. An alternative model entailing association of hydrogen with cation substitution can also be used to fit the data. These
results show that the solubility of hydroxyl in coesite is approximately an order of magnitude lower than in olivines and
pyroxenes, but comparable to that in pyropic garnet. However, FTIR investigations on a variety of ultrahigh pressure metamorphic
rocks have failed in all cases to detect the presence of water or hydrogen in coesite, indicating either that it grew in dry
environments or lost its hydrogen during partial transformation to quartz. On the other hand, micro-FTIR investigations of
quartz crystals replacing coesite show that they contain varying amounts of H2O. These results support the hypothesis that preservation of coesite is not necessarily linked to fast exhumation rates but
is crucially dependent on limited fluid infiltration during exhumation.
Received: 23 August 1999 / Accepted: 10 April 2000 |
| |
Keywords: | Coesite Stishovite Multianvil OH solubility Infrared spectroscopy |
本文献已被 SpringerLink 等数据库收录! |
|