O Water in Olivine, Where Art Thou?
Abstract
A clear understanding of the mechanisms of incorporation of hydrogen ions in olivine is essential for application of laboratory observations of the effect of water on physical properties to processes occurring in Earth's upper mantle. Even though hydrogen ions profoundly influence the physical and chemical behavior of olivine, the primary site for their incorporation within the crystal structure remains a topic of vigorous debate. As the maximum solubility of H+ ions in olivine is ~5 H per 100 Si, either (i) a H+ ion associated with a 3+ cation substituting for a Si4+ ion or (ii) two H+ ions associated with a Si4+ vacancy coupled with a Ti4+ ion substituting for a Mg2+ ion cannot by themselves be the primary sites for protons in olivine. By contrast, the rapid diffusivity and potentially high concentration of vacancies on the cation sublattices suggest that Si4+ and/or Mg2+/Fe2+ vacancies could charge compensate large concentrations of H+ ions. Argument remains, however, as to the identity of the dominant charge-compensating defect -- Si4+ vacancies versus Mg2+/Fe2+ vacancies. One the one hand, first-principle calculations of the lowest energy configuration favor four H+ ions associated with a Si4+ vacancy. The O-H stretching frequencies predicted by such calculations are in good agreement with those observed in infrared spectra. One the other hand, the experimentally determined linear dependence of hydrogen solubility on water fugacity argues for a defect formed from two H+ ions associated with a Mg2+/Fe2+ vacancy as the dominant defect facilitating hydrogen incorporation in olivine. This model is consistent with the measured change in molar volume determined for the addition of hydrogen to olivine. It is also supported by the much higher concentration of Mg2+/Fe2+ vacancies relative to the concentration of Si4+ vacancies inferred from the observed orders of magnitude faster diffusion of magnesium and iron ions than silicon ions. Our recent experimental determination of increasing hydrogen solubility with increasing silica activity further supports the importance of the defect associates formed between H+ ions and Mg2+ vacancies.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMMR22A..02K
- Keywords:
-
- 3904 MINERAL PHYSICS / Defects