CH12 - Structural Phase Transitions Flashcards
(10 cards)
What could the cause of discontinuities be?
Abrupt changes in composition (chemically stratified) or a uniform upper and lower mantle composition with high pressure phase transitions in the constituent minerals
Upper mantle discontinuity due to?
Transformation from open olivine to denser polymorph with spinel structure
Topotactic meaning?
A solid state reaction such that the orientations of the product crystals are determined by the orientation of the inner crystal
what are the 2 transformation mechanisms for olivine to spinel in the mantle?
Nucleation and growth, or Martensitic
Why does nucleation and growth not occur below 700 degrees in the mantle?
If a subducted slab is cold, the activation energy may be sufficiently large to inhibit the transition until higher pressures
What happens if a slab is rapidly subducted?
It will be heated rapidly to above the cut-off transition temperature. Olivine is in a highly metastable state however will explosively transform to spinel. Change in free energy is so great that deep focus earthquakes can form
However if this takes place by Martensitic transformation, Ea may be too small for metastability, hence transformation to spinel can take place at lower temperature
Can we find mantle rocks at the earth’s surface?
xenoliths (down to 100km), ophiolites (few km below Moho), orogenic peridotites (mantle exposures in crust, no oceanic crust), diamonds (can contain mantle minerals)
Types of solid solution?
Omission, interstitial and substitutional
Why do some minerals break down completely while others undergo solid state replacements?
Solid state replacements depend on solid solution rules (radii, charge, bonding), as well as preservation potential. Those that break down are completely different in chemical comp and structure.
Why are depth marker minerals always those that have undergone reactive transformations?
Because reactive transformations are complicated and energetically unfavourable, thus irreversible. Reconstructive reactions are not as kinetically hindered, proceeding as dislocation reactions, and can transform to different phases at standard pressure and temp, hence not useful as a depth marker.
