Does Earth's mantle hold the key to keeping our planet hospitable for life?

[wolram]

http://news.yahoo.com/s/livescience/20071002/sc_livescience/hugestockpileofoxygenfounddeepinsideearth;_ylt=AlP5Y3bcO_CrB7jCXyUGvUkPLBIF

A mineral that acts like a sponge beneath Earth's surface stores more oxygen than expected, keeping our planet from becoming dry and inhospitable like Mars.



The key to the abundant oxygen storage is the mineral majorite, which exists deep below Earth's surface in the mantle. Without the oxygen stockpile, Earth would probably be a barren planet hostile to life, authors of a study suggest in the Sept. 27 issue of the journal Nature.

 

[billiards]

Yes, oxygen is abundant in the mantle, take the upper mantle, composed of 60% olivine (Mg, Fe)₂SiO₄, 18% orthopyroxene (Mg, Fe) SiO₃, 12% garnet (e.g. majorite) (Ca, Mg, Fe)₃Al₂Si₃O₁₂, with about 10% clinopyroxene Ca(Mg, Fe)Si₂O₆. There is a lot of oxygen there. Interesting that they pick out majorite, which I believe (from memory) forms under transition zone conditions - it's probably the most dominant type of garnet along with akimotoite - I'd like to know more about the significance of that. With regards to the water forming hypothesis, I was under the impression that hydrous volatiles were present in the transition zone, perhaps so much so that the rocks there were oversaturated, leading to partial melt (e.g. Bercovici and Kerato). So I would hesitate to accept that the "decomposition" of majorite was releasing oxygen which went on to join with hydrogen to form water. There already seems to be a lot of water in the upper mantle much of which can be accounted for by subducting slabs bringing it down with them. Majorite cannot exist in the lower mantle, it undergoes a phase transition into perovskite under the increased pressure and temperature. So if anything, majorite is just a weird stop gap in the geological water cycle in a very abstract way.