HR: 08:25h
AN: V51B-02 INVITED     [Abstracts]
TI: Dehydration Embrittlement and the H2O Content of Subducting Lithosphere
AU: * Green, H W
EM: harry.green@ucr.edu
AF: Department of Earth Sciences and Institute of Geophysics and Planetary Physics, University of California, Riverside, CA 92521, United States
AB: Dehydration embrittlement has been demonstrated in a variety of hydrous phases and is strongly implicated as the trigger mechanism of intermediate-depth earthquakes (70-300km). Antigorite serpentine is capable of initiating such a shearing instability during dehydration at pressures from 0.1 to 6 GPa in the laboratory, a range over which the volume change accompanying dehydration changes from positive to negative, yet the shearing instability occurs under all conditions. Similarly, exsolution of very small quantities of H2O from nominally anhydrous phases can also trigger instability in the laboratory. It is thus highly likely that dehydration under stress of any reasonably abundant phase in subducting lithosphere would trigger earthquakes. Here I will use this logic to argue that subducting lithosphere is progressively "wrung dry" over the depth interval 100-400 km and that only very small amounts of H2O can exist in such lithosphere below that depth. The evidence is the following: (1) Earthquake frequency declines exponentially between 100 and 300 km (suggesting that the cause of the instability is being exhausted); (2) the resurgence of earthquakes in the transition zone could, in principle, be triggered by dehydration of dense hydrous magnesium silicates, the "alphabet phases", but the continuous production of earthquakes with a maximum at ~ 600 km and the sudden termination before 700 km is inconsistent with the conditions under which the "alphabet phases" exhibit mineral reactions (indicating that this mechanism is unlikely to be involved in deep earthquakes); (3) the pressure/temperature paths followed in subduction zones is such that each of the "alphabet phases" is replaced by another that has a higher H2O content (hence release of free H2O is highly unlikely); (4) the "alphabet phases" are only stable if the highly abundant phases wadsleyite and/or ringwoodite are fully saturated with H2O, which would require so much water that the lithosphere would have been so buoyant above 400 km that it would not have entered into the transition zone (hence saturation is extremely unlikely); (5) if significant H2O is present in ringwoodite, wherever lithosphere passes through into the lower mantle there should be an abundance of earthquakes as that H2O is released during ringwoodite breakdown (the lack of such earthquakes implies that even small amounts of H2O in ringwoodite are unlikely); (6) if H2O gets passed from one "alphabet phase" to another, there should be a flurry of earthquakes in the lower mantle during the dehydration of phase D, the last of these phases (such earthquakes are absent, strongly suggesting that phase D is also absent). The lack of earthquakes in these circumstances where they would be expected if H2O is significantly present implies that (i) dehydration embrittlement is at best a minor trigger of earthquakes in the mantle transition zone and (ii) subduction zones deeper than ~400 km are essentially dry. A corollary is that subduction does not significantly recycle H2O into the deep mantle, at least not at this time.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3630 Experimental mineralogy and petrology
DE: 5104 Fracture and flow
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly