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Are Earth’s Oceans Just the Tip of the Iceberg?
Joseph R. Smyth,
University of Colorado, Boulder
Steven D. Jacobsen,
Carnegie Institution of Washington
(work at BGI supported by Alexander von Humboldt Foundation)
GHz Ultrasonic Interferometry
Ringwoodite (blue) with quartz and
ruby in the diamond anvil cell
Earth’s oceans cover 72 percent of the planet’s surface but constitute only
0.025 percent of its mass. It is possible that deep reservoirs of water
incorporated as hydroxyl into solid silicate minerals of the Earth’s interior
contain the majority of the planet’s hydrogen and have acted as buffers to
maintain ocean volume and continental freeboard over geologic time. One
tenth of one weight percent H2O in subducted oceanic crustal material and
subsequently released to the hydrosphere from mid-ocean ridge basalt is
sufficient to recycle the total ocean volume once over 4.5 billion years. It is
possible that actual fluxes are several times this amount. The nominally
anhydrous minerals of the transition zone (410-660 km depth) may serve
as a large internal reservoir. Seismic velocities in this region are consistent
with one percent H2O by weight in a pyrolite-composition mantle, but not
with anhydrous pyrolite.
New data from ultrasonic measurements in the diamond-anvil cell indicate
that hydration has a larger effect on velocity than temperature within the
uncertainties of each. Lateral velocity variations in the transition zone are
therefore more likely to reflect variations in hydration than variations in
temperature, at least in regions distant from subduction zones.
Quantifying the effects of hydration in the laboratory. The elastic tensor of a
hydrous (1 wt% H2O) Fo90 ringwoodite was measured ultrasonically. Variation of
the isotropic equivalent bulk (Ks) and shear (G) moduli are plotted against
pressure and compared to a nominally anhydrous dataset measured by Brillouin
scattering (Sinogeikin et al. 2001) (left). The new data for hydrous ringwoodite
were made possible by a recent advance in high-frequency (GHz) acoustic
interferometry for the diamond-anvil cell (right). Shear waves are produced by Pto-S conversion, and delivered to the sample through one of the diamond anvils
(Jacobsen et al. 2004).
The measurements yield (linearly fitted) adiabatic bulk modulus (Ks) of 174 ± 1
GPa and dK/dP of 4.9 ± 0.1 and a shear modulus (G) of 103 ± 1 GPa with dG/dP
of 1.8 ± 0.1. Relative to anhydrous ringwoodite, this represents a dramatic
decrease in both bulk and shear moduli and a slight increase in the pressure
derivatives (Fig 2), consistent with static compression measurements (Smyth et al
2004).
Dry and Wet P and S Velocities
Synthesis experiments were conducted in the
5000-ton multi-anvil press at BGI. This facility is
capable of producing single crystals of sufficient
size for GHz ultrasonic interferometry experiments in the diamond anvil cell.
The effect of water on seismic velocities in Earth’s Transition Zone, 410660 km depth. (A) Plot of compressional velocities (Vp) and (B) shear
velocities (Vs) versus depth for hydrous (~1%) ringwoodite and wadsleyite
(blue), and anhydrous (red) ringwoodite wadsleyite, olivine (green) and
PREM (Masters and Shearer, 1995). The figure illustrates that hydrous
ringwoodite and wadsleyite are much closer to PREM than anhydrous
phases.
Conclusions
The observation of the very strong effect of hydration on P and S velocities in ringwoodite, and inferred
from isothermal compression of wadsleyite, indicate that hydration is likely to have a larger effect on
seismic velocities than temperature within the uncertainties in each in the Transition Zone. This means
that tomographic images of the transition zone in regions distant from active subduction, blue coloration is
more likely to indicate dry than cool, and red more likely to indicate wet, than hot. Seismic data are
consistent with a transition zone hydration between 0.5 and 1.5 weight percent H2O for a pyrolitecomposition model.