SEARCHING FOR EARTH’S LOST OCEANS: AN EXPEDITION IN MINERAL CRYSTAL CHEMISTRY

Starting with his work on pectolite in the early 1960’s, one of Prof. Prewitt’s long-term interests has been the crystal chemistry of H incorporation in silicates. These interests have led to new insights on how the planet balances hydrogen between surface and interior reservoirs.

The oceans cover more that 70% of the surface area but compose only 0.025% of the mass. The nominally anhydrous silicate phases thought to compose the upper 660 km of Earth can incorporate more than ten times this much H. Hydrogen is thus the most poorly constrained compositional variable in the Earth. In order to evaluate the possibility of there being a very large reservoir of H in the interior we have conducted experiments to measure the effect of H incorporation on the physical properties of these minerals. At depths less than 410 km, olivine can incorporate 2000 ppm, and possibly as much as 8000 ppm, H2O by weight at pressures above 10 GPa. Clinopyroxene can incorporate a similar amount of H as hydroxyl, and petrologic evidence in natural high-pressure samples suggests it may incorporate 5000 ppm or more. In the subducting basaltic slab after all hydrous phases break down, the pyroxene can carry down 0.1 to 0.2 H2O by weight of the slab. If the ultramafic portion below the crustal portion becomes hydrated, the olivine can carry as much or more water into the interior.

Using single crystal X-ray diffraction, we have measured the effect of hydration on compression of ringwoodite to 12 GPa. Using powder diffraction and synchrotron radiation we have measured compression to 50 GPa. Using GHz ultrasonic measurements of P and S-wave travel times, we observe a reduction of P-wave velocity equivalent to an increase in temperature of 600ºC and on S-wave velocity of 1000ºC. Throughout most of the TZ, hydration has a much larger effect on velocity than does temperature within reasonable ranges of these parameters. In tomographic images of the TZ in regions distant from active subduction, red is more likely to mean ‘wet’ than it is to mean ‘hot’. Observed seismic velocities in the Transition Zone are consistent with a pyrolite composition with 0.5 to 1.0 percent by weight H2O, but not consistent with dry pyrolite compositions. This would allow for Transition Zone storage of two to three times the amount of water currently in the hydrosphere.