Charles Meade

Deep-Focus Earthquakes and Recycling of Water into the Earth's Mantle

For more than 50 years, observations of earthquakes to depths of 100 to 650 kilometers inside Earth have been enigmatic: at these depths, rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces. Laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep-focus seismicity could originate from dehydration and high-pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle. Thus, seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet.

Synchrotron Infrared Absorbance Measurements of Hydrogen in MgSiO3 Perovskite

Micro-infrared spectroscopic measurements on single crystals of MgSiO3 perovskite document two pleochroic hydroxyl absorbance peaks at 3483 and 3423 centimeter–1. These measurements were obtained with the use of a synchrotron infrared source for spectroscopy. These data are consistent with a trace hydrogen content of 700 � 170 hydrogen atoms per 106 silicon atoms in the nominally anhydrous MgSiO3 perovskite. When integrated over the volume of the lower mantle, this concentration is comparable to 12 percent of the mass of hydrogen in the Earths hydrosphere.

Effect of a Coordination Change on the Strength of Amorphous SiO2

Measurements of the yield strength of SiO2 glass to pressures as high as 81 gigapascals at room temperature show that the strength of amorphous silica decreases significantly as it is compressed to denser strctures with higher coordination. Above 27 gigapascals, as the silicon in amorphous SiO2 is continuously transformed from fourfold to sixfold coordination, the strength of the glass decrases by more than an order of magnitude. These data confirm theoretical predictions that the mechanical properties of polymerized amorphous silicates are sensitive to pressure-induced structural transformations and suggest that the viscosity of silica-rich liquids decreases significantly at high pressures. Such a change in melt rheology could enhance the processes of chemical differentiation with depth in the Earths mantle.

High precision powder x‐ray diffraction measurements at high pressures

We describe a new technique for averaging two‐dimensional powder diffraction patterns to form a single profile of the diffracted intensity at different values of the scattering angle 2θ, I(2θ). This approach significantly improves the precision with which d spacings and diffracted intensity can be measured relative to conventional techniques of powder diffraction at high pressures. By analyzing the diffraction patterns from samples inside the high‐pressure diamond cell we find that scattering angles, 2θ, can be determined with an uncertainty of ±0.0068° and the relative intensities of different (hkl) diffraction lines are obtained to within ±0.9%. Combining the measurements of 2θ and intensity we have determined the equation of state and the Debye–Waller (B) factor of gold to 15.4 GPa. These are the highest pressures to which B has been measured to date, and we find that the results are in good agreement with thermodynamic measurements at low pressures.

Ultrahigh-pressure melting of lead - A multidisciplinary study

Measurements of the melting temperature of lead, carried out to pressures of 1 megabar (1011 pascal) and temperatures near 4000 kelvin by means of a laser-heated diamond cell, are in excellent agreement with the results of previous shock-wave experiments. The data are analyzed by means of first principles quantum mechanical calculations, and the agreement documents the reliability of current experimental and theoretical techniques for studies of melting at ultrahigh pressures. These studies have potentially wide-ranging applications, from planetary science to condensed matter physics.