10 research outputs found
Synthesis and characterization of tungsten-incorporated mesoporous molecular sieve MCM-48 by one step
Serpentine Dehydration Curves and their Bearing on Serpentinite Deformation in Orogenesis
Experimental Investigation on the Mechanism of Olivine → Spinel Transformation: Growth of Single Crystal Spinel from Single Crystal Olivine in Ni2SiO4
Thermal study on decomposition of LiBH4 at non-isothermal and non-equilibrium conditions
Brightness temperature of water compressed by a double shock to pressures of 60–79 GPa
Temperatures Induced by Shock Waves in Minerals: Applications to Geophysics
The temperatures of initially transparent minerals of geophysical interest are measured
using a six-channel optical pyrometry system operating over the range 450 to 790 nm. The
radiative temperatures and emissivity of minerals are measured by recording spectral
radiances versus time during the time interval that an intense shock wave is driven through
the sample. The shock wave is induced by the impact of a projectile accelerated by a two-stage light-gas gun. Taken together with the pressure-density Hugoniot data, complete
pressure-density-temperature equations of state may be constructed over the entire pressure
range present within the earth. Shock temperature and shock pressure data for NaCl
extending to 1,040 kbar (104 GPa) demonstrate that a transition from the B1 to B2 phase
occurs below 300 kbar (with a phase transition energy of ~0.2 MJ/kg) and melting of the B2
phase occurs above 550 kbar. Shock temperatures for the high-pressure phase assemblage
of Mg_2SiO_4, believed to be MgO (periclase) and MgSiO_3 (perovskite), are closely matched
by theoretical calculations that assume a phase transition energy from olivine to this
assemblage of ~1.5 MJ/kg. Shock temperature data for α
-quartz and fused quartz
shocked into the stishovite regime display dramatic decreases in shock temperatures at
~700 and ~1,050 kbar, which are interpreted as representing shock-induced melting of
stishovite. The observed data can be fit theoretically by assuming that stishovite is driven
into the super-heated regime ~1,000 K above the melting point and melts suddenly to a
temperature of 4,400 K (at 700 kbar) with a latent heat of melting of 3.5 MJ/kg. Assuming
that SiO_2 stishovite is a component in a ternary MgO-SiO_2-FeO mantle and taking into
account the expected decrease in the solidus of this system relative to the oxides, the
minimum melting point obtained implies a maximum lower mantle temperature of
3,500 K. The slight increase in the melting point of stishovite may be used in conjunction
with a Weertman-type relation between homologous temperature and creep viscosity to
estimate the effect of pressure on viscosity. Such an analysis for SiO_2 suggests an activation
volume for the lower mantle of the earth of ~1 to 4 cm^3 /mole for Mg_2SiO_4. This value is a
factor of 2 to 4 less than inferred from measurements of the activation volume of an upper
mantle mineral such as olivine. This small activation volume implies a maximum increase
of viscosity with depth in the lower mantle of a factor of between ~1 to ~10^4 depending on
the assumed rheological model, activation energy, and the temperature. Whereas a slight
increase with depth of viscosity in the earth's lower mantle as compared with the upper
mantle supports theories of convection throughout the mantle, an increase in viscosity by a
factor of 10^4 probably precludes single-cell, mantle-wide convection