5-18. Study on the nature of crustal fluids
Determining the elastic properties of fluid-bearing mantle
rocks at high pressure and temperature is important for interpreting seismic
velocity structure in terms of fluid fraction and temperature, and
understanding the processes governing earthquake generation and magma
migration, specially in subduction zones, where volatiles may exist in the
mantle wedge. A sharp velocity decrease has commonly been observed upon the
dehydration of rocks containing hydrous minerals.
By using Grüneisen theory and third-order finite strain
theory, elastic-wave velocities in olivine-pyroxene assemblages are accurately
determined as a function of pressure and temperature. Equilibrium distributions
of H2O fluid in mantle rocks have been determined at high pressure
and temperature, and are used to estimate Vp and Vs in the olivine-pyroxene-H2O
system. Importantly, velocities calculated for the peridotite-H2O
system successively reproduce the absolute Vp and Vs values measured in the
laboratory for dehydrated serpentinite. Known equilibrium distributions of H2O-CO2
fluid in peridotite-liquid systems allow one to estimate elastic properties of
fluid-bearing mantle rocks from mineral and liquid elasticity data.
Figure 1. Vs-Vp/Vs diagram in the serpentinites at 1 GPa (circles: 891a,
diamonds: 891b). Note the dramatic changes in the data trend after mineral
dehydration reactions R1 to R3. Before dehydration, Vp/Vs increases with
increasing temperature; however, it rapidly decreases as dehydration proceeds.
Vp/Vs is larger and Vs is smaller in 891a of higher H2O content than
891b (thick arrow).
Figure 2. Vs-Vp/Vs diagram in the serpentinites 891a and 891b (measured, same as
Figure 1) and in the olivine-pyroxene-H2O mixtures (arrows) at 1
GPa. The large circle and diamond are Vs-Vp/Vs in the dry olivine-pyroxene
solids of 891a and 891b, respectively, at 1 GPa and ~900 oC.
Velocities measured in the laboratory are reproduced by numerical calculations
for H2O tube or spheroid distributions. The cross section of tube is
also shown with tube-shape parameter, k.
References
Sato, H. and K. Ito (2001) H2O fluid distribution in mantle rock at 1 GPa: constraints from Vs-Vp/Vs diagram, Bull. Earthq. Res. Inst., Univ. Tokyo, 76, 305-310.
Sato, H. and K. Ito (2002) Olivine-pyroxene-H2O system as a practical analogue for estimating the elastic properties of fluid-bearing mantle rocks at high pressures and temperatures, Geophys. Res. Lett., 29, 39-1-39-4.