Abstract
High pressure and temperature experiments were carried out on
bridgmanite under the hydrous shallow lower mantle conditions (24 – 25
GPa and 1673 – 1873 K with 5 – 10 wt. % of water in the starting
material). Bridgmanite investigated include MgSiO3, (Mg,
Fe)SiO3, (Mg, Al, Si)O3 and (Mg, Fe, Al,
Si)O3. Melting was observed in all runs. The water
content of the melt was estimated from the measurements of the void
space using the equation of state of H2O that occupies
the void space and agree reasonably well with the initial water content.
Melt is enriched in FeO/MgO and
Al2O3/SiO2 compared to
the starting materials. As a consequence, the residual solids contain a
substantial amount of stishovite particularly for the high FeO/MgO and
Al2O3/SiO2 starting
materials. Stishovite-rich materials will cause seismic scattering when
they are transported to ~1100 km depth where stishovite
shows anomalous elastic behavior. The water content in the residual
solids was measured by the FTIR and is 50 – 70 ppm wt. in bridgmanite
and 26 – 670 ppm wt. in stishovite, depending on the starting
composition. However, bridgmanites in these samples contain inclusions
of superhydrous phase B. If these inclusions were formed during cooling,
water content in bridgmanite co-existing with hydrous melt would be
1000-1500 wt ppm that agrees with the previous experimental results on
inclusion-free bridgmanite and with a theoretical study. Some
implications of these results are discussed including the nature of
chemical evolution associated with melting in the shallow lower mantle.