5:15 PM - 6:30 PM
[SCG19-P07] Decomposition of hydrous phase D by the formation of Fe-bearing aluminous bridgmanite
Keywords:hydrous phase D, lower mantle, bridgmanite, subducting slab, water transportation, ferric iron
On the basis of this background, we conducted the following experiment using a Kawai-type multianvil press. A starting materials were used sintered Al-free and aluminous phase D pre-synthesized at 27 GPa and 1100 K, an natural olivine single crystal (Olv) and pyroxene aggregates with bulk composition of pyrolite minus fPc (Pyr-fPc) and (Mg0.9Fe0.1)(Si0.9Al0.1)O3 (FeAl-En) synthesized at 1300 K in a controlled oxygen fugacity (PO2≈IW) and ambient pressure. Pyroxene samples were packed in Mo capsule and sintered at 2 GPa and 1500 K. Sintered phase D was directly attached with Olv, sintered Pyr-fPc or FeAl-En in Au sample capsule. The samples were pressurized to 27 GPa and heated at 1100 K for 1 or 10 hours. Analyses of recovered samples were made with FE-SEM and ATEM.
No reaction between Al-free phase D and Brm + fPc aggregate formed from Olv was found in the SEM observation. The existence of 2-4 µm thick SiO2-rich fine-grained layers were discovered on the boundary between Al-free / aluminous phase D and Brm formed from Pyr-fPc or FeAl-En. TEM observation indicated amorphous phases with Brm composition in fine SiO2 stishovite. These observations strongly indicate that the decomposition of phase D accompanied by the formation of Fe-bearing aluminous Brm. The present result suggests that, although water can be transported into the top of the lower mantle due to the formation of phase D through subduction of slab, phase D will decompose by reduction at the depth to 720 km. The produced H2 should come back to the upper mantle. Thus, water cannot be transported through slab subduction to the majority of the lower mantle.