14:45 〜 15:00
[SIT19-05] Density of subducted terrigenous sediments in the lower mantle
キーワード：堆積物、リサイクル、高圧、密度
The subduction of continental crustal materials makes chemical heterogeneities in the mantle. Indeed, studies on ocean island basalts showed the presence of subducted terrigenous sediments in mantle plumes. However, there are few clues to understand the deep mantle processes that subducted sediments undergo before they are recycled to the surface.
Here we conducted high-pressure experiments using a diamond-anvil cell apparatus to investigate the phase relationships and density of a continental crustal material up to 129 GPa and 2600 K, which corresponds to lowermost mantle conditions. It crystallized into a four-phase assemblage of stishovite + Ca-perovskite + calcium ferrite-type aluminous phase (CF phase) + K-hollandite II at conditions in the upper part of the lower mantle. The K-hollandite II phase disappeared around 60 GPa, and alternatively, the new aluminous phase (NAL phase) appeared as a host of potassium at greater pressures. Furthermore, we observed the assemblage of SiO2 seifertite + Ca-perovskite + CF phase + NAL phase at conditions near the bottom of the mantle. The chemical compositions of Ca-perovskite and the CF phase changed significantly with increasing pressure. In particular, both the Na2O and K2O contents were remarkably enhanced at deep lower mantle conditions.
The density of subducted terrigenous sediments is lower than the surrounding mantle at depths from the top to the middle of the lower mantle and then becomes comparable to that at greater depths due to the change in the mineral assemblage described above. It suggests that the subducted sediments can be trapped around 660 km depth because of the neutral buoyance, along with the underlying MORB materials in slabs (Feng et al., 2021). In the deep lower mantle, unlike the MORBs, sedimental materials become gradually less dense than the surrounding mantle because of the enrichment of SiO2 phases that are less compressible than normal lower mantle minerals. The behavior of subducted sediments may be different from that of MORB materials in the lowermost mantle.
Here we conducted high-pressure experiments using a diamond-anvil cell apparatus to investigate the phase relationships and density of a continental crustal material up to 129 GPa and 2600 K, which corresponds to lowermost mantle conditions. It crystallized into a four-phase assemblage of stishovite + Ca-perovskite + calcium ferrite-type aluminous phase (CF phase) + K-hollandite II at conditions in the upper part of the lower mantle. The K-hollandite II phase disappeared around 60 GPa, and alternatively, the new aluminous phase (NAL phase) appeared as a host of potassium at greater pressures. Furthermore, we observed the assemblage of SiO2 seifertite + Ca-perovskite + CF phase + NAL phase at conditions near the bottom of the mantle. The chemical compositions of Ca-perovskite and the CF phase changed significantly with increasing pressure. In particular, both the Na2O and K2O contents were remarkably enhanced at deep lower mantle conditions.
The density of subducted terrigenous sediments is lower than the surrounding mantle at depths from the top to the middle of the lower mantle and then becomes comparable to that at greater depths due to the change in the mineral assemblage described above. It suggests that the subducted sediments can be trapped around 660 km depth because of the neutral buoyance, along with the underlying MORB materials in slabs (Feng et al., 2021). In the deep lower mantle, unlike the MORBs, sedimental materials become gradually less dense than the surrounding mantle because of the enrichment of SiO2 phases that are less compressible than normal lower mantle minerals. The behavior of subducted sediments may be different from that of MORB materials in the lowermost mantle.