11:15 AM - 11:30 AM
[SIT15-08] Seismic reflectors, scatterers, and water carriers into the lower mantle
Keywords:Seismic reflector, Seismic scatterer, DHMS, NAM, CaCl2-type SiO2 phase, Davemaoite
Seismological studies revealed existence of seismic scatterers and reflectors near subduction zones at depths 700-1900 km (e.g., 1, 2). Niu et al. (2) reported physical properties of reflectors in the upper part of the lower mantle beneath the Mariana subduction zone. Kaneshima (3) reported seismic scatterers at various depths of the lower mantle, and considered that the seismic reflectors and scatterers has been considered to have the same origin.
The seismic scatterers and reflectors may be explained by accumulation of a fluid/melt in the slabs dehydrated from hydrous minerals. DHMS phases such as hydrous phase D and superhydrous phase B dehydrate at pressures around 30-50 GPa. Therefore, Ohtani and Litasov (4) suggested the physical properties of reflectors might be explained by the oceanic crust layer containing fluids supplied by dehydration of these hydrous phases in the hydrous peridotite layer of the slab. There might be several additional explanations for the seismic scatterers and reflectors. Theoretical studies revealed that the seismic scatterers can be explained by the existence of elastic anomaly associated with the second order phase transformation from stishovite to post-stishovite (CaCl2-typeSiO2 phase) (5). Recent experiments by Zhang et al. (6) confirmed that the stishovite-CaCl2 type SiO2 phase transformation shows remarkable reduction of VS due to the shear softening at the phase transformation boundary. Thus, this phase transformation can cause the seismic scatterers. Aluminous CaCl2-type SiO2 phase contains a significant amount of water (7). Variation of the stishovite-CaCl2 type phase transformation pressure in hydrous aluminous silica systems can explain seismic scatterers occurring in a wide depth range of 700-1900 km corresponding to 30-80 GPa because this transformation pressure depends on alumina and water contents (6, 8).
Davemaoite (perovskite-type CaSiO3) also can cause the seismic scatterers in the lower mantle. Davemaoite has tetragonal and cubic structures below and above ~500 K, respectively (9). Kurashina et al. (10) reported that davemaoite can accommodate alumina, crystallizing into the orthorhombic phase below ~1800-2000 K in the lower mantle. They suggested the elastic anomalies of reduction of VS associated with the second order phase transformations in dry aluminous davemaoite from the orthorhombic to cubic phase. On the other hand, Al-free davemaoite CaSiO3 can dissolve significant amounts of water, crystallizing into the tetragonal phase even at high temperatures, and showing no tetragonal-cubic transition up to 2600 K (11). Whereas, davemaoite coexisting with d-AlOOH in an alumina bearing hydrous system showed alumina depletion (11). Therefore, davemaoite coexisting with d-AlOOH under water under-saturated conditions could be dry based on similarity in water partitioning between NAM and DHMS although its water content is not yet measured. Thus, davemaoite may cause the seismic scatterers both in the dry and wet lower mantle.
References:
1. Kaneshima and Helffrich (1999) Science 283:1888–1891; 2. Niu et al. (2003) J Geophys Res 108:2419; 3. Kaneshima (2016) Phys Earth Plane Inter 257:105–114; 4. Ohtani and Litasov (2006) Rev Mineral Geochem 62:397–420; 5. Stixrude (1998) In: Core-Mantle Boundary Region. Geodynamics Series, Gurnis M, Wysession ME, Knittle E, Biffet BA (eds) Am Geophys Uni:p 83-96; 6. Zhang et al. (2022) J Geophys Res Solid Earth, 127: e2021JB023170; 7. Ishii et al., 2022b Proc Nat Acad Sci 119:e2211243119; 8. Criniti et al. 2023 Am Mineral, 108(8):1558-1568; 9. Komabayashi et al. (2007) Earth Planet Sci Let 260(3-4):564-569; 10. Kurashina et al. (2004) Phys Earth Planet Inter 145:67–74; 11. Chen et al. (2020) Phys Earth Planet Inter, 299:1064.
The seismic scatterers and reflectors may be explained by accumulation of a fluid/melt in the slabs dehydrated from hydrous minerals. DHMS phases such as hydrous phase D and superhydrous phase B dehydrate at pressures around 30-50 GPa. Therefore, Ohtani and Litasov (4) suggested the physical properties of reflectors might be explained by the oceanic crust layer containing fluids supplied by dehydration of these hydrous phases in the hydrous peridotite layer of the slab. There might be several additional explanations for the seismic scatterers and reflectors. Theoretical studies revealed that the seismic scatterers can be explained by the existence of elastic anomaly associated with the second order phase transformation from stishovite to post-stishovite (CaCl2-typeSiO2 phase) (5). Recent experiments by Zhang et al. (6) confirmed that the stishovite-CaCl2 type SiO2 phase transformation shows remarkable reduction of VS due to the shear softening at the phase transformation boundary. Thus, this phase transformation can cause the seismic scatterers. Aluminous CaCl2-type SiO2 phase contains a significant amount of water (7). Variation of the stishovite-CaCl2 type phase transformation pressure in hydrous aluminous silica systems can explain seismic scatterers occurring in a wide depth range of 700-1900 km corresponding to 30-80 GPa because this transformation pressure depends on alumina and water contents (6, 8).
Davemaoite (perovskite-type CaSiO3) also can cause the seismic scatterers in the lower mantle. Davemaoite has tetragonal and cubic structures below and above ~500 K, respectively (9). Kurashina et al. (10) reported that davemaoite can accommodate alumina, crystallizing into the orthorhombic phase below ~1800-2000 K in the lower mantle. They suggested the elastic anomalies of reduction of VS associated with the second order phase transformations in dry aluminous davemaoite from the orthorhombic to cubic phase. On the other hand, Al-free davemaoite CaSiO3 can dissolve significant amounts of water, crystallizing into the tetragonal phase even at high temperatures, and showing no tetragonal-cubic transition up to 2600 K (11). Whereas, davemaoite coexisting with d-AlOOH in an alumina bearing hydrous system showed alumina depletion (11). Therefore, davemaoite coexisting with d-AlOOH under water under-saturated conditions could be dry based on similarity in water partitioning between NAM and DHMS although its water content is not yet measured. Thus, davemaoite may cause the seismic scatterers both in the dry and wet lower mantle.
References:
1. Kaneshima and Helffrich (1999) Science 283:1888–1891; 2. Niu et al. (2003) J Geophys Res 108:2419; 3. Kaneshima (2016) Phys Earth Plane Inter 257:105–114; 4. Ohtani and Litasov (2006) Rev Mineral Geochem 62:397–420; 5. Stixrude (1998) In: Core-Mantle Boundary Region. Geodynamics Series, Gurnis M, Wysession ME, Knittle E, Biffet BA (eds) Am Geophys Uni:p 83-96; 6. Zhang et al. (2022) J Geophys Res Solid Earth, 127: e2021JB023170; 7. Ishii et al., 2022b Proc Nat Acad Sci 119:e2211243119; 8. Criniti et al. 2023 Am Mineral, 108(8):1558-1568; 9. Komabayashi et al. (2007) Earth Planet Sci Let 260(3-4):564-569; 10. Kurashina et al. (2004) Phys Earth Planet Inter 145:67–74; 11. Chen et al. (2020) Phys Earth Planet Inter, 299:1064.