9:30 AM - 9:45 AM
[SIT17-03] Role of water in dynamics of slabs and surrounding mantle
Keywords:Water, Partitioning, NAMs, Hydrous minerals, Earthquakes, Seismic reflector
We showed the preferential partitioning of water into hydrous minerals such as hydrous phase A compared to nominally anhydrous minerals such as olivine, wadsleyite, and ringwoodite [1]. Our recent study revealed the similar partitioning behaviors also for the hydrous minerals such as superhydrous phase B, hydrous phase D), and hydrous phase d compared to coexisting bridgmanite [2] and stishovite [3], respectively. Water in the slabs is stored mainly in hydrous minerals, and coexisting nominally anhydrous minerals are essentially dry under the stability field of hydrous minerals.
Here we show the implications for our observations of the water partitioning behaviors. The cold core of the slab is composed of dry nominally anhydrous minerals and hydrous minerals. Therefore, the deformation and seismicity occurring in the cold core of the slabs, such as deep earthquakes (depths greater than 300 km), olivine metastable wedges, and formation of the stagnant slabs caused by large slab deformation were controlled essentially by the dry deformation and transformation dynamics of nominally anhydrous minerals.
The increase of the slab temperature from the upper and lower boundaries of the mantle generates fluids due to dehydration of the hydrous minerals. This dehydration process creates following phenomena in the mantle surrounding the slabs. Water (hydrolytic) weakening in olivine and wadslyite causes softening of the slabs and causes deformation of the slabs. It is well known that the dehydration of serpentine and chlorite can create intermediate earthquakes. Separation and ascent of fluid from the slabs create the deep mantle seismic swarms in the mantle wedge [4]. Fluid/melt separation and accumulation at the top of the lower mantle can create low velocity and low Q region [5]. The seismic reflectors showing density jump, vS decrease, and no vP jump in the upper part of the lower mantle [6] can be accounted for by dehydration of high-pressure hydrous minerals such as super hydrous phase B and hydrous phase D.
References:
[1] Ishii and Ohtani (2021), https://doi.org/10.1038/s41561-021-00756-7. [2] Ishii, Ohtani, Shatskiy (2022), Earth and Planetary Science Letters 583 (2022) 117441. [3] Ishii and Ohtani, this session abstract. [4] White et al. (2020), Earth and Planetary Science Letters 521 (2019) 25–36. [5] Schmandt et al. (2014), Science 344:1265–6. [6] Nie et al. (2003), J. Geophys. Res. 108:2419.
Here we show the implications for our observations of the water partitioning behaviors. The cold core of the slab is composed of dry nominally anhydrous minerals and hydrous minerals. Therefore, the deformation and seismicity occurring in the cold core of the slabs, such as deep earthquakes (depths greater than 300 km), olivine metastable wedges, and formation of the stagnant slabs caused by large slab deformation were controlled essentially by the dry deformation and transformation dynamics of nominally anhydrous minerals.
The increase of the slab temperature from the upper and lower boundaries of the mantle generates fluids due to dehydration of the hydrous minerals. This dehydration process creates following phenomena in the mantle surrounding the slabs. Water (hydrolytic) weakening in olivine and wadslyite causes softening of the slabs and causes deformation of the slabs. It is well known that the dehydration of serpentine and chlorite can create intermediate earthquakes. Separation and ascent of fluid from the slabs create the deep mantle seismic swarms in the mantle wedge [4]. Fluid/melt separation and accumulation at the top of the lower mantle can create low velocity and low Q region [5]. The seismic reflectors showing density jump, vS decrease, and no vP jump in the upper part of the lower mantle [6] can be accounted for by dehydration of high-pressure hydrous minerals such as super hydrous phase B and hydrous phase D.
References:
[1] Ishii and Ohtani (2021), https://doi.org/10.1038/s41561-021-00756-7. [2] Ishii, Ohtani, Shatskiy (2022), Earth and Planetary Science Letters 583 (2022) 117441. [3] Ishii and Ohtani, this session abstract. [4] White et al. (2020), Earth and Planetary Science Letters 521 (2019) 25–36. [5] Schmandt et al. (2014), Science 344:1265–6. [6] Nie et al. (2003), J. Geophys. Res. 108:2419.