Origin of Low-Velocity Zones in the Bottom Mantle Transition Zone
Abstract
The global discontinuities at about 410 km and 660 km have determined the upper and lower boundary of mantle transition zone. A number of seismic studies have identified the presence of low velocity zones (LVZs) with various scales at 570-600 km within the mantle transition zone. The observed shear wave velocity (VS) reduction ranges from -2.2% to -7.4%. To date, the origin of LVZs in the bottom transition zone remains controversial. They have been either explained by water from the sinking slabs or the accumulation of mid-ocean ridge basalt (MORB). Although wadsleyite and ringwoodite, the major mineral in the transition zone, have the ability to store large amount of water, but the water content in the transition zone seems to be lower than previous thought according to the conductivity measurements and seismology observation. Furthermore, previous experimental studies have shown that the VS of MORB along the mantle geotherm is ~1.9% lower than the pyrolitic mantle, which cannot fully explain the observed -2.2% to -7.4% low VS anomaly at the bottom transition zone. However, MORB could have complex composition and oxidation state. How the variation in composition and oxidation state can affect the velocity and density of MORB is still unknown. Here we have examined the velocity and density of hydrous pyrolitic mantle and MORB with varying composition and oxidation state using our obtained elasticity data of Ca-garnet and literature results. Our modeling indicates that LVZs is caused by the subducted MORB rather than water in the transition zone. Further comparison of modeled ρ/VS as well as dlnρ/dlnVS shows that hydrous pyrolitic mantle and MORB exhibit different seismic signatures at the bottom transition zone. These findings provide new insights in our understanding of the origin of LVZs in the bottom transition zone.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMDI003..05W
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 7208 Mantle;
- SEISMOLOGY;
- 8124 Earth's interior: composition and state;
- TECTONOPHYSICS