The mantle lies tens of kilometers to about 2900 kilometers below the earth's crust and surrounds the earth's inner core  According to the different propagation speeds of seismic waves in the earth's interior, the mantle is considered to be divided into upper and lower layers 660 kilometers underground  The upper mantle is composed of minerals with high magnesium content such as olivine, while the composition of the lower mantle is not clear  In the past, researchers believed that, Due to the internal convection movement about 4 billion years after the birth of the earth, the mantle has been homogenized, and the minerals constituting the upper mantle and the lower mantle should be the same
Previous studies suggested that the composition of the mantle was affected by meteorite impact more than 4 billion years ago, but later it was found that the composition of the mantle was consistent with the average composition of other planets in the solar system, and the meteorite impact did not bring significant changes
 However, recent studies have found that the minerals constituting the upper and lower mantle are different