What is the mantle composed of? What are the three sublayers within the mantle? Is there any melt within the mantle?

The Earth's mantle is a layer of the planet that lies between the crust and the outer core. It is predominantly composed of solid rock, but also contains small amounts of other elements. The mantle is divided into three sublayers: the upper mantle, the transition zone, and the lower mantle. Let's explore each of these sublayers, and address the presence of melt within the mantle.

The upper mantle is the topmost sublayer of the mantle and extends from about 35 km to 410 km depth beneath the Earth's surface. It is primarily composed of solid peridotite, which is a dense, coarse-grained rock consisting mostly of the minerals olivine and pyroxene. The upper mantle also features the asthenosphere, a partially molten layer within the mantle. This partial melt allows the rigid lithosphere (the crust and the uppermost mantle) to move over the asthenosphere, resulting in tectonic plate movements.

The transition zone lies beneath the upper mantle and ranges from about 410 km to 660 km depth. Within this zone, the pressure and temperature conditions lead to changes in the crystal structures of the mantle minerals, resulting in a phase transformation of olivine into more dense and stable forms, such as wadsleyite and ringwoodite.

The lower mantle extends from the base of the transition zone (at approximately 660 km depth) to the core-mantle boundary (at about 2,890 km depth). The primary composition of the lower mantle is a solid material consisting of silicate minerals, predominantly bridgmanite (a high-pressure magnesium-iron silicate) and ferropericlase (an iron-magnesium oxide). The lower mantle is characterized by high pressure and temperature conditions that can reach up to 3,500 K.

The mantle is primarily composed of solid rock, but there is evidence of partial melt within the mantle, particularly within the asthenosphere of the upper mantle. This partial melt allows the movement of the lithosphere, driving tectonic processes. However, the overall melt fraction within the mantle is very low, generally less than 1%. In summary, the Earth's mantle is primarily composed of peridotite in the upper mantle, transitioning to denser forms of olivine in the transition zone, and bridgmanite and ferropericlase in the lower mantle. The three sublayers of the mantle are the upper mantle, the transition zone, and the lower mantle. There is a small amount of melt within the mantle, particularly within the asthenosphere of the upper mantle, which is crucial for the movement of tectonic plates.