The Mantle

It builds up most of the earths volume and makes up 84% of its volume. It is solid but behaves like an extremely slow-flowing material. Furthermore, it is the largest chemical reservoir in the planet, which governs the volcanic activity, crust formation and geochemical cycles.

Composition

The mantle is made of silicate minerals, build from:

• Oxygen (O) - 50 wt%
• Silicon (Si) - 22 wt%
• Magnesium (Mg) - 22 wt%
• Iron (Fe) - 4 wt%

These elements are among the most abundant in the planet after differentiation. The mantle is ultraMafic which describes that it has the highest proportions of Mg and Fe compared to the crust, and less Si and Al.

Upper mantle

The upper mantle is mostly made of a rock called Peridotite, a mixture of:

• Olivine ($\mathrm{Mg}{\phantom{A}}_2\mathrm{SiO}{\phantom{A}}_4-\mathrm{Fe}{\phantom{A}}_2\mathrm{SiO}{\phantom{A}}_4$)
• Orthopyroxene
• Clinopyroxene
• Spinel or garnet (depending on pressure)

These minerals reflect the mantle's chemical environment, low water, low alkali elements, but abundant Mg, Fe, Si and O. As pressure increases with depth, these minerals rearrange into new structures, different crystal forms, leading to seismic discontinuities in the mantle.

Mantle melting

The mantle governs the Magma formation. When parts of it partially melts, the melt does not have the same composition as the source rock. Instead, melt is enriched in incompatible elements. These elements do not fit well into the crystal structures of the mantle minerals.

• K, Rb, Cs
• U, Th
• Light rare-earth elements (LREEs)

This selective extraction of incompatible elements changes the mantle over time. There are depleted mantle parts that have less of these incompatible elements and enriched mantle parts that have more.

Transition Zone

Between 410 km and 660 km depth lies the transition zone, where olivin and pyroxene convert into high-pressure forms. This wont change the chemistry but the mineralogy.

Lower mantle

The lower mantle is dominated by:

• Bridgmanite ($\mathrm{MgSiO}{\phantom{A}}_3$)
• Ferropericlase (MgO - FeO)

These minerals are extremely stable under enormous pressures.

Question and Answers

What rock type best represents the composition of the upper mantle?::Peridotite, composed mainly of olivine, pyroxenes, and spinel or garnet.

Why is the mantle described as “ultramafic”?::Because it is rich in Mg and Fe and relatively poor in Si and Al compared to crustal rocks.

What elements dominate mantle composition?
?
Elements:

• Oxygen (O) - 50 wt%
• Silicon (Si) - 22 wt%
• Magnesium (Mg) - 22 wt%
• Iron (Fe) - 4 wt%

What happens to incompatible elements during partial melting of the mantle?::They preferentially enter the melt, enriching magmas relative to the source rock.

What is enriched mantle?::Mantle containing added incompatible elements, often from recycled crustal materials or deep-mantle heterogeneity.

Why is melt from the mantle not the same composition as its source rock?::Because incompatible elements are concentrated in melt rather than in solid mantle minerals.

What mineral group dominates the lower mantle?::Bridgmanite (MgSiO₃ perovskite) and ferropericlase (MgO–FeO).