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Deep Earth Elastic Properties and a Universal Pressure Scale

Final Report Summary - DEEP (Deep Earth Elastic Properties and a Universal Pressure Scale)

The Earth’s lower mantle which extends from depths of 660 km down to 2900 km comprises 55 % of the Earth by volume. This region is inaccessible to direct sampling but high pressure experiments indicate that it must contain a significant portion of a mineral called bridgmanite, which is, therefore, the most common mineral on Earth. The amount of bridgmanite in the lower mantle and its chemical composition are unknown and as a consequence the composition of the Earth as a whole is unknown. However, by measuring the speed of vibrational waves through minerals from which the lower mantle is likely comprised, the seismic velocity in the lower mantle can be predicted for a given bulk chemical composition. This can then be compared with actual measurements of the velocities of seismic waves within the lower mantle made by seismologists. This is the only direct method for determining the bulk composition of the Earth. In this project the speed of vibrational waves through minerals at the extremely high pressure and temperature conditions of the lower mantle have been measured. The results show that, contrary to recent proposals, the Earth’s lower mantle has the same chemical composition as the upper mantle. This means that the silicate portion of the Earth is silicon poor in comparison to the earliest meteorite types, which are generally considered to have been the building blocks of the Earth.

Metallic iron forms the Earth’s core but further results of this study show that seismic wave velocities within the lower mantle are consistent with the presence of a small portion of iron metal mixed with the more dominant silicate and oxide minerals. This observation is important because it supports a mechanism whereby the Earth was rapidly oxidised internally soon after its formation. This oxidation process changed the composition of volcanic gases leaving the interior and influenced the development of the early atmosphere.

Water from the surface trapped in rocks of the oceanic plates is cycled into the Earth’s interior at subduction zones. As the rocks penetrate the interior they heat up and water release water, which rises back to the surface in volcanic magmas. However, an important component of water is potentially subducted into the deep mantle. In this study two quite unexpected observations were made that help to constrain this deep water cycle. The first was that the speed of seismic waves through a water-bearing mineral called lawsonite were found to match very closely wave speeds within subduction zones observed by seismologists. This allows constraints to be placed on the amount of water that is currently being deeply subducted into the Earth and implies that this is greater today than in the past. Secondly a new water bearing compound was discovered that has a higher thermal stability than any solid water bearing material yet discovered. This phase is stable at conditions of the Earth’s lower mantle where previously no other potential water reservoir rocks were thought to exist. This observation explains why lavas, thought to arise from material that rose out of the lower mantle, have raised water contents. This mineral therefore forms an important link in the geochemical cycling of water through the interior.