Ca-phosphates in the deep Earth

The elements phosphorus and the halogen elements fluorine and chlorine are essential for the evolution of life on Earth. Phosphorus is indispensible for the energy supply of cells so that no forms of higher life would be possible without this element. Fluorine and phosphorus are also of critical importance for bio-mineralization in the form of bone and teeth. Chlorine plays an essential role in signal transmission within nerves and in the development of tissue. The availability of these elements for the biosphere is controlled by an exchange with the Earths interior, a process that has been operating ever since life appeared on Earth. Weathering of rocks and volcanism are major processes that make phosphorus and halogens available to the biosphere. The transport of these elements from the biosphere back into the Earths interior is effected by means of a process called subduction that presumably started already in a very early stage of the Earths history more than 4 billion years ago. Subduction causes a continuous albeit almost unimaginably slow homogenization of the uppermost 2900 km of the Earths interior. This region involves the Earths crust and mantle, both representing together with the core the three fundamental building blocks of the Earth. Many aspects of the distribution and transport of phosphorus and the halogens in the mantle, in particular in the lower mantle extending to depths greater than 600 km, are still poorly understood. It is known that a special group of minerals called calcium phosphates, which are essentially compounds of calcium, phosphorus and oxygen, play a major role in the transport and storage of phosphorus and halogens within the uppermost 600 km of the Earths interior. Whether the calcium phosphates play any role at still greater depths, which minerals may act as alternative carriers for phosphorus and halogens and how these elements are incorporated into the structures of any suitable phases is still completely unknown. Such knowledge, however, is of crucial importance for any correct estimate of the amounts of phosphorus and the halogens exchanged between the biosphere and the Earths interior throughout the history of the Earth. This research project aims at making an essential contribution to a solution to these questions and, thus, to obtain a better understanding of the global phosphorus and halogen cycles.

Phosphorus plays a crucial role for the emergence of life on Earth because phosphorus compounds are amongst the building blocks of the molecules that contain the genetic information and are also indispensable for the energy supply of living organisms. Phosphorus is made available to the biosphere by weathering of phosphorus-bearing rocks. These are part of a global cycle that, over millions of years, carries rocks from the Earth's interior to the surface and returns them back. This process that causes a continuous mixing of the Earth's interior to ca. 3000 km depth is called subduction. The most important carrier of phosphorus in all major rock types to depths of ca. 270 km is the calcium phosphate mineral apatite. This phase with the formula Ca5(PO4)3(OH, F, Cl), is not only a major phosphorus carrier but also an important host for water and halogens and, thus, also plays a vital role in the global water and halogen cycles. Apatite is also an important host for trace elements including uranium and thorium both of which play a significant role in the production of heat in the Earth's interior. When apatite-bearing rocks are transported to depths exceeding ca. 270 km, apatite breaks down to form the mineral tuite with the formula Ca3(PO4)2. The major aim of this project was to better understand how phosphorus and trace elements are stored in the deep Earth's interior and the role that tuite and possibly other phosphate phases thereby play. To achieve this goal high pressure high temperature experiments were conducted using a peridotite which is the most important type of rock in the deep Earth's interior as a model rock. These experiments demonstrate that tuite is stable to depths and temperatures of at least 880 km and 2000C, respectively, confirming the stability of tuite in the lower mantle. The experiments also show that to ca. 650 km depth not only tuite but also some of the typical silicate minerals of mantle peridotites, namely garnet, olivine and its high pressure modifications, can store significant amounts of phosphorus Therefore in a depth range of ca. 100-650 km phosphorus is jointly stored and transported by phosphates and silicates whereby the role of silicates becomes increasingly more important as depth increases. This situation changes at ca. 660 km depth which marks the transition between upper and lower mantle. Here an entirely new association of silicate and oxide mineral phases become stable none of which can store significant amounts of phosphorus. This indicates that in the Earth's lower mantle phosphate phases such as tuite must play an important role as phosphorus storage phases in case the bulk phosphorus content of a peridotite exceeds ca. 0.01 wt.%.