The role of Ca-phosphates in the deep Earth’s phosphorus, volatile and incompatible trace element cycle (2020-2023, FWF P33124-N)
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 Earth’s 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 Earth’s interior is effected by means of a process called subduction that presumably started already in a very early stage of the Earth’s history more than 4 billion years ago. Subduction causes a continuous albeit almost unimaginably slow homogenization of the uppermost 2900 km of the Earth’s interior. This region involves the Earth’s 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 Earth’s 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 Earth’s 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.
Preliminary regions of the stability of the Ca-phosphates apatite [Ca5(PO4)3(OH, F, Cl)] and tuite [gamma-Ca3(PO4)2] in subduction zones constrained by experiments by Konzett & Frost (2009) (J. Petrol. 50, 2043-2062), Konzett et al. (2012) (Contrib. Mineral. Petrol. 163, 277-296) and by experiments conducted for this study
Backscattered electron photomicrograph of a tuite-bearing high P-T experiment containing the phase assemblage tuite + bridgmanite [MgSiO3] + majorite + ringwoodite + ferropericlase + quenched melt using a synthetic lherzolite as starting material and conducted at 20 GPa and 1600°C
Publications:
Pausch T, Joachim-Mrosko B, Withers AC, Ludwig T, Vazhakuttiyakam J, Konzett J (2024): The role of calcium phosphate in the storage and transport of the phosphorous at the upper-to-lower mantle transition: An experimental study to 25 GPa in a model peridotitic bulk. Geochimica et Cosmochimica Acta. https://doi.org/10.1016/j.gca.2024.04.026