Ca-phosphates play a vital role in the global phosphorus, halogen and incompatible trace element cycle. During subduction, apatite, which is the most abundant terrestrial Ca-phosphate, breaks down to form tuite [γ-Ca3(PO4)2] at depths of ~220-270 km. Whereas previous studies have already explored the stability and phase relations of Ca-phosphates and their capacity to carry phosphorus and incompatible trace elements to depths of approx. 550 km, no such data are available so far for the P-T range corresponding to the upper-to-lower mantle transition.
In this study, multi-anvil experiments were performed at 15 to 25 GPa and 1600 to 2000 °C using a spinel lherzolite doped with 3wt% synthetic β-Ca3(PO4)2, selected incompatible trace elements and halogens as starting material. In this bulk tuite breaks down at or near the solidus between 1750 and 1800°C at 15 GPa and between 1600 and 1700°C at 25 GPa, yielding a negative slope of the tuite-out reaction(s). This indicates that tuite can be stable to depths of at least ~750 km in subduction zone P-T regimes and along the ACMA, allowing a potential stability in convecting mantle. Phases coexisting with tuite include bridgmanite, Ca-perovskite, majoritic garnet, ringwoodite, forsterite, clinoenstatite, ferropericlase, and melt. In the absence of tuite at 20 GPa, the major phosphorus carriers are melt and majoritic garnet. Beyond the stability of majoritic garnet and tuite, and for a bulk P content of 5800 µg/g, Ca-perovskite and bridgmanite are the major mineral hosts for phosphorus that can incorporate ~1400 µg/g and ~87-131 µg/g, respectively.