Diamond evidence for limited subduction of phosphorus into the lower mantle

The cycling of phosphorus, a life-essential element, is fundamental to Earth’s biogeochemical system. While crustal-level phosphorus cycling is well constrained, mantle-level recycling, particularly the efficiency of phosphorus transport into the deep mantle, remains poorly constrained. Here, we report ferropericlase, calcite (retrogressed aragonite), and tuite (high-pressure phosphate) inclusions in a diamond from Juína, Brazil. The geochemistry of these inclusions combined with thermodynamic modeling indicates that their source rock was subducted altered oceanic crust (AOC) residing at lower-mantle depths. These inclusions provide mineralogical confirmation that phosphorus is transported into Earth’s lower mantle as tuite. Formation of this diamond and its inclusions requires the subducting slab to have followed the coldest end member of recognized slab geotherms among modern subduction systems. The Ca-rich nature of the protolith allows this type of AOC to remain subsolidus, retaining phosphate that would otherwise dissolve during partial melting and move into the shallow mantle. The requirements of both a very cold slab geotherm and a Ca-rich composition for the AOC to transport phosphorus to the lower mantle are stringent conditions that effectively minimize deep recycling. This limiting effect would have been even more pronounced prior to the Paleoproterozoic, when the surface temperatures of AOC would have been higher, leading to enhanced melting. Consequently, phosphorus is dominantly retained in the crust through shallow mantle recycling, now and especially in the ancient past, maintaining an accessible surface phosphorus reservoir critical for nutrient availability and the origin of life.