The Phosphorus Budget of the Silicate Earth Based on an Updated Estimate of the P/Nd Ratio

• Published in: Journal of geophysical research. Solid earth Vol. 128; no. 2
• Main Authors: Ma, Chao, Tang, Yanjie, Ying, Jifeng, Zhang, Hongfu
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-02-2023
• Subjects: Apatite; Basalt; Biological evolution; Biosphere; Continental crust; Differentiation; Earth; Earth mantle; Evolution; Garnet; Geochemistry; Geophysics; Isotopes; Lava; mantle; Mineralogy; Neodymium; Oceanic crust; Oceans; P/Nd ratio; Phosphorus; Ratios; Reservoirs; silicate Earth; Silicates; Subduction
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2022JB025384

Calculating the phosphorus (P) budget of the bulk silicate Earth (BSE) is necessary to understand the global P cycle and the origin and evolution of life. Covariations of P and neodymium (Nd) contents suggest that P is 1.1–1.4 times more incompatible than Nd during mantle melting and magmatic differentiation. As such, the P/Nd ratios of mantle sources can be estimated using inverse modeling, and the P budget of the BSE can be calculated from the P/Nd ratios of the continental crust, mid‐ocean ridge basalts (MORBs), and ocean island basalts (OIBs). The P/Nd ratios for the MORB and OIB mantle sources and continental crust are 62.5 ± 0.8 (2σm), 57.0 ± 0.5 (2σm), and 28.4 ± 12.0 (2σm), respectively. These reservoirs have different P/Nd ratios due to subduction, magmatic differentiation, and mineralogy. Subduction can significantly affect the global P and Nd cycles. Magmatic differentiation, especially in the continental crust, may redistribute P by changing the contents of garnet, apatite, and other minerals. The fractional contributions of these global reservoirs suggest the modern BSE has a P/Nd ratio of 57.7 ± 3.0 (2σm), equating to ∼3.1 × 1020 kg of P in the BSE.
Plain Language Summary
P is an essential nutrient element that underpins biological evolution. As such, knowledge of how P is distributed in different reservoirs of the bulk silicate Earth (BSE) allows us to understand the biosphere and Earth system. We estimated the P budget of the BSE based on the P/Nd ratio using geochemical data for the continental crust, mid‐ocean ridge basalts (MORBs), and ocean island basalts (OIBs). The fractional contributions of continental crust and MORB and OIB mantle sources with different P/Nd ratios indicate the P budget of the modern BSE is ∼3.1 × 1020 kg. In addition, the mantle contains >95% of the P budget of the BSE. This latest estimate could lead to a better understanding of the geochemical differentiation of the silicate Earth, the distribution of P in nature, and the present‐day Earth system.
Key Points
The mid‐ocean ridge basalt and ocean island basalt sources and continental crust have different P/Nd ratios
The different P/Nd ratios may be due to mineralogy, magmatic differentiation, and subduction
An updated estimate of the P/Nd ratio indicates the modern silicate Earth contains about 3.1 × 1020 kg of P