Characterizing Neodymium Isotopes in Phosphate Rock and Fertilizer Resources

<p data-start="16" data-end="675">Phosphate rock is an essential non-renewable resource used in the production of agricultural fertilizers, yet it also contains trace metals and rare earth elements that may be transferred into the environment during fertilizer use. Isotopic systems such as Sr, U, and Pb have previously been used to trace phosphate sources, but the potential of neodymium (Nd) isotopes as a provenance tracer for phosphate rock and fertilizers remains poorly understood. This study evaluates whether Nd isotopic compositions (&epsilon;Nd) are preserved from phosphate rock into fertilizer products and whether they can distinguish materials derived from different geologic sources. <p data-start="677" data-end="1218">Nine samples were analyzed, including seven phosphate rocks from major phosphogenic provinces: the Permian Phosphoria Formation (USA), Miocene Hawthorn Group (USA), and South Tethyan province (Israel), along with two commercial fertilizers. Phosphate rock samples were sequentially leached to isolate the phosphate-bound fraction, while fertilizers underwent complete digestion. Rare earth element (REE) concentrations were measured by ICP-MS, and Nd isotopic compositions were determined using thermal ionization mass spectrometry (TIMS). <p data-start="1220" data-end="1758">Results show that phosphate rocks preserve marine REE+Y signatures, including negative Ce anomalies and positive Y anomalies, but exhibit substantial variability in &epsilon;Nd values (&minus;9.15 to &minus;4.82), even within a single Permian core. No consistent relationship was observed between Nd concentration and &epsilon;Nd, indicating heterogeneous source inputs. Fertilizer samples displayed more radiogenic &epsilon;Nd values than their corresponding rocks, whereas previously published Sr and U isotope data remained consistent between rock and fertilizer pairs. <p data-start="1760" data-end="2299" data-is-last-node="" data-is-only-node="">These findings suggest that Nd isotopes are sensitive to local depositional heterogeneity and fertilizer processing effects, limiting their straightforward use as a direct tracer linking phosphate rock to fertilizer products. While &epsilon;Nd provides valuable information on provenance and source variability, further work is needed to constrain within-deposit heterogeneity and the effects of processing before Nd isotopes can be reliably applied as an environmental tracer in phosphate fertilizer systems.