A new phosphate purification method for igneous weathering profiles

Abstract

RationaleThe oxygen isotope composition of phosphate (δ18OPO4) is widely employed for reconstructing paleotemperature and tracing biogeochemical phosphorus cycling. However, existing phosphate purification protocols do not work well for igneous rocks and igneous weathering profiles (IWPs). A reliable purification method is needed for measuring δ18OPO4 in these materials.MethodsOur phosphate purification method includes two steps of cation exchange resin treatment separated by a step of calcium phosphate precipitation (R‐Ca‐R method). In addition, the silver phosphate precipitation in our procedure is featured by slow evaporation to crystallization until the appearance of ammonium nitrate or silver nitrate crystals. We evaluated our methods on weathered and pristine igneous rocks, phosphorite rocks, KH2PO4, and (NH4)2HPO4 solutions.ResultsOur purification method converted over 99.9% phosphate in solution to calcium phosphate, which can be easily decalcified by cation resin. The improved silver phosphate precipitation method produced high phosphate yields (97.1%–99.5%) and retained original δ18OPO4 within analytical uncertainty (2σ = 0.6‰). We applied the purification and precipitation method on five igneous rocks and IWPs, and obtained δ18OPO4 values ranging from 6.4‰ to 8.0‰. Duplicate phosphate extractions yielded δ18OPO4 values differing by less than 0.3‰.ConclusionsWe developed a new phosphate purification method that is applicable to phosphate extraction in igneous rocks and IWPs. We also proposed a reliable indicator for the termination of silver phosphate precipitation. Our method can achieve high phosphate yield and reproducible δ18OPO4 value.