Evolution of magma compositions and phosphorus availability in early Earth’s crust: New constraints from zircon-melt partitioning experiments

Zircons are the oldest remaining witnesses of Earth’s near-surface processes, and conditions in the Hadean and Archaean crust are derived predominantly from their trace element and isotopic compositions. However, quantitative assessment of element and isotope partitioning between zircon and melt remains incomplete. We experimentally determined the effect of phosphorus (P) abundance on zircon-melt partition coefficients of Al (DAl), Li (DLi), and P (DP). Results indicate that P content has opposite effects on DAl and DLi, whereas the partitioning of P itself and other trace elements is independent of P abundance. Parametrization of our results yields new assessments of the aluminum saturation index (ASI) and P and Li contents of Hadean magmas. Magma ASI values are ∼0.5 lower than previously thought and consistently remain below 1 during the first ∼1 Ga of Earth evolution, suggesting involvement of only igneous protoliths. First peraluminous (ASI > 1) melts do not appear until ca. 3.6 Ga, supporting the hypothesis that a transition from a tectonic style dominated by vertical motion to horizontal tectonics accompanied by partial melting of sediments did not occur before that time. New calculated magma Li concentrations for young zircons are in much better agreement with the continental crust Li abundance. Average calculated Archaean and Hadean magma Li concentrations are unrealistically large (>1000 ppm), suggesting that Li in zircons formed before 2 Ga is not primary. Calculated magma P abundances are uniform (∼1900 ± 400 ppm) throughout Earth history, suggesting sufficient crustal P was available throughout the Hadean to support the origin of life.