Insight into the Global Phosphorus Cycle from Apatite in Ash from the 2018 Kilauea Eruptions

Abstract

Volcanic ash eruptions are recognised as an important source of phosporus (P) for the global P cycle, delivering P to soils and the ocean. At volcanoes, P is hosted in primary phases such as melt-precipitated apatite, glass and rarely other phases (e.g., sanidine with 5 wt% P at Tolbachik volcano [1]). Data for P in volcanic gases is scarce and suggests concentrations on the order of ~0.1-10’s ppm [2-3]. However, some condensates record higher P contents, as do some ash samples that include fragments formed by gas-solid (high T contact metamorphic) reactions in the conduit [4]. Interestingly, at atmospheric pressures and high temperatures P is readily released from P2O5 [i.e. it is ‘volatile’; 5], but P is reasonably soluble in basaltic melts [6]. Here, we consider the role of P-bearing volcanic gas in condensation and gas/fluid-solid reactions. We observed apatite crystals attached to sulfate-silica rinds and decorating the interior walls of glass vesicles in ash from the 2018 Kilauea eruptions. These crystals appear to have formed after the primary phases as a result of gas-rich fluid reactions with solid surfaces (rinds or glass). We propose that surface Ca has reacted with P in the gas phase to form these crystals. To test this hypothesis we modelled the formation of apatite using a Gibbs Free Energy minimization approach from a starting composition that included relevant gas and solid phases. The modelling shows apatite is effectively produced from reactions between P-bearing gases and solids. These results indicate that sequestration of P in condensates or products of gas-solid reactions needs to be included in assessing the global P cycle and primary magmatic fluids may have more P than volcanic gases.