THE ROLE OF PHOSPHORUS IN CRYSTALLIZATION PROCESSES OF BASALT - AN EXPERIMENTAL-STUDY

To evaluate the effects of phosphorus on differentiation of evolved ba saltic magmas, a series of one-atmosphere experiments on a ferrobasalt ic composition were carried out over a range of P2O5 contents and at o xygen fugacities from 2 log(10) units below to 2 log(10) units above t he fayalite-magnetite-quartz (FMQ) buffer curve. The experiments were performed isothermally, the investigated variables being the amount of added P2O5 and the oxygen fugacity (f(O2)) The results confirm the hi gh solubility of phosphorus in basaltic magmas and show that, at fixed temperature, the progressive addition of phosphorus causes: (1) the d isappearance of olivine at reducing conditions, ( 2) the disappearance of magnetite at oxidising conditions, (3) the stabilisation of pigeon ite throughout the studied range of f(O2), (4) an increase in the moda l plagioclase/pyroxene ratio, and (5) increased melt proportion and la rge changes in the composition of the coexisting melt, in particular t he SiO2 content. The destabilisation of magnetite with increasing P2O5 content may be accounted for by the formation of Fe3+(PO4)(3-) comple xes, rather than by a large change of the redox ratio of the melt; we suggest that the formation of Fe3+(PO4)(3-) complexes dominates that o f P-O-M complexes (where M is a network-modifying cation). The effect of P on the modal proportions of plagioclase and pigeonite may help to explain the mineralogy of some anorthositic rocks and KREEP basalts, as well as the presence of pigeonite as an intercumulus phase in the S kaergaard intrusion (resulting from enrichment of the trapped liquid i n phosphorus). These new results thus provide insights into the effect s of phosphorus in lunar and terrestrial systems, as well as providing information regarding the structural role of phosphorus in silicate m elts.