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

To evaluate the effects of phosphorus on differentiation of evolved basaltic magmas, a series of one-atmosphere experiments on a ferrobasaltic composition were carried out over a range of P2O5 contents and at oxygen fugacities from 2 log(10) units below to 2 log(10) units above the 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 high solubility of phosphorus in basaltic magmas and show that, at fixed temperature, the progressive addition of phosphorus causes: (1) the disappearance of olivine at reducing conditions, ( 2) the disappearance of magnetite at oxidising conditions, (3) the stabilisation of pigeonite throughout the studied range of f(O2), (4) an increase in the modal plagioclase/pyroxene ratio, and (5) increased melt proportion and large changes in the composition of the coexisting melt, in particular the SiO2 content. The destabilisation of magnetite with increasing P2O5 content may be accounted for by the formation of Fe3+(PO4)(3-) complexes, 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 of 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 Skaergaard intrusion (resulting from enrichment of the trapped liquid in phosphorus). These new results thus provide insights into the effects of phosphorus in lunar and terrestrial systems, as well as providing information regarding the structural role of phosphorus in silicate melts.