LAUSR

Abstract In order to understand late magmatic processes that occur in the deep oceanic crust, we performed a phase-equilibria study in a representative late-stage system at a pressure of 200 MPa with a special focus on the role of water and oxygen fugacity. The starting composition for the experiments was evaluated based on a statistical approach using evolved fresh MORB glasses from the database PETDB highest in FeO and TiO2 (in average 17.92 wt% and 3.73 wt%, respectively), assumed to represent frozen liquids erupted at the seafloor generated by extensive differentiation of MORB. We conducted crystallization experiments in a range of temperatures from 850 to 1050 °C with water activities from 0.1 to 1 and under redox conditions from FMQ-1.1 to FMQ + 3.2 (FMQ = fayalite-magnetite-quartz oxygen buffer). The results show that in this Fe- and Ti-rich late-stage system, Fe-Ti-oxides are the liquidus phases followed by clinopyroxene, apatite, and plagioclase, which is more stable at low water activity. Amphibole is stable at high aH2O and at temperatures lower than 900 °C. The evolution of the melt composition with decreasing temperature and aH2O follows in general the liquid lines of descent observed in other experiments in ferrobasaltic compositions. Orthopyroxene, which occurs as late crystallizing phase in many oceanic gabbros, was absent among the experimental mineral assemblages. Based on the evolution of coexisting clinopyroxene and plagioclase compositions, our experiments define a trend similar to the trends from other suites of oceanic gabbros from various locations. Our experimental results shed new light on the formation of highly evolved lavas from fast- and intermediate spreading mid-ocean ridges implying a relatively simple two-step differentiation model. First, primitive MORBs differentiate along the 1 atm cotectic trend to typical ferrobasaltic compositions by fractionation of olivine, plagioclase, clinopyroxene. This trend continues as long as Fe-Ti oxides are not saturated, which is strongly dependent on the prevailing oxygen fugacity. When such ferrobasaltic magmas are cut off from replenishment by fresh MORB, the possibility for further differentiation to highly evolved melts is given by fractionation of oxides, clinopyroxene, plagioclase, apatite and finally amphibole and apatite. Phase relations obtained in our experiments help to understand the formation of oxide gabbros from the ultra-slow spreading Southwest Indian Ridge (IODP drill core Hole 735B). The experimental evaluated crystallization order with temperature is in accord with the record of crystallization of the Hole 735B oxide gabbros.