An experimental study of focused magma transport and basalt-peridotite interactions beneath mid-ocean ridges: implications for the generation of primitive MORB compositions

摘要

We performed experiments in a piston-cylinder apparatus to determine the effects of focused magma transport into highly permeable channels beneath mid-ocean ridges on: (1) the chemical composition of the ascending basa and (2) the proportions and compositions of solid phases in the surrounding mantle. In our experiments, magma focusing was supposed to occur instantaneously at a pressure of 1.25 GPa. We first determined the equilibrium melt composition of a fertile mantle (FM) at 1.25 GPa-l,310 deg C; this composition was then synthesised as a gel and added in various proportions to peridotite FM to simulate focusing factors OMEGA equal to 3 and 6 (OMEGA = 3 means that the total mass of liquid in the system increased by a factor of 3 due to focusing). Peridotite FM and the two basalt-enriched compositions were equilibrated at 1 GPa-1,290 deg C; 0.75 GPa-1,270 deg C; 0.5 GPa-1,250 deg C, to monitor the evolution of phase proportions and compositions during adiabatic decompression melting. Our main results may be summarised as follows: (1) magma focusing induces major changes of the coefficients of the decompression melting reaction, in particular, a major increase of the rate of opx consumption, which lead to complete exhaustion of orthopyroxene (and clinopyroxene) and the formation of a dunitic residue. A focusing factor of approx 4-that is, a magma/rock ratios equal to approx = 0.26-is sufficient to produce a dunite at 0.5 GPa. (2) Liquids in equilibrium with olivine (+ - spinel) at low pressure (0.5 GPa) have lower SiO_2 concentrations, and higher concentrations in MgO, FeO, and incompatible elements (Na_2O, K_2O, TiO_2) than liquids produced by decompression melting of the fertile mantle, and plot in the primitive MORB field in the olivine-silica-diopside-plagioclase tetrahedron. Our study confirms that there is a genetic relationship between focused magma transport, dunite bodies in the upper mantle, and the generation of primitive MORBs.