An experimental study of focused magma transport and basalt–peridotite interactions beneath mid-ocean ridges: implications for the generation of primitive MORB compositions |
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Authors: | Sarah Lambart Didier Laporte Pierre Schiano |
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Institution: | (1) Laboratoire Magmas et Volcans, OPGC, Université Blaise Pascal, CNRS, IRD, 5 rue Kessler, 63038 Clermont-Ferrand Cedex, France |
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Abstract: | 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 basalt; 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-1,310°C; this composition
was then synthesised as a gel and added in various proportions to peridotite FM to simulate focusing factors Ω equal to 3
and 6 (Ω = 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°C; 0.75 GPa-1,270°C; 0.5 GPa-1,250°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 ≈4—that is, a magma/rock ratios equal to ≈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 SiO2 concentrations, and higher concentrations in MgO, FeO, and incompatible elements (Na2O, K2O, TiO2) 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. |
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Keywords: | Dunite Peridotite Partial melting Focused magma transport Primitive MORB Magma/rock interactions |
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