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Contributions to Mineralogy and Petrology

, Volume 157, Issue 4, pp 429–451 | Cite as

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

  • Sarah Lambart
  • Didier Laporte
  • Pierre Schiano
Original Paper

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.

Keywords

Dunite Peridotite Partial melting Focused magma transport Primitive MORB Magma/rock interactions 

Notes

Acknowledgments

This study has benefited from discussions with Andréa Tommasi, Marguerite Godard and Muriel Laubier. Special thanks are due to the following persons: Jean-Luc Devidal for technical assistance with the electron microprobe; Jean-Marc Hénot for technical assistance with the scanning electron microscope; Ariel Provost for his mass-balance program; Kenneth Koga for assistance with algorithm pMELTS; Mhammed Benbakkar for the ICP-AES analysis of the synthetic basalt in Table 1. This study was supported by the program DyETI of the Institut National des Sciences de l’Univers (INSU-CNRS), through grants to D. Laporte and A. Tommasi. Many thanks to T. L. Grove and two anonymous reviewers for their constructive comments.

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Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  1. 1.Laboratoire Magmas et Volcans, OPGCUniversité Blaise Pascal, CNRS, IRDClermont-Ferrand CedexFrance

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